Classes

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros(C++20)

Language support library

Concepts library(C++20)

Diagnostics library

Memory management library

Metaprogramming library(C++11)

General utilities library

Containers library

Iterators library

Ranges library(C++20)

Algorithms library

Strings library

Text processing library

Numerics library

Date and time library

Input/output library

Filesystem library(C++17)

Concurrency support library(C++11)

Execution control library(C++26)

Technical specifications

Symbols index

External libraries

 

C++ language

General topics
Preprocessor Comments	Keywords Escape sequences
Flow control
Conditional execution statements
if	switch
Iteration statements (loops)
for range-for(C++11)	while do-while
Jump statements
continue - break	goto - return
Functions
Function declaration
Lambda function expression
inline specifier
Dynamic exception specifications(until C++17*)
noexcept specifier(C++11)
Exceptions
throw-expression try block	catch handler
Namespaces
Namespace declaration	Namespace aliases
Types
Fundamental types Enumeration types Function types	Class/struct types Union types
Specifiers
const/volatile decltype(C++11) auto(C++11)	constexpr(C++11) consteval(C++20) constinit(C++20)
Storage duration specifiers
Initialization
Default-initialization Value-initialization Zero-initialization Copy-initialization Direct-initialization	Aggregate initialization List-initialization(C++11)   Constant initialization Reference initialization

Expressions
Value categories Order of evaluation	Operators Operator precedence
Alternative representations
Literals
Boolean - Integer - Floating-point
Character - String - nullptr(C++11)
User-defined(C++11)
Utilities
Attributes(C++11)
Types
typedef declaration
Type alias declaration(C++11)
Casts
Implicit conversions static_cast const_cast	Explicit conversions dynamic_cast reinterpret_cast
Memory allocation
new expression	delete expression
Classes
Class declaration Constructors this pointer	Access specifiers friend specifier
Class-specific function properties
Virtual function override specifier(C++11)   final specifier(C++11)	explicit(C++11) static
Special member functions
Default constructor Copy constructor Move constructor(C++11)	Copy assignment Move assignment(C++11) Destructor
Templates
Class template Function template	Template specialization Parameter packs(C++11)
Miscellaneous
Inline assembly	History of C++

 

Classes

General

Overview

class/struct types

union types

Injected-class-name

Class property specifiers(C++26)

Members

Data members

Static members

The this pointer

Nested classes

Member templates

Bit-fields

using-declarations

Member functions

Member access specifiers

Constructors and member initializer lists

Default member initializer(C++11)

friend specifier

explicit specifier

Converting constructor

Special member functions

Default constructor

Copy constructor

Move constructor(C++11)

Copy assignment operator