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High Integrity C++ Coding Standard Manual

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High Integrity C++ Coding Standard Manual

This is the Programming Research High-Integrity C++ Coding Standard Manual. It is designed for use by organizations that aim to produce high quality C++ software. You can download the PDF format from here: High Integrity C++ Coding Standard Manual

Table of Contents






































1 Introduction

High quality code is portable, readable, clear and unambiguous. This document defines a set of

rules for the production

of high quality C++ code. An explanation is provided for each rule. Each rule shall be enforced

unless a formal

deviation is recorded. Note that Rule 2.1 outlines the process for deviation where this is deemed

necessary. The

guiding principles of this standard are maintenance, portability, readability and safety. This

standard adopts the view

that restrictions should be placed on the ISO C++ language standard1 in order to limit the

flexibility it allows. This

approach has the effect of minimising problems created either by compiler diversity, different

programming styles, or

dangerous/confusing aspects of the language. Different compilers may implement only a subset of the


standard or interpret its meaning in a subtly different way that can lead to porting and semantic

errors. Without applying

good standards, programmers may write code that is prone to bugs and/or difficult for someone else

to pick up and


1 International Standard ISO/IEC 14882 First Edition 1998-09-01

1.1 Typographical Conventions

Throughout this document, a rule is formatted using the following structure.


This statement describes a rule for C++. Adherence is mandatory.


This statement describes a guideline for C++. Adherence is recommended.


This paragraph explains the rationale behind the rule or guideline.


This paragraph explains cases where the rule or guideline does not apply.

Exclusive with

This section lists references to rules or guidelines that should be disabled if this rule or

guideline is


See also

This section lists references to rules or guidelines that are relevant to the current rule or



This section lists sources of relevant material.


C++ keywords and code items are shown in single quotes in the text.

1.2 Escalation policy

This standard aims to enforce current best practice in C++ development by applying semantic and


recommendations, including controlling the use of language features of C++ which can lead to

misunderstanding and/or

errors. In each case a justification is presented as to why the restriction is being applied.

However, in view of the fact

that research into usage of languages in general and C++ in particular is ongoing, this standard

will be reviewed and

updated from time to time to reflect current best practice in developing reliable C++ code.

1.3 Base Standard and Policy

1.3.1 ISO Standard C++

The Base Standard for this document is the ISO/IEC C++ Standard 14882 with no extensions allowed

and further

restrictions as detailed in the rules.

1.3.2 Statically detectable restrictions

This Standard requires that the use of the C++ language shall be further restricted, so that no

reliance on statically

detectable1 undefined or unspecified behaviour listed in this standard is allowed. Coding practice

that results in

undefined behaviour is dangerous and must always be avoided. Where undefined behaviour can be

identified statically,

coding rules limit the potential for introducing it. The rules also prohibit practice which,

although well defined, is known

to cause problems.

1 That is, at compile time

1.3.3 Allowable environments

In general, only ISO C++ compliant compilers should be used. However, at the current time,

compilers do not achieve

full ISO compliance, and it may be some time before the mainstream compilers become completely ISO

C++ compliant.

Hence only the features of a compiler that are proven to be ISO C++ compliant should be used.

Compiler validation is a

useful way to gauge the compliance of a compiler with the ISO C++ standard.

1.3.4 Rule subsets

Some of the rules in this standard are mutually exclusive, hence only a subset of rules should be

selected from this


1.3.5 Examples

This standard contains many example code fragments which are designed to illustrate the meaning of

the rules. For

brevity some of the example code does not conform to all best practices, e.g. unless the rule or

guideline relates

explicitly to exception specifications, the example code may not be exception safe.

1.4 Basis of requirements

Requirements in this standard express:

(a) restrictions on the use of language constructs or library functions that are not completely

defined by the ISO

C++ Standard.

(b) restrictions on language constructs that permit varied compiler interpretation.

(c) restrictions on the use of language constructs or library functions that are known to be


misunderstood or misused by programmers thereby leading to errors.

(d) restrictions on the use of language constructs that inhibit the capabilities of static


The basis of these requirements is that by meeting them it is possible to avoid known problems and

thereby reduce the

incidence of errors.

1.5 Inconsistencies across file boundaries

The rules in this standard refer directly to inconsistencies which can arise within a single

translation unit, i.e. a file which

is being checked or compiled. In C++, owing to its independent compilation model, many such

inconsistencies arise

across file boundaries, (this standard includes inter translation unit rules).

1.6 Policy on non-C++ code and non-standard pre-processors

The embedding of code, written in languages other than C++, within C++ code is forbidden unless

accompanied by a

written justification for its use. The generally poor definition of interfaces to embedded, non C++

code, can lead to

problems. Any necessary use should therefore be localised as much as possible. Embedded code for


other than the Standard C++ pre-processor shall be similarly restricted.

1.7 Deviations

Notwithstanding the requirements of this standard, it may be necessary, and in some cases desirable

to tolerate limited

non-compliance. Such non-compliance shall, without exception, be the subject of a written deviation

supported by a

written justification.

1.8 Compliance matrices for C++ development

Good practice advocates using a compliance matrix to accompany all C++ development projects. A

compliance matrix

shall detail the following information about the project.


Description of development.


Compiler release(s) for development and whether this compiler is validated or not. Compiler

conformance to ISO or

pre-ISO standards shall be stated.


Any compiler switches used (compilers must be validated under these conditions).


Hardware type for which the development is intended.


Operating system for the development, including version number and any patches applied.


Third party libraries used for the development, including version numbers.


Maximum number of characters assumed for unique identification of coded identifiers.


Any software metric limits in force.


List of compiler flaws if available (to ensure portability).


All dependence on implementation defined behaviour.


Conformance to Annex B (Implementation Quantities) of the ISO Standard.

Such a matrix should be laid out as a simple table.

For example:-

Description of Development

Edge-detection algorithm library

Compiler release

GNU compiler version x.x.x.x

Compiler validation status

Yes; ISO

Compiler switches



2 General

High Integrity CPP Rule 2.1 Thoroughly document in the code any deviation from a standard rule.

Justification This standard addresses most situations, however a specific situation may require

deviation from the

standard introducing unexpected anomalies in system behaviour or affecting other system qualities.

Since there are usually several ways to address such requirements, it is important to consider the

benefits and drawbacks of each approach. Alternatives should be documented so that approaches

are not taken, during maintenance, that have already been discarded. All the consequences of the

choice should be documented so that correct assumptions can be made in maintenance.

High Integrity CPP Guideline 2.2 Specify in your compiler configuration that plain 'char' is

implemented as

'unsigned char'.

Justification Support 8-bit ASCII for internationalisation. The size and sign of char is

implementation-defined. If

the range of type char corresponds to 7-bit ASCII, and 8-bit characters are used, unpredictable

behaviour may result. Otherwise prefer to use wchar_t type.

See also Rule 8.4.5

3 Class

3.1 General

High Integrity CPP Rule 3.1.1 Organise 'class' definitions by access level, in the following order

: 'public',

'protected', 'private'.

(QACPP 2108, 2109, 2191, 2192, 2195)

Justification Order by decreasing scope of audience. Client program designers need to know public


designers of potential subclasses need to know about protected members; and only implementors of

the class need to know about private members and friends.

class C // correct access order



// ...


// ...


// ...


Reference Industrial Strength C++ A.12, A.13;

High Integrity CPP Rule 3.1.2 Define class type variables using direct initialisation rather than



(QACPP 5012)

Justification In constructing both objects 'a1' and 'b1', a temporary String( "Hello" ) is

constructed first, which is

used to copy construct the objects. On the other hand, for 'c1' only a single constructor is


Note, some compilers may be able to optimise construction of 'a1' and 'b1' to be the same as 'c1';

however, conversion rules would still apply, e.g. at most one user-defined conversion.

String a1 = "Hello"; // avoid

String b1 = String( "Hello" ); // avoid

String c1( "Hello" ); // prefer

See also Rule 8.4.4

High Integrity CPP Rule 3.1.3 Declare or define a copy constructor, a copy assignment operator and


destructor for classes which manage resources.

(QACPP 2110, 2111, 2112, 2113)

Justification The compiler provided functions that perform copying (i.e. copy constructor and copy


operator), perform bitwise or shallow copy. This will result in copied objects pointing to the same

resource (after copy) and both will share the resource when a duplicated resource may have been

necessary. On destruction each object will free its copy of the resource, which may lead to the


resource being freed more than once.

The destructor should be declared because the implicit destructor will not release resources (


dynamically allocated memory).

Explicitly declare your intentions when writing copy constructors and assignment operators. Make it

clear when you wish to use a shallow copy in your assignment operator by explicitly coding the

function even when your compiler generates the same code by default.

When a copy constructor and a copy assignment operator for a class with pointer types in its member

data are considered impractical, declare the functions private, but do not define them hence

preventing clients from calling them and preventing the compiler from generating them.

See also Guideline 3.1.13

Reference Effective C++ Item 11;Industrial Strength C++ 5.11;

High Integrity CPP Rule 3.1.4 Use an atomic, non-throwing swap operation to implement the copy-

assignment operator ('operator=')

(QACPP 2074, 2081, 2082, 4620, 4621)

Justification Herb Sutter recommends implementing the copy-assignment operator with a non-throwing


member and the copy constructor. This has the advantage that copy assignment is expressed in

terms of copy construction, does not slice objects, handles self-assignment and is exception safe.


relies on the Swap() member being guaranteed not to 'throw' and to swap the object data as an

atomic operation.

class A



A& operator=( const A& rhs )


A temp( rhs );

Swap( temp ); // non-throwing

return *this;



void Swap( A& rhs ) throw ();


Exclusive with Rule 3.1.5

Reference Exceptional C++ Item 41;

High Integrity CPP Rule 3.1.5 Ensure copy assignment is implemented correctly in terms of self

assignment, inheritance, resource management and behaves consistently with the built in

assignment operator.

(QACPP 2074, 2081, 2082, 4072, 4073, 4620, 4621)

Justification Scott Meyers recommends the following:

A& A::operator=( const A& rhs )


if ( this != &rhs ) // 1.


Release_All_Resources_Of( this ); // 2.

Base::operator=( rhs ); // 3.

Copy_Member_Data( this, rhs ); // 4.


return *this; // 5.


1. Prevent assignment to self. Assignment to self is inefficient and potentially dangerous, since

resources will be released (step 2) before assignment (step 4).

2. Release all resources owned by the current object ('this'). Apply delete to any pointers to data

owned and referenced solely by the current object. (This includes all pointers which point to space

allocated using the new operator, unless a reference counting idiom is used.) Owned resources

should be released to prevent problems such as memory leaks.

3. If the function is a member of a derived class, invoke operator=() for the base class. Invoking


assignment operator of the base class, instead of setting base class attribute values, reduces


between classes in the same inheritance hierarchy, improving maintainability.

4. Copy all member data in the argument object according to the copy semantics for the class.


all data members are assigned. If a pointer value is simply copied and the copy semantics do not

support multiple references to an object through a mechanism such as reference counting, a

subsequent delete of one of the objects will cause the pointer in the other object to be invalid.


will cause a problem either through an access attempt via the invalid pointer, or through an

attempt to

delete the pointed-to object in the destructor of the containing object.

5. Return *this as a reference. Returning *this provides behaviour consistent with the built-in

assignment operators.

When maintenance results in the addition of a data item to a class, all assignment operators must



Exclusive with Rule 3.1.4

Reference Effective C++ Items 15, 16, 17;Industrial Strength C++ 5.12, 7.7;

High Integrity CPP Rule 3.1.6 Do not inline virtual functions.

(QACPP 2131)

Justification Virtual functions cannot be inlined due to polymorphism. A compiler cannot know which

instance of a

virtual function will be called at compile time so the inline keyword will be ignored.

class A



virtual ~A() {} // ok destructor must be defined

virtual void foo() {} // avoid function is virtual so

// never inlined


See also Guideline 3.1.7, Guideline 3.2.6, Guideline 8.1.2, Rule 11.8

Reference Industrial Strength C++ A.15;

High Integrity CPP Guideline 3.1.7 Do not use the 'inline' keyword for member functions, inline


by defining them in the class body.

(QACPP 2133)

Justification The inline keyword is a hint to the compiler, its use does not mean that the function

will actually be

inlined. By putting the definition of a function in the class body the compiler will implicitly try

to inline

the function.

In order for a function to be inlined its definition must be visible when the function is called,

by placing

the definition inside the class body it will be available where needed.

class C



int bar() { return 1; } // prefer

inline int car() { return 1; } // avoid

inline int foo(); // avoid


inline int C::bar() // avoid


return 1;


See also Rule 3.1.6, Guideline 3.2.6, Guideline 8.1.2, Rule 11.8

Reference Industrial Strength C++ A.15;

High Integrity CPP Rule 3.1.8 Declare 'const' any class member function that does not modify the


visible state of the object.

(QACPP 4211, 4214)

Justification Although the language enforces bitwise const correctness, const correctness should be

thought of as

logical, not bitwise.

A member function should be declared const if it is impossible for a client to determine whether


object has changed as a result of calling that function.

The 'mutable' keyword can be used to declare member data which can be modified in const functions,

this should only be used where the member data does not affect the externally visible state of the


class C



const C& foo() { return * this; } // should be declared const

const int& getData() { return m_i; } // should be declared const

int bar() const { return m_mi; } // ok to declare const


int m_i;

mutable int m_mi;


Reference Effective C++ Item 21;Industrial Strength C++ 7.13;

High Integrity CPP Guideline 3.1.9 Behaviour should be implemented by only one member function in a


Justification If two functions implement the same behaviour, they should be implemented in terms of

each other or

through a common helper function. An example is a binary 'operator+', which should be implemented

in terms of 'operator+='.

class A



A operator+( const A& rhs )


A temp( *this );

temp += rhs;

return temp;


A& operator+=( const A& rhs );


This will increase code reuse and improve maintainability.

See also Rule 11.1, Rule 12.1

High Integrity CPP Rule 3.1.10 Do not declare conversion operators to fundamental types.

(QACPP 2181)

Justification Conversion operators should not be used, as implicit conversions using conversion

operators can take

place without the programmers knowledge.

This standard advocates the declaration of all one argument constructors as 'explicit' to avoid


conversion by constructor. This rule extends the requirement by disallowing type conversion by

conversion operators.

class B;

class C



operator B(); // conversion operator


See also Rule 3.1.11, Rule 3.2.3

Reference More Effective C++ Item 5;Industrial Strength C++ 7.19;

High Integrity CPP Rule 3.1.11 Do not provide conversion operators for class types.

(QACPP 2181)

Justification Conversion operators should not be used because implicit conversions using conversion


can take place without the programmers knowledge. Conversion operators can lead to ambiguity if

both a conversion operator and a constructor exist for that class. In most cases it is better to

rely on

class constructors.

class C;

class D



D( C ); // 1


class C



operator D(); // 2


void foo( D );

void bar()


C c;

foo( c ); // ambiguous (convert to D by 1 or 2?)


See also Rule 3.1.10

Reference Effective C++ Items 18, 27;More Effective C++ Item 5;Industrial Strength C++ 7.19;

High Integrity CPP Guideline 3.1.12 Provide an output operator ('operator<<') for ostream for all


Justification Providing an output stream operator is useful for the debugging and testing of code.

High Integrity CPP Guideline 3.1.13 Verify that all classes provide a minimal standard interface

against a

checklist comprising: a default constructor; a copy constructor; a copy assignment operator

and a destructor.

(QACPP 2110, 2111, 2112, 2114, 2142, 2185, 2610, 2611, 2612, 2613, 2614, 2615, 2616, 2617,

2618, 2631, 2632, 2633)

Justification The following functions are key to making a class behave like a fundamental type and

providing for

easier comprehension and maintenance.

class X


X(); // default constructor

X( const X& ); // copy constructor

X& operator=( const X& ); // copy assignment operator

~X(); // destructor


The compiler will provide default versions for some or all of these functions depending on what


declared versions exist. The behaviour of the compiler-generated default constructor is not always

appropriate because it does not initialise members that are of POD type.

The behaviour of the other compiler-generated functions is satisfactory only if a class has no


member variables and if each of these implicitly generated functions may have public access.

Defining these functions results in a more consistent interface and a more maintainable and

extensible implementation, and carries few penalties. If a class design does not require these

functions then explicitly declare them private; this will prevent the compiler generated functions


being used.

Exception A default constructor is only implicitly generated by the compiler if there is no user-



See also Rule 3.1.3

Reference Effective C++ Item 11, 33;Industrial Strength C++ 5.11, 14.2;ISO C++ 12.1/5;

3.2 Constructors and Destructors

High Integrity CPP Rule 3.2.1 Ensure all constructors supply an initial value (or invoke a

constructor) for

each virtual base class, each non virtual base class and all non-static data members.

(QACPP 4050, 4051, 4052, 4054, 4206, 4207)

Justification Each constructor must initialise all member data items. Explicit initialisation

reduces the risk of an

invalid state after successful construction. All virtual base classes, direct non virtual base

classes and

non-static data members should be included in the initialisation list for the constructor, for many

constructors this means that the body becomes an empty block.

Regardless of how explicit initialisers are specified, the order of initialisation is as follows:

1. Virtual base classes in depth and left to right order as they, or a class that derives from


appear in the inheritance list.

2. Base classes in left to right order of inheritance list.

3. Non-static member data in order of declaration in the class definition.

class B {};

class VB : public virtual B {};

class C {};

class DC : public VB, public C




: B(), VB(), C(), i( 1 ), c() // correct order of initialization



int i;

C c;


See also Rule 3.2.2

Reference Effective C++ Item 12;Industrial Strength C++ 5.5;

High Integrity CPP Rule 3.2.2 Write members in an initialisation list in the order in which they

are declared.

(QACPP 4053)

Justification Data members are initialised in the order in which they are specified in the class

definition, not the

order they appear in the initialisation list of the constructor. Similarly destructors of members


called in reverse construction order.

See also Rule 3.2.1

Reference Effective C++ Item 13;Industrial Strength C++ 5.6;

High Integrity CPP Rule 3.2.3 Declare all single argument constructors as explicit thus preventing

their use

as implicit type convertors.

(QACPP 2180)

Justification By making single argument constructors explicit they cannot be used accidentally in

type conversions.

class C



C( const C& ); // ok copy constructor

C(); // ok default constructor

C( int, int ); // ok more than one non-default argument

explicit C( int ); // prefer

C( double ); // avoid

C( float f, int i=0 ); // avoid, implicit conversion constructor

C( int i=0, float f=0.0 ); // avoid, default constructor, but

// also a conversion constructor


void bar( C const & );

void foo()


bar( 10 ); // compile error must be 'bar( C( 10 ) )'

bar( 0.0 ); // implicit convserion to C


Exception This rule does not apply to copy constructors as they do not perform a conversion.

See also Rule 3.1.10, Rule 7.1, Rule 7.8, Guideline 10.7, Rule 11.4

Reference Industrial Strength C++ 7.18;

High Integrity CPP Guideline 3.2.4 An abstract class shall have no public constructors.

Justification Abstract classes cannot be used to declare objects, by making constructors protected

it is explicit that

the class can only be used from derived classes.

High Integrity CPP Rule 3.2.5 Ensure destructors release all resources owned by the object.

Justification Failure to release resources owned by the object could result in resource leaks.

Reference Industrial Strength C++ 12.8;

High Integrity CPP Guideline 3.2.6 Do not inline constructors or destructors.

Justification A constructor will implicitly call the constructors for its bases and will initialise

some or all of its

members (potentially calling more constructors). If a constructor is inlined, the initialisation


for the members and bases will appear for every object declaration. Where space is a concern, the

resultant bloat of initialisation code may be a problem. Similarly for destructors.

class A



inline A();


int m_i;

int m_j;

int m_k;


class B : public A



inline B();


int m_m;

int m_n;


// Code typically produced for:

// B b;

b.m_i = 0;

b.m_j = 0;

b.m_k = 0;

b.m_m = 0;

b.m_n = 0;

See also Rule 3.1.6, Guideline 3.1.7, Guideline 8.1.2, Rule 11.8

Reference Industrial Strength C++ A.15;

3.3 Inheritance

High Integrity CPP Rule 3.3.1 Use public derivation only.

(QACPP 2193, 2194)

Justification Using public derivation maintains visibility of public base members in an intuitive

way. Public

derivation indicates the "is-a" relationship. Private derivation indicates the "is-implemented-by"

relationship, which can also be indicated by containment (that is, declaring a private member of


class type instead of inheriting from it). Containment is the preferred method for "is-implemented-


as this leaves inheritance to mean "is-a" in all cases.

class A {};

class B : private A {}; // avoid private derivation

class C : protected A {}; // avoid protected derivation

class D : A {}; // avoid implicitly private derivation

class E : public A {}; // prefer public derivation

Reference Effective C++ Item 42;

High Integrity CPP Rule 3.3.2 Write a 'virtual' destructor for base classes.

(QACPP 2116)

Justification If an object will ever be destroyed through a pointer to its base class, then that

base class should have

a virtual destructor. If the base class destructor is not virtual, only the destructor for the base

class will

be invoked. In most cases, destructors should be virtual, because maintenance or reuse may add

derived classes that require a virtual destructor.

class Base {};

class Derived : public Base



~C() {}


void foo()


Derived* d = new Derived;

delete d; // correctly calls derived destructor


void boo()


Derived* d = new Derived;

Base* b = d;

delete b; // problem! does not call derived destructor!


See also Guideline 17.7

Reference Effective C++ Item 14;Industrial Strength C++ 10.4;

High Integrity CPP Rule 3.3.3 Avoid downcasting base class object pointers to derived class.

(QACPP 3070)

Justification Use virtual functions instead. The most common reason for casting down the

inheritance hierarchy is

to call methods particular to a class in the hierarchy when a pointer to the base class is passed


stored. This may be better achieved by the use of virtual functions.

class A


virtual void bar();


class B : public A


virtual void bar();

virtual void foo();


void foo()


A* a = new B;

static_cast< B* >( a )->foo(); // avoid

a->bar(); // prefer


Reference Effective C++ Item 39;

High Integrity CPP Rule 3.3.4 Avoid casting to a virtual base class as this is irreversible.

(QACPP 3071)

Justification Do not cast a pointer up an inheritance hierarchy to a virtual base class as this

pointer may not be

cast back down the hierarchy.

class A {};

class B : public virtual A {};

A* foo()


B* b = new B;

return static_cast< A* >( b ); // casting to virtual base


Exception A dynamic_cast can be used to cast down a polymorphic hierarchy. In order to use


RTTI is required and this incurs a runtime overhead.

Reference ISO C++ 5.2.9/5, 5.2.9/8;

High Integrity CPP Rule 3.3.5 Override all overloads of a base class virtual function.

(QACPP 2120, 3820)

Justification When a virtual function is overridden then the overloads of that function in the base

class are not

visible from the derived class. If all overloaded functions are not brought into the derived class,


overriding them or with a using declaration, then you can get surprising results when calling


functions of that name.

class Base



virtual void foo( short );

virtual void foo( double );


class Derived : public Base



virtual void foo( short );

void bar()


foo( 0.1 ); // calls Derived::foo( short )!



Reference Industrial Strength C++ 7.16;

High Integrity CPP Rule 3.3.6 If a virtual function in a base class is not overridden in any

derived class then

make it non virtual.

Justification If each derived class is using the base class implementation of the virtual function

then the function

probably does not need to be virtual. Making it non virtual will improve performance by reducing


cost of calling the function.

See also Rule 3.3.7, Rule 3.3.8, Rule 3.3.9, Rule 3.3.11

High Integrity CPP Rule 3.3.7 Only define virtual functions in a base class if the behaviour will

always be

valid default behaviour for derived classes.

Justification Virtual functions in the derived class may or may not override the base class

function implementation.

If the behaviour will not be appropriate for most derived classes then it probably should not be


in the base class.

Exception Destructors must always be defined in the base class.

See also Rule 3.3.6, Rule 3.3.8, Rule3.3.9

Reference Effective C++ Item 36;

High Integrity CPP Rule 3.3.8 Declare a function pure virtual in the base class if each derived

class has to

provide specific behaviour.

Justification If a function is pure virtual in a base class then derived classes must define it.

Use pure virtual

functions and abstract classes to create abstractions that are implemented in derived classes.

See also Rule 3.3.6, Rule 3.3.7, Rule3.3.9

High Integrity CPP Rule 3.3.9 If a virtual function is overridden in each derived class with the


implementation then make it a non virtual function in the base class.

Justification If each derived class has the same implementation for a function then that function

can be

implemented non virtually in the base class, this improves performance, code reuse and eases


See also Rule 3.3.6, Rule 3.3.7, Rule 3.3.8

High Integrity CPP Rule 3.3.10 Ensure that the return type of the virtual function being overridden



Justification A virtual function must be written in the derived class with the same signature as

the virtual function it

overrides in the base class, except that a covariant return type is allowed. This means that the


type of the derived function can be a type derived from the base class return type. If the original

return type was B* or B&, then the return type of the overriding function may be D* or D&, provided


is a public base of D.

class Base



virtual Base* clone() { return new Base( *this ); }


class Derived : public Base



virtual Derived* clone() { return new Derived( *this ); }


void fn( Derived* d, Base* b )


Derived* p1 = d->clone();

Derived* p2 = b->clone(); // error, downcast needed here


Reference Stroustrup;

High Integrity CPP Rule 3.3.11 Do not overload or hide inherited non-virtual functions.

(QACPP 2121)

Justification Overloading or hiding non-virtual member functions can result in unexpected behaviour

as non-virtual

functions are statically bound. This results in the declaration type of the pointer or reference

determining the selection of member functions and not what the pointer or reference is actually

pointing at.

See also Rule 3.3.6

Reference Effective C++ Item 37;Industrial Strength C++ 7.16;

High Integrity CPP Rule 3.3.12 When redeclaring and overriding functions use the same default


values as in other declarations.

(QACPP 2018)

Justification An overridden virtual function should have the same default values as the base class


Default parameter values are determined by the static type of the object. This means that the


values used may not match those of the virtual function being called.

class Base



virtual void goodvFn( int a = 0 );

virtual void badvFn( int a = 0 );


class Derived : public Base



virtual void goodvFn( int a = 0 );

virtual void badvFn( int a = 10 );


void foo( Derived& obj )


Base& baseObj = obj;

// Ok - derived and base have the same default value


baseObj.goodvFn(); // calls Derived::goodvFn with a = 0

obj.goodvFn(); // calls Derived::goodvFn with a = 0

// Uses default value from base even though calls derived function


baseObj.badvFn(); // calls Derived::badvFn with a = 0

obj.badvFn(); // calls Derived::badvFn with a = 10


See also Rule 12.1

Reference Effective C++ Item 38;

High Integrity CPP Rule 3.3.13 Do not invoke virtual methods of the declared class in a constructor



(QACPP 4260, 4261)

Justification Invoking virtual methods in a constructor always invokes the method for the current

class, or its base,

even when the constructor is invoked as part of the construction of a derived class. This also


to virtual methods called in a destructor.

class B




virtual void func();


class D : public B


D() : B() {}

virtual void func();




func(); // B::func called not D::func


High Integrity CPP Rule 3.3.14 Declare the copy assignment operator protected in an abstract class.

(QACPP 2080)

Justification By ensuring that the copy assignment operator is protected, it can only be, and

should only be, called

by the assignment operator of the derived class.

class Base



Base& operator=( const Base& ); // should have protected access


class Derived : public Base



Derived& operator=( const Derived& );


void foo()


Derived obj1;

Derived obj2;

Base* ptr1 = &obj1;

Base* ptr2 = &obj2;

*ptr1 = *ptr2; // problem; partial assignment


Reference More Effective C++ Item 33;

High Integrity CPP Rule 3.3.15 Ensure base classes common to more than one derived class are


(QACPP 2151)

Justification If a class derives non-virtually from more than one class with a common non-virtual

base class, then

multiple copies of that base class will be created. Virtual inheritance ensures that there is only


instance of the base class object, making calls to its member functions unambiguous.

class base



void f();


class derived_left: public base {};

class derived_right: public base {};

class derived: public derived_left, public derived_right {};

void test()


derived d;

d.f(); // ambiguous - derived_left::base::f()

// or derived_right::base::f()


If the intent was that the call should not be ambiguous, then derived should probably inherit base

using virtual inheritance from both of it's immediate base classes. For example:

class derived_left: public virtual base {};

class derived_right: public virtual base {};

Reference Industrial Strength C++ 10.5;

High Integrity CPP Rule 3.3.16 Explicitly declare polymorphic member functions virtual in a derived


(QACPP 2132)

Justification When examining the class definition of a derived class, documentation is needed to

determine which

members are virtual. Specifying 'virtual' explicitly helps to document the class.

class A



virtual void f();

virtual void operator+( A const& );

virtual ~A();


class B1 : public A



virtual void f(); // virtual: make explicit

virtual void operator+( A const& ); // virtual: make explicit

virtual ~B1(); // virtual: make explicit


3.4 Object Oriented Design

High Integrity CPP Rule 3.4.1 Make member data private.

(QACPP 2100, 2101)

Justification By implementing class interfaces with member functions the implementor achieves

precise control

over how the object state can be modified and allows a class to be maintained without affecting

clients. If direct access to object state is allowed through public member data then encapsulation



class C


int a; // avoid (implicitly private)


int b; // avoid


int c; // avoid


int d; // prefer


Reference Effective C++ Item 20, Industrial Strength C++ 10.1;

High Integrity CPP Rule 3.4.2 Do not return non-const handles to class data from const member


(QACPP 4024, 4626, 4628)

Justification Non-const handles returned from const member functions indirectly allow modification

of class data.

Const functions returning pointers or references to member data should return const pointers or


class A



int* foo() const


return m_pa; // permits subsequent modification of private data



int* m_pa;


void bar()


const A a;

int* pa = a.foo();

*pa = 10; // modifies private data in a!


Exclusive with Rule 3.4.3

Reference Effective C++ Items 21, 29;Industrial Strength C++ 7.12;

High Integrity CPP Rule 3.4.3 Do not write member functions which return non const pointers or


to data less accessible than the member function.

(QACPP 2011)

Justification Member data that is returned by a non const handle from a more accessible member


implicitly has the access of the function and not the access it was declared with. This reduces

encapsulation and increases coupling.

Member functions returning pointers or references to member data should return const pointers or


class A



A () : m_private_i(0) {}

int& modifyPrivate()


return m_private_i;


int const& readPrivate()


return m_private_i;



int m_private_i;


void bar()


A a;

// m_private_i is modified.

// m_private_i implicitly has the same access

// as the member function modifyPrivate, i.e. public.


a.modifyPrivate() = 10; // avoid

// Generates a compile error as value is not modifiable.


a.readPrivate() = 10; // prefer


Exclusive with Rule 3.4.2

Reference Effective C++ Items 21, 29, 30;Industrial Strength C++ 7.12;

High Integrity CPP Rule 3.4.4 Ensure friends have a legitimate basis in design, otherwise avoid.

(QACPP 2107)

Justification A function or class should not be made a friend simply for programmer convenience.


increase coupling, complicate interfaces and reduce encapsulation.

High Integrity CPP Rule 3.4.5 When publicly deriving from a base class, the base class should be


(QACPP 2153)

Justification When thinking about object design it is common practice to take the commonality of

each object and

define an abstraction on these features. Leaf classes that inherit from this abstraction are then

concerned primarily with object creation.

class Abstract



virtual ~Abstract() = 0;

// ...


Abstract& operator=( const Abstract& rhs );


class Concrete1 : public Abstract



Concrete1& operator=( const Concrete1& rhs );

// ...


class Concrete2 : public Abstract



Concrete2& operator=( const Concrete2& rhs );

// ...


Reference More Effective C++ Item 33;

High Integrity CPP Rule 3.4.6 Write derived classes to have at most one base class which is not a


abstract class.

Justification Inheriting from two or more base classes, that are not pure abstract classes, is

rarely correct. It also

exposes the derived class to multiple implementations, with the risk that subsequent changes to any

of the base classes could invalidate the derived class.

A pure abstract class is one for which all members are pure virtual functions. The purpose of a


abstract class is to define an interface that one or more concrete classes may implement. It is

reasonable that a concrete class may implement more than one interface.

High Integrity CPP Guideline 3.4.7 All members of a public base class must be valid for a derived


Justification Public inheritance should implement the subtype relationship, in which the subtype or

derived type is a

specialisation of the supertype or base type.

Hence the behaviour of the sub type as determined by its member functions and (the object state) by

its member variables should be entirely applicable to the supertype.

3.5 Operator Overloading

High Integrity CPP Rule 3.5.1 Avoid overloading the comma operator (','), operator AND ('&&'), and


OR ('||').

(QACPP 2077, 2078, 2079)

Justification The behaviour that users expect from these operators is evaluation from left to

right, in some cases

with shortcut semantics. When an operator is overloaded function call semantics come into play,


means that the right and left hand sides are always evaluated and become parameters to a function

call. The order of evaluation of the parameters to the function is unspecified and it is possible

that the

right hand operand is evaluated before the left.

Reference More Effective C++ Item 7;

High Integrity CPP Rule 3.5.2 Always write operations, that are normally equivalent, to be

equivalent when


Justification Users of a class expect that overloaded operators will behave in the same way as the


built-in operator.

a += b // should give the same result as a = a + b

a += 1 // should give the same result as ++a

Reference Effective C++ Item 15;

High Integrity CPP Rule 3.5.3 Ensure that overloaded binary operators have expected behaviour.

(QACPP 2071, 2072, 2073, 4222)

Justification Write overloaded operators such that the behaviour is understandable based on the

behaviour of the

operator on fundamental types.

As far as possible when overloading built-in operators they should follow the behaviour that the


has come to expect. This promotes reuse and maintainability. This does not mean that overloaded

operators should have meanings identical to that of the normal usage.

Operator + should have an additive effect (e.g. string concatenation).

Equivalence operators ( ==,!= ) should only be used to determine object equivalence. If operator!=


defined, operator== should be defined as well.

class Complex



Complex operator+( const Complex& c );


// This will be very confusing:


Complex Complex::operator+( const Complex& c )


cout << "this function does nothing close to addition";

return *this;


Reference Effective C++ Items 21, 22, 23;

High Integrity CPP Rule 3.5.4 Make binary operators non-members to allow implicit conversions of

the left

hand operand.

(QACPP 2070)

Justification By making binary operators members, a conversion to the left hand side of the binary

operator is not


class complex



complex( float r, float i = 0 );

complex operator+( const complex& rhs );


void Add()


complex a( 1, 0 );

a = a + 2; // fine: 2 is converted to complex

a = 2 + a; // error: no applicable operator +


Reference Effective C++ Item 19;

High Integrity CPP Guideline 3.5.5 When overloading the subscript operator ('operator[]') implement

both const and non-const versions.

(QACPP 2140, 2141)

Justification Allow the operator to be invoked on both const and non-const objects.

class Array





for ( int i = 0; i < Max_Size; ++i )


x[ i ] = i;



int& operator[] ( const int a )


std::cout<< "nonconst" << std::endl;

return x[ a ];


int operator[] ( const int a ) const


std::cout << "const" << std::endl;

return x[ a ];



enum { Max_Size = 10 };

int x[ Max_Size ];


int main()


Array a;

int i = a[ 3 ]; //non-const

a[ 3 ] = 33; //non-const

const Array ca;

i = ca[ 3 ]; //const

ca[ 3 ] = 33; //compilation error

return 0;


Reference Effective C++ Item 18;

4 Complexity

High Integrity CPP Rule 4.1 Do not write functions with an excessive McCabe Cyclomatic Complexity.

(QACPP 5040)

Justification The McCabe Cyclomatic Complexity is a count of the number of decision branches within

a function.

Complex routines are hard to maintain and test effectively. Recommended maximum in this standard

is 10.

This rule will highlight complex code which should be reviewed.

High Integrity CPP Rule 4.2 Avoid functions with a high static program path count.

(QACPP 5041)

Justification Static program path count is the number of non-cyclic execution paths in a function.

Functions with a

high number of paths through them are difficult to test, maintain and comprehend. The static


path count should not exceed 200.

High Integrity CPP Rule 4.3 Avoid functions with many arguments.

(QACPP 5042)

Justification Functions with long lists of arguments are difficult to read, often indicate poor

design, and are difficult

to use and maintain. The recommended maximum in this standard is six parameters.

5 Control Flow

High Integrity CPP Rule 5.1 Follow each flow control primitive ('if', 'else', 'while', 'for', 'do'

and 'switch') by

a block enclosed by braces, even if the block is empty or contains only one line.

(QACPP 4013, 4014, 4016, 4060, 4061, 4062, 4063, 4064, 4065, 4066, 4068)

Justification The consistent application of braces to delimit a block makes the code clearer, more

consistent, and

less error prone.

See also Rule 5.11

Reference Industrial Strength C++ 4.3;

High Integrity CPP Rule 5.2 For boolean expressions ('if', 'for', 'while', 'do' and the first

operand of the

ternary operator '?:') involving non-boolean values, always use an explicit test of equality or


(QACPP 3054)

Justification The explicit test clarifies intent, and is more precise. If a boolean expression

involves an object (e.g. a

database pointer or smart pointer), the implicit test will have different behaviour than an

explicit test if

operator==() is overloaded.

If the expression contains an assignment, the explicit test indicates that the assignment was


int bar();

void foo()


if ( bar() ) // avoid


if ( 0 != bar() ) // prefer



High Integrity CPP Rule 5.3 Avoid conditional expressions that always have the same result.

(QACPP 3260, 4090, 4091, 4092, 4093, 4094)

Justification If a conditional expression always has the same result, there is no need for the


void bar( unsigned int ui )


// By definition ui cannot be less than zero hence

// this expression is always false.


if ( ui < 0U )


// never reached




// always executed



Exception It is possible to have an expression that always evaluates to the same result on a given

platform but

not another platform.

void bar( unsigned int ui )


if ( ui <= 0xFFFFU )




// only reached depending on platform



High Integrity CPP Rule 5.4 Follow each non-empty case statement block in a switch statement with a

break statement.

(QACPP 4011, 4612)

Justification This practice has safety advantages and encourages maintainability. If only part of

the action for

multiple cases is identical, place that part in a separate function.

This rule does not require each case statement to have a unique statement block. It does prohibit


through from one case statement block to another.

void foo( int i )


switch ( i )


case 0:

case 1:

++i; // non-empty case statement needs break





Reference Industrial Strength C++ 4.4;

High Integrity CPP Rule 5.5 Do not alter a control variable in the body of a for statement.

(QACPP 4235)

Justification Users expect loop control variables to be modified in the for statement, and also

that the variable is

modified for every iteration. Changing this behaviour makes the code difficult to maintain and


Reference Industrial Strength C++ 4.1;

High Integrity CPP Rule 5.6 Do not alter a control variable more than once in a for, do or while


(QACPP 4236)

Justification The behaviour of iteration statements with multiple modifications of control

variables is difficult to

maintain and understand.

void foo()


for ( int i = 0; i != 10; ++i ) // does this loop terminate?


if ( 0 == i % 3 )






Reference Industrial Strength C++ 4.1;

High Integrity CPP Guideline 5.7 The control variable in a for loop should be tested against a

constant value,

not a function or expression.

(QACPP 4244)

Justification Efficiency

// Avoid:


for ( int i = 0; i < xxx.size(); ++i )


// Prefer:


const int list_size = xxx.size();

for ( int i = 0; i < list_size; ++i )


High Integrity CPP Rule 5.8 Do not use 'goto'.

(QACPP 4000)

Justification 'goto' should never be used to branch into a block, or to branch around a variable

definition. There is

always an alternative using the principles of structured programming.

Reference Industrial Strength C++ 4.6;

High Integrity CPP Rule 5.9 Ensure that every compound statement except the body of a switch


has a single entry point and (barring the propagation of C++ exceptions) a single exit point.

(QACPP 4020)

Justification A single entry and exit simplifies the control graph for the compound statement and

reduces the

overall complexity. A single exit point for a function (whose body is also a compound statement)

makes it easier for reviewers to check that the exit conditions (such as updating of output


are always satisfied. It also provides a single point for post-condition assertions and for


trace instructions.

Exclusive with Rule 5.10

High Integrity CPP Rule 5.10 For functions with non-void return type, ensure all paths have a


statement that contains an expression of the return type.

(QACPP 4022, 4023)

Justification Exiting a function without an explicit return statement is undefined behaviour.

Exclusive with Rule 5.9

Reference ISO C++ 6.6.3/2;

High Integrity CPP Rule 5.11 Include explicit cases for all alternatives in multi-way conditional


(QACPP 4010, 4070)

Justification Forces programmers to consider all cases and reduces the risk of an unexpected value


incorrect execution.

See also Rule 5.1

Reference Industrial Strength C++ 4.5;

High Integrity CPP Rule 5.12 Declare for loop control variables within the for statement instead of

using an

existing variable.

(QACPP 4230)

Justification This is a best practice rule. The main advantage is that the scope of the loop

control variable is

naturally limited to the for loop statement, using this construct achieves this minimum scope.

See also Rule 8.2.2, Rule 8.4.4

6 Constants

High Integrity CPP Rule 6.1 Use suffixes L, U, and UL for all constants of type 'long', 'unsigned

int' and

'unsigned long'.

Justification It is good practice to be explicit with constant values. Use upper-case suffixes.

const unsigned int a = 0U;

const unsigned int b = 0u; // avoid

const unsigned int c = 0; // avoid

const long d = 0L;

const long e = 0l; // avoid

const long f = 0; // avoid

const unsigned long g = 0UL;

const unsigned long h = 0Ul; // avoid

const unsigned long i = 0; // avoid

See also Rule 6.2

High Integrity CPP Rule 6.2 Use suffixes F and L for all constants of type 'float' and 'long


(QACPP 3012)

Justification It is good practice to be explicit with constant values. Use upper-case suffixes.

const float PI = 3.1415F;

const long double R = 0.003; // avoid

const long double A = 0.0L;

const long double Z = 0.0l; // avoid

See also Rule 6.1

High Integrity CPP Rule 6.3 Write the value of a character constant to be in the range of its type.

Justification If the value exceeds the range it will be truncated, but this truncation is not


char c = 'abcde'; // avoid

int i = 'abcde'; // avoid

High Integrity CPP Rule 6.4 Only use escape sequences defined by the ISO C++ Standard.

(QACPP 0076, 0077, 0446, 0447)

Justification Escape sequences (those beginning with \) other than those defined by the standard

have undefined


The ISO C+++ Standard defines the following escape sequences:

Name ASCII Name C++ Name


newline NL(LF) \n

horizontal tab HT \t

vertical tab VT \v

backspace BS \b

carriage return CR \r

form feed FF \f

alert BEL \a

backslash \ \\

question mark ? \?

single quote ' \'

double quote " \"

octal number ooo \ooo

hex number hhh \xhhh

High Integrity CPP Rule 6.5 Do not write character string literal tokens adjacent to wide string



(QACPP 5065)

Justification This results in undefined behaviour.

#define WW "World"

#define LH L"Hello"

char* txt1 = LH WW; // undefined

const char* txt2 = "hello" L"world"; // undefined

High Integrity CPP Guideline 6.6 Global and static data should be const.

Justification Functions that use non-const global or static data are not re-entrant. This causes

problems with

recursion and multi threading. Global variables frequently cause problems in maintenance.

Exception Singleton functions use static data to ensure only one instance of an object is created.

7 Conversions

High Integrity CPP Rule 7.1 Always use casting forms: 'static_cast', 'const_cast', 'dynamic_cast'


'reinterpret_cast' or explicit constructor call. Do not use any other form.

(QACPP 3080)

Justification These casting forms are easier to identify in code and their more narrowly specified

purpose makes it

possible for compilers to diagnose usage errors. The older, C-style cast - "(type) expr" used to

convert one fundamental type to another is subject to implementation-defined effects. For scalar

types it can result in silent truncation of the value. For pointers and references, it does not

check the

compatibility of the value with the target type.

The function style cast - "type( expr )" is equivalent to the C-style cast, is equally difficult to

find in

code, and has the same problems as the C-style cast. This standard does not preclude constructor

style conversions, which use the same syntax as the function style cast. Thus, only function style

casts that make use of a conversion operator are prohibited.

See also Rule 3.2.3, Guideline 7.2, Rule 8.3.5

Reference More Effective C++ Item 2;Industrial Strength C++ 6.2;

High Integrity CPP Guideline 7.2 Minimise the use of casts.

(QACPP 3081)

Justification Excessive use of casts in an implementation may be an indication of a poor design.

See also Rule 7.1, Guideline 10.7

Reference Industrial Strength C++ 6.1;

High Integrity CPP Rule 7.3 Avoid casting away volatile qualification.

(QACPP 3061)

Justification A volatile object is specified as modifiable outside the program, such as with memory

mapped I/O.

Casting away volatile means that the compiler may perform optimisations that are not valid, this


lead to unexpected results in optimised builds.

High Integrity CPP Rule 7.4 Avoid casting away const qualification.

(QACPP 3060)

Justification The existence of a 'const' attribute on a data member or variable is an indication to

programmers that

the given item is not expected to be changed. Casting away 'const'-ness for an object allows non-

const methods to be called for that object which may lead to unexpected behaviour.

Reference Industrial Strength C++ 6.3;

High Integrity CPP Rule 7.5 Avoid using pointer or reference casts.

(QACPP 3030, 3031)

Justification Avoid using pointer or reference casts. They have been referred to as the goto of OO


'goto' tends to complicate the control flow of a program making it difficult to statically

determine the

flow of control. Pointer and reference casts complicate the type flow making it harder to determine


type of an object. This, at best, produces difficult to maintain and very error prone code, as it


control away from the compiler.

Most pointer and reference casts may be eliminated by using virtual functions and tighter control


typing so there is less ambiguity between the declared type of the pointer or reference and the


that is really there.

Exception If your compiler supports run time type information you may use dynamic_casts. This

operator will

check that the type you are asking for is really the type of the pointer or reference and if it is

not it will

return a null. 'dynamic_cast' throws an exception in the case of a reference target type.

High Integrity CPP Rule 7.6 Do not convert floating values to integral types except through use of

standard library routines.

(QACPP 3011)

Justification Since mixed precision arithmetic involves implementation defined and undefined

behaviour, and since

implicit conversions violate this standard, use of specific conversion functions is safer. This


prohibits the use of casts on floating values.

See also Rule 7.8, Guideline 10.7

High Integrity CPP Rule 7.7 Do not cast pointers to and from fundamental types.

(QACPP 3036, 3037)

Justification This occurs most in situations where pointers to objects are passed as integral types

or stored as

integral types. This practice disables the ability of the compiler to perform strong type checking.

See also Rule 7.8

High Integrity CPP Rule 7.8 Do not write code that relies on implicit conversions of arguments in



(QACPP 2180, 3050)

Justification For user defined types, implicit type conversions imply construction and destruction

of temporary

objects. This can create unexpected side-effects and is often inefficient. Remove use of implicit


by overloading the function(s) in question with respect to any arguments which are implicitly cast.

See also Rule 3.2.3, Rule 7.6, Rule 7.7

Reference Industrial Strength C++ 6.1, 7.18;

8 Declarations and Definitions

8.1 Structure

High Integrity CPP Guideline 8.1.1 With the exception of object definitions and unnamed namespace

declarations and definitions, declare in header files: all non-inline functions, classes,

variables, enumerations and enumerators, which are named at namespace scope and which

have external linkage.

(QACPP 5005)

Justification Note that the global scope is included in the term "at namespace scope", as this is

the global


Include the following declarations and definitions in header files

enum level { low, med, high }; // enumeration

extern int a; // data declaration

int foo( int ); // function declaration

class Org; // type declaration

struct Line{ float dx; float dy; }; // type definition

const float s = 3.0E8; // constant definition

// (implicitly internal

// linkage)

Do not include external object definitions in header files

float f = 3.0E8; // global variable definition

See also Rule 11.2

Reference Stroustrup;

High Integrity CPP Guideline 8.1.2 With the exception of unnamed namespace declarations and

definitions, define in header files all inline functions which are at namespace scope and which

have external linkage.

(QACPP 5006)

Justification Note that the global scope is included in the term "at namespace scope", as this is

the global


Where inline function definitions are to be visible to more than one translation unit, place them


header files.

inline int get( char* s ); // inline function

// declaration

inline int get( char* s ) { return ( *s )++; } // inline function

// definition

See also Rule 3.1.6, Guideline 3.1.7, Guideline 3.2.6

Reference Stroustrup;

High Integrity CPP Guideline 8.1.3 With the exception of unnamed namespace declarations and

definitions, define in header files all template definitions which are at namespace scope and

which have external linkage.

(QACPP 5007)

Justification Where explicit instantiation is not the instantiation model, the template definition

must be visible where

it is used. Placing all template definition code in headers will mean that any usage of a template


always have the template available.

Reference Stroustrup;

8.2 Scope

High Integrity CPP Rule 8.2.1 Do not hide declarations in other scopes.

(QACPP 2500, 2501, 2502)

Justification Hiding variables is confusing and difficult to maintain. Changes in variable names

may cause errors

not detectable at compile time. Variables declared in function scope cannot be accessed if they are

hidden by a declaration in an inner scope.

int i;

void foo()


int i; // avoid - hides i in global scope

i = 10;


See also Rule 8.3.4

High Integrity CPP Rule 8.2.2 Avoid global variables.

(QACPP 2300, 2311)

Justification A global variable is one which is declared outside any function, class or unnamed

namespace, and

has external linkage. Such objects can be accessed directly by any module which contains the

appropriate declaration, creating uncontrollable linkage between modules.

Order of initialisation across translation units is unspecified. During program start up,

initialisation of

globals in other translation units cannot be relied upon. If you need a global variable, use a



class Application




Application& theApp()


static Application app;

return app;


See also Rule 5.12, Rule 8.4.4

Reference Effective C++ Item 28;Industrial Strength C++ 1.4, 9.1;

High Integrity CPP Guideline 8.2.3 Always use using declarations or write explicit namespace


Do not use using directives.

(QACPP 5134)

Justification Namespaces are an important tool in separating identifiers and in making interfaces


Using directives, i.e. 'using namespace', allow any name to be searched for in the namespace

specified by the using directive.

Using declarations are better than using directives as the name is treated as if declared in the


containing the using declaration, it will always be considered by lookup not just when a

declaration for

that name does not exist in the current scope. Only the name specified by the using declaration is

brought in from the namespace, so the compiler will not attempt to find other names declared in


namespace as it would with the using directive.

Exclusive with Guideline 8.2.4

High Integrity CPP Guideline 8.2.4 Only have using namespace directives in the main source file,


all include directives.

(QACPP 5135)

Justification A using namespace directive means that names declared in the nominated namespace can

be used

in the scope of the using namespace directive. This greatly increases the likelihood of hiding


and if an include file contains a using namespace directive then every file that uses that include


suffer the effects of the additional names. If there is more than one using namespace directive


there is a possibility of name collisions.

If a using directive occurs above an include directive then the include file contents may be


on names from the nominated namespace. This may lead to maintenance and reuse problems.

Exclusive with Guideline 8.2.3

Reference Herb Sutter, Migrating to namespaces;

8.3 Language Restrictions

High Integrity CPP Rule 8.3.1 Avoid using the keyword 'static' when declaring objects in


(QACPP 2313, 2314)

Justification The use of keyword static in declaration of objects in namespaces is deprecated by

the C++ standard.

Use unnamed namespaces instead.

High Integrity CPP Guideline 8.3.2 Restrict the use of the 'extern' keyword. Do not write 'extern'

where it

is implicit.

Justification Keyword 'extern' is used to specify external linkage. It is implicit in function

declarations written at

global and namespace scope and should not be used in such declarations. Global and namespace

scope const objects and typedefs have internal linkage. Recommended practice is to define all const

objects with internal linkage in header files only. Hence extern qualification is only necessary


declaring data objects with external linkage.

// Header file:

const float s = 3.0E8F; // internal linkage constant definition

extern int a; // external linkage object declaration

int foo( int ); // external linkage function declaration

// Implementation file:

int a = 2; // external linkage object definition

See also Rule 11.2

Reference Stroustrup;

High Integrity CPP Rule 8.3.3 Do not use the 'auto' or 'register' keywords.

(QACPP 5069)

Justification The keyword 'auto' is redundant and 'register' is a hint to the compiler. Most modern

compilers can do

a better job of register allocation than a programmer.

High Integrity CPP Rule 8.3.4 Ensure each identifier is distinct.

(QACPP 1710)

Justification Names should not differ only in case (foo/Foo) or in use of underscores (foobar/

foo_bar). Similarity of

identifiers impairs readability, can cause confusion and can lead to mistakes.

Do not exploit ISO C++ Standard tolerance of the same identifier being declared as different types


the same scope.

// Valid C++:

class C;

int C; // object C hides type of same name

See also Rule 8.2.1

High Integrity CPP Rule 8.3.5 Avoid ambiguous grammar between function style casts and


Justification Function style casts to fundamental types are not allowed by this standard per Rule

7.1 ('static_cast'

should be used). However the following C++ grammar ambiguity remains, see example. All such

ambiguities are resolved to declarations. The essence of this rule is do not write declarations


unnecessary brackets which potentially render the declaration ambiguous.

In the following example the declaration of b is ambiguous. It could be taken to mean conversion of


to type A, or a declaration of an A called b. In such cases of ambiguity, a declaration is always


class A {};

A a; // ok

A (b); // ambiguous

See also Rule 7.1

8.4 Object Declarations and Definitions

High Integrity CPP Rule 8.4.1 Do not write the characters 'l' (ell) and '1' (one) or 'O' (oh) and '

0' (zero) in the

same identifier.

(QACPP 5217)

Justification The characters are similar and may be confused by the reader.

High Integrity CPP Rule 8.4.2 Declare each variable on a separate line in a separate declaration


If the declaration is not self-explanatory, append a comment describing the variable.

(QACPP 4107, 4108, 5075)

Justification Declaring each variable on a separate line makes it easier to find the declaration of

a particular

variable name. Determining the types of variables becomes confusing when pointers and access

specifiers are used for multiple declarations on the same line.

Reference Industrial Strength C++ 5.3;

High Integrity CPP Rule 8.4.3 Initialise all objects at definition. Never use an object before it

has been given

a value.

(QACPP 4101, 4102, 4104, 4105, 4200, 4201, 4204, 4205, 4231, 4238)

Justification Evaluating unset objects is guaranteed to cause problems.

Some compilers do not warn when variables are used before they are assigned a value. When the

initialise-immediately-before-first-access strategy is used, maintenance invariably adds an access

before the initialisation.

void foo1( const char array[] )


int i;

array[ i ]; // undefined behaviour


void foo2( const char array[] )


int i = 0;

array[ i ]; // logically still wrong,

// but at least behaviour is defined


Initialising an object at its definition with a reasonable default value should help avoid compiler


undefined behaviour and limit testing to catching only errors of logic.

See also Rule 8.4.4

Reference More Effective C++ Item 12;Industrial Strength C++ 5.2, 5.5;

High Integrity CPP Rule 8.4.4 Postpone variable definitions as long as possible.

Justification Avoids unnecessary cost of construction and destruction when a variable is unused (e.

g. when an

exception is raised). Allows objects to be initialised when declared, hence avoiding default

constructor being used followed by later initialisation. This assists in documenting the purpose of

variables by initialising them in the context in which their meaning is clear.

#include "MyClass.h"

void initialiseBeforeFirstAccess( SomeType value )


MyClass obj; // call default constructor

obj = value; // call operator=


void initialiseAtDeclaration( SomeType value )


MyClass obj( value ); // call constructor taking SomeType


See also Rule 3.1.2, Rule 5.12, Rule 8.2.2, Rule 8.4.3

Reference Industrial Strength C++ 5.1;

High Integrity CPP Rule 8.4.5 Do not use the plain 'char' type when declaring objects that are

subject to

numeric operations. In this case always use an explicit 'signed char' or 'unsigned char'


Justification Numeric operations that assume signedness of plain char are not portable as it is


defined whether plain char is signed or unsigned.

A good way to handle this issue is to have a project wide typedef for a byte type.

typedef unsigned char byte;

See also Guideline 2.2

High Integrity CPP Guideline 8.4.6 Use class types or typedefs to indicate scalar quantities.

Justification Using class types to represent scalar quantities exploits compiler enforcement of

type safety. If this is

not possible typedefs should be used to aid readability of code for manual checking.

#include "class_time_stamp.h";

ClassTimeStamp start_time; // prefer (compiler type checking)

long start_time; // avoid

typedef long TimeStamp;

TimeStamp start_time; // prefer

Reference Industrial Strength C++ 15.12;

High Integrity CPP Rule 8.4.7 Declare one type name only in each typedef declaration.

(QACPP 5078)

Justification The '&' and '*' in a typedef declaration only apply to the declarator they are

adjacent to. Therefore,

multiple declarations in a typedef can be confusing and difficult to maintain.

// It is not intuitive that value is an int type

// whereas pointer is an int* type.


typedef int* pointer, value;

High Integrity CPP Rule 8.4.8 Do not typedef array types.

(QACPP 2411)

Justification Using typedefs of array types can cause problems relating to bounds checking and


typedef int ARRAY_TYPE[ 10 ];

void foo ()


int* array = new ARRAY_TYPE; // calls new[]

delete array; // incorrect should be delete[]


Exclusive with Rule 8.4.9

See also Rule 12.3

Reference Industrial Strength C++ 13.6;

High Integrity CPP Rule 8.4.9 Do not use unbounded (C-style) aggregate types.

(QACPP 0227)

Justification Array bounds checking is not performed on C-style arrays and any attempt to access

memory outside

the bounds of an array gives rise to undefined behaviour. Also C-style arrays do not maintain a

record of the size of the array.

Array semantics should be provided by C++ classes that enforce appropriate bounds. Prefer to use

STL vector template where possible.

Exclusive with Rule 8.4.8, Rule 10.2

See also Rule 17.9

Reference Industrial Strength C++ 13.6;

High Integrity CPP Guideline 8.4.10 Avoid pointers to members.

(QACPP 5070, 5071)

Justification The syntax of pointer to members is obscure and there are inconsistencies between

different compiler


High Integrity CPP Rule 8.4.11 Use 'const' whenever possible.

Justification This allows specification of semantic constraint which a compiler can enforce. It

communicates to

other programmers that value should remain invariant - by explicit statement. For example, specify

whether a pointer itself is const, the data it points to is const, both or neither:

char* p1; // non-const pointer, non-const data

const char* p2; // non-const pointer, const data

char* const p3; // const pointer, non-const data

const char* const p4; // const pointer, const data

Reference Effective C++ Item 21;

High Integrity CPP Guideline 8.4.12 Directly append the '*' and '&' to type names in declarations



Justification This helps to emphasise that these tokens are part of the type specification.

char* str; // preferred

char *str; // avoid

High Integrity CPP Guideline 8.4.13 Prefer to use signed numeric values, not unsigned.

(QACPP 3084)

Justification Conversions between signed and unsigned types can lead to surprising results.

9 Exceptions

High Integrity CPP Rule 9.1 Do not throw exceptions from within destructors.

(QACPP 4032, 4631)

Justification When an exception is thrown, stack unwinding will call the destructors of any local

objects from where

the exception is thrown to where the exception is caught. Should one of these destructors throw

another exception, the program will immediately terminate.

Reference More Effective C++ Item 11;Industrial Strength C++ 12.5, ISO C++ 15.5.1;

High Integrity CPP Rule 9.2 Only throw objects of class type.

(QACPP 3500)

Justification Exceptions pass information up the call stack to a point where error handling can be


Class types can have member data with information about the cause of the error, and also the class

type itself is further documentation of the cause. User exception types should derive from

std::exception or one of its derived classes.

Reference Industrial Strength C++ 12.11;

High Integrity CPP Rule 9.3 Catch exceptions by reference.

(QACPP 4031)

Justification Using pass-by-pointer for exceptions requires extra calls for memory allocation and

deletion which

may themselves cause further exceptions or memory loss if the exception object is not deleted. If


exception object is caught by value, information in a derived class may be sliced from the

exception in

this exception handler.

See also Rule 11.4

Reference More Effective C++ Item 13;Industrial Strength C++ 12.13;

High Integrity CPP Guideline 9.4 Only use the C++ exception handling mechanism to handle error


Justification Do not rely on exceptions in normal operation of code. Using exception handling as a


mechanism violates principles of structured programming and can complicate maintenance.

High Integrity CPP Guideline 9.5 Each application must have some scheme for ensuring that all


resources are properly released when an exception is thrown.

Justification Orphaned resources are resources that are created between the time the try block is

entered and the

time the exception is thrown. This includes any objects created on the heap (using new) and

resources acquired through function calls (e.g. a call to open a database).

Ensure that the application functions correctly when an exception is thrown and that an error


does not corrupt persistent resources such as databases. Standard exception handling behaviour

only invokes destructors for local objects.

See also Guideline 9.6

Reference Stroustrup;

High Integrity CPP Guideline 9.6 Each application that acquires resources that are not

automatically freed at

program termination must use some mechanism to ensure that acquired resources are freed if

the program unexpectedly terminates.

Justification This ensures that an error condition does not corrupt persistent resources such as


See also Guideline 9.5

10 Expressions

High Integrity CPP Rule 10.1 Use symbolic names instead of literal values in code. Do not use "



(QACPP 4400, 4401, 4402, 4403, 4404)

Justification By eliminating "magic" numbers from the body of the code and placing them in header

files, the code

becomes easier to maintain. Symbolic names should be self documenting.

Exception Literals with intuitive meaning: the character literal '\0', numeric literals 0 & 1 and

the boolean literals

true and false.

String literals that only occur in the code once. This exception does not apply where there is a

requirement for internationalisation.

Reference Industrial Strength C++ 5.4;

High Integrity CPP Rule 10.2 Access to an array should be demonstrably within the bounds of the


(QACPP 4307)

Justification This improves robustness and security. This applies to indices and also to C library

functions that

modify arrays, such as sprintf() and scanf(). Functions that do not provide a means of bounds

checking, such as gets(), should not be used.

Exclusive with Rule 8.4.9

High Integrity CPP Rule 10.3 Do not assume the order of evaluation of operands in an expression.

(QACPP 3220, 3221)

Justification The C++ language standard does not guarantee the order of evaluation of sub-

expressions within an

expression between sequence points. Sequence points are those points in the evaluation of an

expression at which all previous side-effects can be guaranteed to have taken place.

The following example has implementation defined results:

x = foo( ++i, ++i ); // either ++i could be evaluated first

Reference Industrial Strength C++ 15.1, 15.22;

High Integrity CPP Rule 10.4 Use parentheses in expressions to specify the intent of the


(QACPP 3700)

Justification Rather than letting the operator precedence specify the order, use parentheses to

ensure clarity and

correctness. Remove doubt about behaviour of complex expressions. What is obvious to one

programmer may not be to another, and may even be incorrect.

Each pair of operands of a binary operator, except for arithmetic and assignment operators, should


surrounded by parentheses. Each operand to a relational or boolean operator should be either a

single element (no exposed operators) or should be enclosed in parentheses.

High Integrity CPP Rule 10.5 Always discard the result of an assignment operator.

(QACPP 4071)

Justification Assignment operators are frequently mistaken for comparison operators.

Assignment operators should not be used in any type of statement other than an assignment

statement, where the result of the assignment operator is discarded and only the side effect (


the value referenced by the left-hand side) is retained.

int main( int argc, char** argv )


int i = 1;

int j = 2;

// Confusing use of assignment operator, always discard the result


if ( ( j = i ) == 1 )


std::cout << "hit" << std::endl;


// Prefer to write


j = i;

if ( 1 == j )


std::cout << "hit" << std::endl;


return 1;


High Integrity CPP Guideline 10.6 When comparing variables and constants for equality always place


constant on the left hand side.

Justification A common mistake in C++ is to write '=' for '==' in comparisons. By placing the

constant on the left

hand side the compiler protects against this mistake.

int a = getValue();

if ( a == 10 ) // avoid: error prone


if ( 10 == a ) // prefer: compiler will warn if '=' is used


High Integrity CPP Guideline 10.7 Do not use expressions which rely on implicit conversion of an


(QACPP 0150, 3000, 3001, 3010, 3011, 3012, 3050, 3051, 3054, 3062, 3072, 3073)

Justification The effect of implicit conversions are frequently either undefined or implementation-

defined. Be

explicit about any type conversions that are required.

Implicit conversions include those resulting from implicit use of a user-defined constructor and

conversion operator.

See also Rule 3.2.3, Guideline 7.2, Rule 7.6

Reference Industrial Strength C++ 6.1;

High Integrity CPP Rule 10.8 Ensure expressions used in assertions are free from side-effects.

Justification Neither insertion nor removal of the assertion should affect the execution of the

system when the

routine is used correctly.

Reference Industrial Strength C++ 11.1;

High Integrity CPP Rule 10.9 Do not code side effects into the right-hand operands of '&&', '||', '

sizeof' or


(QACPP 3230, 3240, 3241)

Justification The right-hand operands of the logical AND and logical OR operators are conditionally

executed with

the result that side-effects present in these operands might not be executed. The operand of sizeof


never evaluated so that the side-effects that would normally occur from evaluating the expression


not take place. The operand of typeid is evaluated only if it represents a polymorphic type.

bool doSideAffect();

class C



virtual ~C(); // polymorphic class


C& foo();

void foo( bool condition )


if ( false && doSideAffect() ) // doSideAffect not called!


if ( true || doSideAffect() ) // doSideAffect not called!


sizeof( doSideAffect() ); // doSideAffect not called!

typeid( doSideAffect() ); // doSideAffect not called!

typeid( foo() ); // foo called to determine the

// polymorphic type


Reference ISO C++ 5.2.8, 5.3.3;

High Integrity CPP Rule 10.10 Avoid statements that have no side effects.

(QACPP 3242, 3243, 3244, 3245)

Justification For example: The left hand side of a comma operator is evaluated for its side effects

only, and does

not affect the value of the expression. If the left hand side has no side effects it is redundant.

Removing it makes the expression more readable.

static void foo( void )


unsigned int a = 0U;

unsigned int b = 0U;

a = (0U, b); // left side of comma operator has no side effect




High Integrity CPP Rule 10.11 Do not apply the following bitwise operators to signed operands:


operators ('<<', '>>'), bitwise AND ('&'), exclusive OR ('^') and inclusive OR ('|').

(QACPP 3003)

Justification Although left-shift is defined for signed operands, right-shift applied to a negative

operand is

implementation defined. This asymmetry can cause confusion unless shift operations are restricted


unsigned operands only. Bitwise operations on signed operands rely on the representation used for

integral types and should be avoided.

High Integrity CPP Rule 10.12 Validate arguments to be used in shift operators.

(QACPP 3321, 3322)

Justification Right hand side operands to a shift operator which are negative or are larger than

the number of bits

in the left hand side will lead to undefined behaviour.

Reference ISO C++ 5.8;

High Integrity CPP Rule 10.13 Do not mix signed and unsigned data items in the same expression.

(QACPP 3000, 3002)

Justification Conversion from unsigned to signed integral types, taking integral promotion into

account, involves

implementation defined behaviour and is a portability risk.

High Integrity CPP Rule 10.14 Do not mix arithmetic precision in expressions.

(QACPP 3000, 3001, 3010, 3011, 3012, 3051, 3054)

Justification Since mixed precision arithmetic involves implementation defined and undefined

behaviour, it is safer,

for portability reasons, to consistently use double precision for floating point expressions,

unless the

application specifically requires single or extended precision, or homogeneous integral types.

High Integrity CPP Rule 10.15 Do not write code that expects floating point calculations to yield



(QACPP 3270, 4234)

Justification Equivalence tests for floating point values should use <, <=, >, >=, and not use ==

or !=. Floating

point representations are platform dependent, so it is necessary to avoid exact comparisons.

bool double_equal( const double a, const double b )


const double scale = ( std::fabs( a ) + std::fabs( b ) ) / 2.0;

return std::fabs( a - b ) <= ( std::numeric_limits::epsilon()

* scale );


void foo( double f )


if ( f != 3.142 ) // avoid


if ( double_equal( f, 3.142 ) ) // prefer



High Integrity CPP Rule 10.16 Do not use the increment operator ('++') on a variable of type 'bool'


(QACPP 3291)

Justification This is deprecated. Use specific assignment or user functions like 'toggle()', 'set()

' and 'clear()'.

High Integrity CPP Rule 10.17 Guard both division and remainder operations by a test on the right


operand being non-zero.

(QACPP 0015, 0435, 4308)

Justification For defensive programming purposes, either a conditional test or an assertion should

be used.

int doDivide( int number, int divisor )


assert( 0 != divisor );

return number / divisor;


Reference Rule ;

High Integrity CPP Guideline 10.18 Guard the modulus operation to ensure that both arguments are



Justification Use defensive programming to reduce the effect of implementation defined and

undefined behaviour.

Reference Rule ;

High Integrity CPP Rule 10.19 Do not use the comma operator.

(QACPP 3243)

Justification Using the comma operator is confusing and is nearly always avoidable without any loss

of readability,

program size or program performance.

High Integrity CPP Rule 10.20 Do not use the ternary operator (?:) in expressions.

(QACPP 3380, 3381, 3382, 3383, 3384, 3385, 3386)

Justification Evaluation of a complex condition is best achieved through explicit conditional

statements. Using the

conditional operator invites errors during maintenance.

High Integrity CPP Rule 10.21 Apply unary minus to operands of signed type only.

(QACPP 3002)

Justification Unary minus on an unsigned expression, after applying integral promotion, gives an

unsigned result

which is never negative.

11 Functions

High Integrity CPP Rule 11.1 All functions that have the same name should have similar behaviour,


only in the number and/or types of parameters.

Justification This aids maintainability, reuse and conceptual clarity. An overloaded function

should represent a set

of variations on the same behaviour.

See also Guideline 3.1.9, Rule 12.1

High Integrity CPP Rule 11.2 Enclose all non-member functions that are not part of the external

interface in

the unnamed namespace in the source file.

Justification The preferred method of making functions non-linkable from other translation units is

to place the

definitions inside an unnamed namespace; explicitly declaring functions static is now deprecated.


other non-member functions shall not use a storage class specifier and hence by default are

externally visible.

See also Guideline 8.1.1, Guideline 8.3.2

High Integrity CPP Rule 11.3 Specify the name of each function parameter in both the function


and the function definition. Use the same names in the function declaration and definition.

(QACPP 2017)

Justification This helps to document the function, reducing the need for comments and making it

easier to refer to

a parameter within documentation.

Exception However, names of unused parameters may be omitted to avoid "unused variable" warnings.


where the implementor of a function does not have control over the function interface.

High Integrity CPP Rule 11.4 Use pass-by-reference in preference to pass by value or pass by


(QACPP 2010, 2013, 2014)

Justification Pass by reference is more efficient than pass by value as the copy constructor of the

object will not be

invoked. Passing class objects by value can result in an object of a base class being passed


of a copy of the actual object, reducing extendibility (not to mention slicing of the object).

The C-style use of pointer types as function formal parameters in order to update object(s) in the

calling function should be avoided. These formal parameters should be declared as reference types.

See also Rule 3.2.3, Rule 9.3, Rule 11.5, Rule 17.5

Reference Effective C++ Item 22;Industrial Strength C++ 7.5, 7.6;

High Integrity CPP Rule 11.5 Declare read-only parameters of class type as const references. Pass


value read-only parameters that are of a fundamental type.

Justification Declaring parameters as const references allows for compile time checking that the

object is not


There is no advantage to passing a read-only argument of fundamental type by reference, since the

size of most fundamental types is less than or equal to the size of a pointer.

See also Rule 11.4

Reference Industrial Strength C++ 7.3;

High Integrity CPP Rule 11.6 Do not use ellipsis '...' in function parameters.

(QACPP 3074)

Justification Use of the ellipsis notation (...) to indicate an unspecified number of arguments

should be avoided. It

is better to develop specific methods for all situations. Use of ellipsis defeats the type checking

capability of C++. The use of ellipsis for non-POD types is undefined.

Reference ISO C++ 5.2.2/7;

High Integrity CPP Rule 11.7 A function should not return a reference or a pointer to an automatic


defined within the function. Instead, it should return a copy of the object.

(QACPP 4026, 4027, 4028)

Justification Memory for the variable will be deallocated before the caller can reference the

variable. This error

might not cause an error in testing. Returning local objects by value is ok.

For example:

// Do not return a pointer or reference to a local variable :

class String



String( char* A );

String( const String& );


String& fn1( char* myArg )


String temp( myArg );

return temp; // temp will be destroyed before

// the caller gets it


String fn2( char* myArg )


String temp( myArg );

return temp;


Reference Effective C++ Items 23, 29;Industrial Strength C++ 5.9;

High Integrity CPP Rule 11.8 Only declare trivial functions 'inline'.

(QACPP 2133, 2134, 4120, 4121)

Justification The 'inline' keyword is only a hint, and a compiler may not inline every function

declared with the

'inline' keyword.

Inline functions do not necessarily improve performance and they can have a negative effect.

Inappropriate use will lead to longer compilation times, slower runtime performance and larger


See also Rule 3.1.6, Guideline 3.1.7, Guideline 3.2.6

Reference More Effective C++ Item 24;Industrial Strength C++ 7.1, 7.2;

High Integrity CPP Rule 11.9 Do not overload on both numeric and pointer types.

(QACPP 2020)

Justification When there are both pointer and numeric overloads of a function it is not obvious

which function is

called when there is a numeric argument. The ambiguity and confusion is best avoided altogether.

void f( char );

void f( class X* );

void fn()


f( 0 ); // ambiguous

f( 1 ); // calls f( char )

f( '1' ); // calls f( char )


Reference Effective C++ Item 25;

12 Memory Management

High Integrity CPP Rule 12.1 Do not use default arguments with overloaded functions.

Justification Default arguments or overloading allow for the same function to be called in more

than one way. If an

overloaded function has default arguments, ambiguities may arise when calling that function. It is

better to avoid the problems that this creates in code comprehension and choose between using

overloaded functions or a single function with default arguments.

// Avoid, calls to foo with 1 arg are ambiguous


void foo( int );

void foo( int, char c = 10 );

// Prefer, bar(int) is implemented in terms of bar( int, char )


void bar( int, char c );

void bar( int );

// Prefer, default arg is okay here as there are no overloads of car


void car( int, char c = 10 );

See also Guideline 3.1.9, Rule 3.3.12, Rule 11.1

Reference Effective C++ Item 24;

High Integrity CPP Rule 12.2 Allocate memory using 'new' and release using 'delete'. Do not use the


memory management functions malloc(), realloc(), and free().

(QACPP 3332, 3334, 3901)

Justification Functions 'new' and 'delete' invoke constructors and destructors.

Undefined results will occur if 'delete' is invoked on a malloc'ed pointer, or free is invoked on

an object

created with 'new'.

C functions such as strdup() that use any of the C memory management functions should also not be


Reference Effective C++ Item 3;Industrial Strength C++ 13.1;

High Integrity CPP Rule 12.3 Ensure the form used when invoking 'delete' is the same form that was


when memory was allocated using 'new'.

(QACPP 3330, 3331)

Justification For every allocation of an array using new[], the corresponding delete of the array

shall use delete[].

If delete without the array operator is used on memory allocated using the array new operator, the

behaviour is undefined.

void foo()


int * array = new int[ 10 ];

delete array; // undefined behaviour


void bar()


int * obj = new int;

delete[] obj; // undefined behaviour


See also Rule 8.4.8

Reference Effective C++ Item 5;More Effective C++ Item 9;Industrial Strength C++ 8.1, 8.2;

High Integrity CPP Rule 12.4 Do not specify the number of elements when deleting an array of


(QACPP 0013)

Justification This is an obsolete feature which was never added to the ISO C++ standard.

High Integrity CPP Rule 12.5 Do not return a dereferenced pointer initialised by dynamic allocation

within a


Justification In resource management it is important that ownership of resources is clearly

documented. If a

resource is returned from a dereferenced pointer, it will not be clear to the caller of the

function that a

resource is changing ownership.

Reference Effective C++ Items 29, 31;

High Integrity CPP Rule 12.6 Write operator delete if you write operator new.

(QACPP 2160)

Justification Operator new and operator delete should work together. Overloading operator new means

that a

custom memory management scheme is in operation, if the corresponding operator delete is not

provided the memory management scheme is incomplete.

Reference Effective C++ Item 10;Industrial Strength C++ 8.5;

High Integrity CPP Rule 12.7 Document that operator new and operator delete are static by declaring



(QACPP 2162)

Justification 'Operator new' and 'operator delete' are implicitly static functions, however

specifying 'static' explicitly

helps to document the class.

High Integrity CPP Rule 12.8 On use of delete always set the pointer to zero after the delete.

Justification Setting the pointer to zero after a delete operation helps to trap continued use of

that pointer as well

as giving a clear indication that it no longer points to anything.

Note that zeroing is not necessary:

1. When the pointer is assigned immediately after the delete.

2. On deallocating in a destructor as the object goes out of scope.

13 Portability

High Integrity CPP Guideline 13.1 Avoid implementation defined behaviour.

(QACPP 0027, 0029)

Justification Implementation-defined behaviour can vary dramatically across compilers, this causes


problems between different compilers and different versions of the same compiler.

See also Rule 13.4

Reference Industrial Strength C++ 15.1;

High Integrity CPP Guideline 13.2 Use standard language features and standard library functions in


to extra functionality provided by the operating system or environment.

Justification The extra functionality may not be available on different compilers or different


Reference Industrial Strength C++ 15.2;

High Integrity CPP Rule 13.3 Do not exceed the translation limits imposed by the ISO C++ Standard.

Justification Exceeding the translation limits may hamper the compilation of the source code and

make the code


This rule requires that the code comply with the limits stated in Annex B of the ISO C++ Standard.

Reference ISO C++;

High Integrity CPP Rule 13.4 Do not use compiler specific language or pre-processor extensions.

(QACPP 0027, 0028, 0029, 0060, 0095, 1040)

Justification Portability and compiler compatibility, including upward compatibility with future

versions of the same


Examples include compiler-specific pre-processor directives, functions and keywords beginning with


double underscore. Note that #pragma is also non-portable, but is sometimes essential.

See also Guideline 13.1

Reference Industrial Strength C++ 15.2;

High Integrity CPP Rule 13.5 Do not use the 'asm' declaration.

(QACPP 1100)

Justification Use of inlined assembler should be avoided since it restricts the portability of the

code. If it is

essential to use inlined assembler, the assembler dependency should be abstracted out to function(


that contain inlined assembler only.

High Integrity CPP Rule 13.6 Do not make any assumptions about the internal representation of a

value or


(QACPP 2176, 3033)

Justification This rule helps ensure portability of code across different compilers and platforms.

Here are some

recommendations on assumptions that can be made about the target architecture or compiler.

- Do not assume that you know the size of the basic data types, an int is not always 32 bits in


- Do not assume byte order, you may wish in the future to port your code from a big endian

architecture to a little endian architecture or vice versa.

- Do not assume that non POD class data members are stored in the same order as they are


- Do not assume that two consecutively declared variables are stored consecutively.

- Only use built-in shift operators on unsigned fundamental types.

Reference Industrial Strength C++ 13.7;

High Integrity CPP Rule 13.7 Do not cast a pointer to fundamental type, to a pointer to a more


aligned fundamental type.

(QACPP 3033)

Justification Aids portability. Different hardware architectures may have different byte alignment

rules. In most

cases, this rule is equivalent to saying that a pointer to a fundamental type should not be cast to


pointer to a longer fundamental type.

Reference Industrial Strength C++ 15.8;

14 Pre-processor

High Integrity CPP Rule 14.1 Use the C++ comment delimiters "//". Do not use the C comment

delimiters "/*

... */".

(QACPP 1050)

Justification The scope of C++ comments is clearer. Errors can result from nesting of C comments.

Reference Effective C++ Item 4;Industrial Strength C++ 3.4;

High Integrity CPP Guideline 14.2 Do not use tab characters in source files.

(QACPP 5200)

Justification Use \t in string and character literals instead of the tab character.

Tab width is not consistent across all editors and tools. The conventional C++ tab width is 4, but


tools use 8. Not all tools provide the ability to change tab widths and making sure that tab widths


correct across all tools can be challenging. Storing spaces ensures that formatting is preserved on

printing and editing using different tools.

This does not mean that tabs may not be used when editing, provided the editor can convert tabs to

spaces when the file is stored.

High Integrity CPP Guideline 14.3 Write pre-processor directives to begin in column 1 with no


between the '#' and the pre-processor directive.

(QACPP 5229)

Justification It is good practice to adopt a common approach to writing pre-processor statements

and this is a

common convention.

#ifdef SOME_FLAG





Exclusive with Guideline 14.4

High Integrity CPP Guideline 14.4 Write pre-processor directives to begin in column 1 with

whitespace between

the '#' and the pre-processor directive representing nesting in preprocessor conditionals.

(QACPP 5230)

Justification It is good practice to adopt a common approach to writing pre-processor statements.

#ifdef SOME_FLAG





Exclusive with Guideline 14.3

High Integrity CPP Rule 14.5 Control conditional compilation by the use of, or absence of, a pre-


token definition.

(QACPP 0016)

Justification Control of conditional compilation through a specific value of a pre-processor token

is prone to error.

Use #ifdef and #ifndef, rather than #if. Tokens which control conditional compilation should be

checked only for the presence of a definition.

Exception In some cases it is necessary to use a specific value, for example if conditionally

compiled code is

specific to a particular version of a compiler.

High Integrity CPP Rule 14.6 Use the '__cplusplus' identifier to distinguish between C and C++


Justification C++ compilers are required to define '__cplusplus' to indicate a C++ environment,

which can be used

to select function prototypes and differentiate between C and C++ environments.

High Integrity CPP Guideline 14.7 Do not include comment text in the definition of a pre-processor


(QACPP 5117)

Justification Comments can be written in pre-processor macros, they are replaced by a space before

the pre-

processor translation phase. However they are often hard to read and resulting compiler diagnostics

(if any) can be very hard to interpret. Also, some compilers have implementation-defined limits on


length of the fully expanded line and do not necessarily report errors when this limit is exceeded.

Place comments just before the line containing the '#define' directive and not in the body of the


High Integrity CPP Rule 14.8 Ensure that the last line of all files containing source code is

followed by a


(QACPP 5118)

Justification Behaviour is undefined where a source file, that is not empty, does not end in a new-

line character.

Reference ISO C++ 2.1/2;

High Integrity CPP Rule 14.9 Use <> brackets for system and standard library headers. Use "" quotes


all other headers.

(QACPP 1011, 1012)

Justification It is important to distinguish the two forms of #include directive not only for

documentation purposes

but also for portability. Different compilers may have different search methods and may not find


correct header file if the wrong form of #include is used.

Reference Industrial Strength C++ 15.4;

High Integrity CPP Rule 14.10 Do not include a path specifier in file names supplied in #include


(QACPP 1010, 1013)

Justification Specific directory names and paths may change across platforms and compilers. For


limits.h is in the sys subdirectory for the Sun compiler, but in the normal include directory for


The include directory path should be specified in the makefile. Paths in include file names are not



Put paths in the makefile; #include only the filename:

cc -I$(INCLUDE) -I$(INCLUDE)/sys


Reference Industrial Strength 15.5;

High Integrity CPP Rule 14.11 Incorporate include guards in header files to prevent multiple

inclusions of

the same file.

(QACPP 0063, 0103, 1000, 1001)

Justification This resolves the problem of multiple inclusion of the same header file, since there

is no way of

knowing in which sequence the header files will be included or how many times they will be


This prevents compiler and linker errors resulting from redefinition of items. It also prevents


file inclusion.

The defined macro should be the same as the name of the file, with any '.' changed to '_', and all

characters in upper-case.

// File example.h:

#ifndef EXAMPLE_H

#define EXAMPLE_H

// All declarations and definitions


Reference Industrial Strength C++ 1.7;

High Integrity CPP Rule 14.12 Use lower-case for file names and references to file names (such as



(QACPP 5121)

Justification This rule results from an incompatibility between mono-case and case-sensitive file

systems. For

example, an include directive with a mixed-case file name will work successfully in DOS, but fail


UNIX unless the case of the file name matches.

Reference Industrial Strength C++ 15.6;

High Integrity CPP Rule 14.13 Write header files such that all files necessary for their

compilation are


Justification This means that every header file is self sufficient: a programmer who puts #include

"header.h" should

not have to #include any other headers prior to that file.

High Integrity CPP Rule 14.14 Enclose macro arguments and body in parentheses.

(QACPP 1030, 1031)

Justification If the body of a macro contains operators and these operators and the macro arguments

on which

they operate are not enclosed within parentheses then use of the macro in certain contexts could


rise to unexpected results.

For example:

#define BAD_SQ( A ) A * A

#define OK_SQ( A ) ( ( A ) * ( A ) )

int x = BAD_SQ( 6 + 3 ); // expands to: 6 + 3 * 6 + 3

int y = OK_SQ( 6 + 3 ); // expands to: ( ( 6 + 3 ) * ( 6 + 3 ) )

Exception It is not necessary to place the body of an object-like macro in parentheses if it

consists of a single

token. For example, a macro body comprising one token which is a literal or an identifier should


be parenthesized.

Exclusive with Rule 14.17, Rule 14.19

Reference Effective C++ Item 1;

High Integrity CPP Rule 14.15 Do not use pre-processor macros to define code segments.

(QACPP 1023)

Justification Macro expansion is performed by textual substitution, without regard for the

underlying syntax or

semantics of the language.

Use inline expansion and/or function templates to achieve the desired effect. These obey all the

normal language rules.

High Integrity CPP Rule 14.16 Do not use the NULL macro.

(QACPP 1024)

Justification The NULL macro is defined by the C++ standard as an implementation defined C++ null


constant, however when interfacing to C code, that C code will have NULL defined as:

#define NULL ((void*) 0 )

Varying definitions of NULL in third-party libraries may be incompatible and lead to significant



Avoid a mixture of definitions of NULL by not using it. Use 0 instead as it is valid for any

pointer type.

Reference ISO C++ 4.10;Effective C++ Item 25;

High Integrity CPP Rule 14.17 Use const objects or enumerators to define constants, not #define.

(QACPP 1020, 1021)

Justification This should be done for type safety and maintainability. Preprocessor constants do

not have a type

other than the literal type and this allows for misuse. In addition, most debuggers do not


#define'd values, whereas values with symbolic names can be accessed.

Exclusive with Rule 14.14

See also Rule 14.19

Reference Effective C++ Item 1;Industrial Strength C++ 13.5;

High Integrity CPP Rule 14.18 Do not use digraphs or trigraphs.

(QACPP 5210)

Justification Trigraphs are special three character sequences, beginning with two question marks

and followed by

one other character. They are translated into specific single characters like \ or ^. Digraphs are

special two character sequences that are similarly translated.

Do not use '??' at any point in the code as in combination with some characters this will be


and cause confusion. Be aware of digraph character sequences and avoid them. It is possible to

avoid such sequences arising accidentally by using spaces.

Trigraph Equivalent Digraph Equivalent

??= # %:%: ##

??( [ %: #

??< { <: [

??) ] <% {

??> } :> ]

??/ \ %> }

??' ^

??! |

??- ~

// Here the ??/??/?? becomes \\?? after trigraph translation


cout << "Enter date ??/??/??";

// Here the <::std::pair becomes [:std::pair


::std::vector<::std::pair > vector_of_pairs;

High Integrity CPP Rule 14.19 Do not use function macros, use inline functions instead.

(QACPP 1020)

Justification Inline functions are as 'efficient' as function macros and have the predictable

behaviour and type

safety advantages of a regular function.

#define MAX(a,b) ( ( a ) > ( b ) ? ( a ) : ( b ) ) // avoid

template< typename T > // prefer

inline const T& max( const T& a, const T& b )


return a > b ? a : b;


Exclusive with Rule 14.14

See also Rule 14.17

Reference Effective C++ Item 1;Industrial Strength C++ 13.5;

15 Structures, Unions and Enumerations

High Integrity CPP Rule 15.1 Do not use variant structures (unions).

(QACPP 2176)

Justification Unions provide a way to alter the type ascribed to a value without changing its

representation. This

reduces type safety and is usually unnecessary. In general it is possible to create a safe


using polymorphic types.

Reference Industrial Strength C++ 13.7;

High Integrity CPP Rule 15.2 Do not include member functions or access specifiers in struct types.

(QACPP 2171, 2173, 2175)

Justification Treating struct as a true class type has no advantages over using the class

specifier. Struct should be

used to designate the POD type equivalent to a C struct. Note that the default access specifier for

structs and unions is public.

Reference Industrial Strength C++ A.14;

High Integrity CPP Rule 15.3 Do not rely on the value of an enumerator.

Justification When using an enumerator to represent a constant value, using the symbolic name


documentation and maintainability.

enum Colours { RED = 0xA, GREEN, BLUE };

bool foo()


Colours colour = GREEN;

if ( 11 == colour ) // avoid


else if ( BLUE == colour ) // prefer




// colour is red?



See also Rule 15.4

High Integrity CPP Rule 15.4 Avoid casting an integer to an enumeration as the result of this cast


unspecified if the value is not within the range of the enumeration.

(QACPP 3013)

Justification The underlying type chosen to represent an enumeration is implementation defined. The

type is only

required to be large enough to hold all the values defined in the enumeration set. A cast from an

integral value to an enumeration may cause overflow on the integral value.

Even when an overflow does not occur, an enumerator may not exist for that integral value and the

enum object will not have a symbolic name for its value.

enum Colours { RED, GREEN = 2, BLUE };

void bar()


Colours colour = static_cast< Colours >( 1000 ); // may cause overflow

if ( 1000 == colour )


// may not be reached



void foo()


Colours colour = static_cast< Colours >( 1 ); // value not in set

switch ( colour )


case RED:

case GREEN:

case BLUE:



break; // value not handled



See also Rule 15.3

16 Templates

High Integrity CPP Rule 16.1 Avoid implicit conversions from class templates to non-dependent types


this ensures that clients cannot bypass the class interface.

(QACPP 2183)

Justification Each instantiation of a class template is a different type, but when there is a

conversion operator to a

non dependent type then different instantiations can be treated as if they were that type. This is


particular problem when an implicit conversion to a fundamental type is available, as any two


instantiations may be operands to every built-in operator.

// Example with smart pointers:

template< typename T >

class SmartPointer



SmartPointer( T* );

operator void const*();


// Here the designer added conversion to void

// const* to help comparison to the null pointer.

// However a bad side effect to this feature

// is that it is possible to generate equality

// comparison between two smart pointers on

// different types:

void doIt( AA* pa, BB* pb )


SmartPointer spa( pa );

SmartPointer spb( pb );

if (spa == spb) // problem! comparing different pointer types



Reference More Effective C++ Item 28;

High Integrity CPP Rule 16.2 Do not define a class template with potentially conflicting methods.

Justification Defining a template with potentially conflicting methods will cause problems with

some instantiations

of that template.

template< typename T >

class A



void foo( T );

void foo( int );


template class A< int >; // error void foo(int) declared twice

High Integrity CPP Rule 16.3 Only instantiate templates with template arguments which fulfill the


requirements of the 'template'.

Justification Using a template argument where some of the requirements for the argument are not met

may cause

compilation errors. Implicit instantiation only occurs for the parts of a template definition that

are used.

If an instantiation error is contained in a function definition that is not called then the error

will not be

seen unless maintenance leads to that function being called. Explicit template instantiation will

instantiate all parts of the template defintion, ensuring that the template argument is valid.

class person



int getAge( void );


template< class T >

class singleVal



bool isMatch( T t )


return ( singleton == t );



T singleton;


void foo( person const& other )


singleVal< person > emperor; // no error as isMatch not yet

// called

if ( emperor.isMatch( other ) ) // instantiation error,

{} // no 'op==' for person


// explicit instantiation of complete template definition


template class singleVal< person >; // error! no op== for person

High Integrity CPP Guideline 16.4 Only use templates when the behaviour of the class or function

template is

completely independent of the type of object to which it is applied.

Justification If behaviour varies with the type of object, inheritance with virtual functions

should be used.

Templates should implement genericity and should be applicable to any type that meets the interface

requirements specified in the template definition.

template< typename T > void foo( T t )


// Avoid: a template definition should not have behaviour

// defined by the dynamic types of template arguments


if ( 0 != dynamic_cast< SomeType* >( t ) )



template< typename T > void bar( T t )


// Prefer: this template works with any type that provides

// the member someFunction in its interface




17 Standard Template Library (STL)

High Integrity CPP Rule 17.1 Use Standard C++ Library headers defined by the language standard and


outdated .h headers. For example, use and not , and not


(QACPP 1014)

Justification ISO C++ defines the standard implemention of library components. Programmars should use these

versions of the library rather than vendor-specific or C library versions.

High Integrity CPP Rule 17.2 Use Standard Template Library containers and algorithms in preference to

custom designs.

Justification The STL forms part of the language standard and represents a well-tested library of re-usable code.

High Integrity CPP Guideline 17.3 Make copying efficient for objects in containers.

Justification It is important to be aware that STL containers use copy operations and to implement these

operations as efficiently as possibly. When an object is added to a container it is copied and the copy

is stored inside the container; for objects of class type this copy is made by the class copy constructor.

For some container types, including vector, objects may be moved inside the container; this move will

use the class copy assignment operator.

class MyClass




MyClass( MyClass const& rhs ); // copy ctor

MyClass& operator=( const MyClass& rhs ); // copy assignment


void foo( const MyClass& obj, const MyClass& anotherObj )


std::vector< MyClass > vec;

// Call to push_back will call the copy constructor.


vec.push_back( obj );

// Call to insert will call the copy assigment operator for

// each object stored after the insert iterator.


vec.insert( vec.begin(), anotherObj );


See also Guideline 17.4

Reference Effective STL Item 3;

High Integrity CPP Guideline 17.4 Where copying is expensive use containers of pointers or smart pointers.

Justification Pointers are small and have builtin operators for copying values. Because of this containers of

pointers are efficient for insertion, sorting and other operations.

If containers of pointers are used then it is the responsibility of the programmer to manage the lifetime

of the objects. This can be done with a reference counting smart pointer class.

class BigClassWithLotsOfData {};

void badVectorUsage( std::vector< BigClassWithLotsOfData >& vec )


BigClassWithLotsOfData newObj;

vec.push_back( newObj ); // calls expensive copy constructor!


void goodVectorUsage( std::vector< BigClassWithLotsOfData* >& vec )


// This only copies a pointer so insertions are cheap.


BigClassWithLotsOfData* pNewObj = new BigClassWithLotsOfData;

vec.push_back( pNewObj );


See also Guideline 17.3, Rule 17.5

Reference Effective STL Item 3;

High Integrity CPP Rule 17.5 Do not attempt to insert derived class objects in a container that holds base

class objects.

Justification If you attempt to insert an object of derived type into a container of base type objects then slicing will

occur and the container will not hold the intended object. The problem of slicing is eliminated when

pointers to base class objects are stored.

See also Rule 11.4, Guideline 17.4

Reference Effective STL Items 3, 7;

High Integrity CPP Guideline 17.6 Use empty() instead of checking size() against zero.

Justification Testing empty() and comparing size() to zero are the same thing, however for some containers it is

expensive to calculate the number of elements so it is less efficient to compare the size to zero. It is

always better to use empty when testing if a container has elements.

std::list< Node > myList;

if ( false == myList.empty() ) // constant time test




if ( 0 == myList.size() ) // linear complexity operation




Reference Effective STL Item 4;

High Integrity CPP Guideline 17.7 Do not use STL containers as public base classes.

Justification All STL containers lack a virtual destructor. If a class with a non virtual destructor is used as a base

class it is possible to get undefined behaviour on destruction, this happens if the derived class is

allocated on the heap and later deleted through a base class reference.

class MyVector : public std::vector {};

void doSomething()


MyVector* pHeapVec = new MyVector; // allocate derived obj on heap

std::vector* pBaseVec = pHeapVec; // access through base class

delete *pBaseVec; // undefined behaviour!


See also Rule 3.3.2

High Integrity CPP Rule 17.8 Never create containers of auto_ptrs.

Justification 'auto_ptr' has destructive copy semantics, this means that when you copy an auto_ptr the source

loses its value. STL containers require that element types provide copy semantics such that the

source and destination are equivalent after a copy.

The C++ Standard prohibits containers of auto_ptrs so they should not compile. However some STL

implementations and some compilers do not reject them.

class MyClass {};

void foo( vector< auto_ptr< MyClass > >& myVec )


// After myObj2 is initialised myObj1 is a 0 ptr!


auto_ptr< MyClass > myObj1 = myVec[ 0 ];

auto_ptr< MyClass > myObj2 = myObj1;


Reference Effective STL Item 8;

High Integrity CPP Rule 17.9 Use vector and string in place of dynamically allocated arrays.

Justification vector and string automatically manage their storage requirements so the programmer does not need

to manage dynamically allocated memory. This removes the potential for inefficiency and memory

related bugs that can occur with dynamically allocated arrays.

vector and string contain commonly needed operations and are interoperable with STL algorithims so

programmers can avail themselves of a large body of efficient and reliable code.

See also Rule 8.4.9

Reference Effective STL Item 13;

High Integrity CPP Guideline 17.10 Where possible pre-allocate in containers to save unnecessary


Justification STL containers grow as needed when elements are inserted. However, increasing the capacity of a

container can be costly as it involves allocation of memory and potentially moving previously inserted

elements. While this overhead is not always an issue it is better to reserve the required storage space

in advance as this reduces the number of memory allocation requests and limits having to move


void badPushBackManyNumbers( vector< int >& vec )


// This code may result in the vector increasing

// its capacity several times.


for ( int i = 0; i < 100; ++i )


vec.push_back( i );



void goodPushBackManyNumbers( vector< int >& vec )


// This code cleverly preallocates so the vector only

// increases its capacity once.


vec.reserve( vec.size() + 100 );

for ( int i = 0; i < 100; ++i )


vec.push_back( i );



Reference Effective STL Items 14, 30;

High Integrity CPP Rule 17.11 When passing vector types to C style functions use '&v[ 0 ]'.

Justification The STL class vector is designed to be usable as a C style array. The elements in a non-empty

vector are guaranteed to be stored contiguously so it is possible to use the address of the first

element in the container as a pointer to an array of elements. The best way to do this is by '&v[0]'

where v is a vector of some object with a C compatible type. Other methods of treating a vector as an

array are implementation defined and not portable.

extern "C" void functionTakingArrayOfInt( int i[] );

extern "C" void functionTakingPointerToArrayOfInt( int* pvi );

void goodWayToUseCFunctionWithVector( vector< int >& vec )


assert( false == vec.empty() && "this doesnt work with empty vectors!"


functionTakingArrayOfInt( &vec[ 0 ] ); // ok

functionTakingPointerToArrayOfInt( &vec[ 0 ] ); // ok


void badWayToUseCFunctionWithVector( vector< int >& vec )


functionTakingArrayOfInt( vec.begin() ); // may not work as intended!


Reference Effective STL Item 16;

High Integrity CPP Rule 17.12 Only use STL string's member c_str to get a const char* to use with legacy


Justification The c_str method is defined to return a valid, null terminated C style string. Other methods of getting

a C style representation are implementation defined and not portable.

Reference Effective STL Item 16;

High Integrity CPP Rule 17.13 Do not use vector.

Justification vector does not conform to the requirements of a container and does not work as expected in

all STL algorithms.

In particular &v[0] does not return a contiguous array of bools as it does for other vector types.

Reference Effective STL Item 13;ISO C++ 23.1;

High Integrity CPP Rule 17.14 Return false for equivalent values in relational predicates.

Justification Sorted containers, and algorithms that operate on sorted containers, require comparison predicates

that define the sort order of the elements. These predicates are used to test if elements are equal,

they do so by checking that neither element preceeds the other in the sort order.

Returning true from a comparison predicate for equivalent elements means the container will never

detect that elements are equal, resulting in an invalid state.

// Potential algorithm determining equivalent elements for sorted

// containers.


bool areElementsEqual( T& a, T& b )


// pred is a comparison predicate that defines the sort order

// of a & b.


if ( !pred( a, b ) && !pred( b, a ) )


// a and b are equivalent



See also Rule 17.15

Reference Effective STL Item 21;

High Integrity CPP Rule 17.15 Never modify the key part of a set or multiset element.

Justification sets and multisets sort elements as they are inserted into the container, therefore any change to an

element that affects its sort position will corrupt the container and result in very hard to find bugs.

See also Rule 17.14

Reference Effective STL Item 22;

High Integrity CPP Guideline 17.16 Minimise mixing of iterator types.

Justification Iterator types are implementation defined. Portability issues may arise as different STL

implementations may have different operations defined for particular iterators.

Efficiency may suffer where different iterator types are used as operands in operator expressions.

Potentially the operator is a function call for which one or both of the iterators must undergo a


Certain container member functions may not be called as they only accept the plain iterator type as a


// May or may not be a member in some implementations.


template< typename T >

bool operator== ( vector< T >::const_iterator& lhs,

vector< T >::const_iterator& rhs );

void bar()


vector< int > v;

vector< int >::iterator lhs = v.begin();

vector< int >::const_iterator rhs = v.end();

// Should operator== be implemented

// as a member of const_iterator then

// this this code will not compile.


// rhs implicitly converted

// to const_iterator followed by

// function call to operator ==.


if ( lhs == rhs )



void foo( vector< int >& v,

vector< int >::const_iterator& iter )


// Error cannot convert from

// const_iterator to iterator.


v.insert( iter, 10 );


Reference Effective STL Item 26;

High Integrity CPP Rule 17.17 The result of a predicate should depend only on its parameters.

Justification For certain algorithms there is no requirement that the order of evaluation, or even that the same

predicate object be used, when iterating through a container. A predicate should not be dependent on

the order of evaluation; it should return the same result for an element regardless of previous calls or

any external state.

In addition, algorithms can copy predicates so there is no guarantee that the state of the predicate will

be maintained.

By declaring operator() const it is explicit that the state of the predicate is not modified by calling the


class Bad_Predicate



Bad_Predicate() : m_count( 0 ) {}

bool operator()( const int& )


return ++m_count == 5;



int m_count;


// Irrespective of the number of elements in the deque m_count

// may never reach 5 as the predicate might be copied.


bool is_large_deque( std::deque< int >& d )


return d.end() !=

std::find_if( d.begin(), d.end(), Bad_Predicate() );


Reference Effective STL Item 39;

High Integrity CPP Guideline 17.18 Use STL algorithms rather than hand-written loops.

Justification Implementations may optimize algorithms for particular container types to improve efficiency. Code

using algorithms is generally clearer, more straightforward, less error prone, and easier to maintain.

See also Rule 17.19

Reference Effective STL Item 43;

High Integrity CPP Rule 17.19 Use container member functions rather than algorithms with the same name.

Justification Where particular container operations are implemented as members these members should be used

instead of the generic algorithm. Member implementations can take advantage of internal container

structure and this leads to a more efficient implementation.

void foo( std::set< int >& s )


std::set< int >::iterator iter;

// Generally std::find cannot take advantage of the structure

// of the container it operates on and executes with linear

// complexity.


iter = std::find( s.begin(), s.end(), 10 );

// set.find takes advantage of the structure of the set and

// executes with logarithmic complexity.


iter = s.find( 10 );


See also Guideline 17.18

Reference Effective STL Item 44;

High Integrity CPP Rule 17.20 Directly include necessary STL headers.

Justification Some implementations may include extra STL headers not explicitly specified by the standard. Code

that is dependent on these indirect inclusions and does not directly include the appropriate header in

the source file will be non portable.


void foo()


// May work with some STL implementations which include

// < string > in < vector >


std::string s;


Reference Effective STL Item 48;

High Integrity CPP Guideline 17.21 Minimise use of the Standard Template Library 'auto_ptr'.

Justification 'auto_ptr' has destructive copy semantics which may be non-intuitive and can lead to erroneous


void foo( auto_ptr ai );

void bar()


auto_ptr ai( new int );

foo( ai ); // destructive copy

*ai = 10; // error ai no longer exists


Exception If you use 'auto_ptr' take note of the following:

- Do not use auto_ptr with array types.

- Only use auto_ptr with dynamically allocated variables.

- Be aware that implicit conversions can take place between auto_ptrs of different types where a

conversion exists for the underlying pointer types.

- Use auto_ptr where "ownership-transfer semantics" are required, for example do not use auto_ptr

where you need two pointers to the same object concurrently.

18 Future Direction of Standard

This section provides some guidance on the future direction of this standard as this may affect the way you currently

program. It lists Rules and Guidelines which are marked for review in subsequent editions of this standard. This means

that items mentioned here may be promoted to Rules, demoted to Guidelines or dropped altogether.

No issues in this release.



Object created from a functor class. Also known as a function object.

Functor class

Any class that overloads the function call operator (operator() ) is a functor class.


An acronym for Plain Old Data.

A POD-struct is ¡®an aggregate class that has no non-static data members of type pointer to

member, non-POD-struct, non-POD-union (or array of such types) or reference, and has no

user-defined copy assignment operator and no user-defined destructor.¡¯

A POD-union is ¡®an aggregate union that has no non-static data members of type pointer to

member, non-POD-struct, non-POD-union (or array of such types) or reference, and has no

user-defined copy assignment operator and no user-defined destructor.¡¯

A POD-class is a class that is either a POD-struct or a POD-union.

Arithmetic types, enumeration types, pointer types, and pointer to member types are

collectively called scalar types.

Scalar types, POD-class types, and arrays of such types are collectively called POD types.


A predicate is a function that returns either bool or a type that can be implicitly converted to


Predicate class

A predicate class is a functor class whose operator() function is a predicate, i.e. its

operator() returns true or false.


[Stroustrup, 2000]

Bjarne Stroustrup: The C++ Programming Language. Addison-Wesley. 2000

[C++ Standard, 1999]

International Standard ISO/IEC 14882:1998(E) Programming Language C++.

[Effective C++, 1996]

Scott Meyers: Effective C++. Addison-Wesley. 1996

[More Effective C++, 1996]

Scott Meyers: More Effective C++. Addison-Wesley. 1996

[Effective STL, 2001]

Scott Meyers: Effective STL. Addison-Wesley. 2001

[Industrial Strength C++, 1997]

Mats Henricson, Erik Nyquist, Ellemtel Utvecklings AB: Industrial Strength C++.

Prentice Hall. 1997

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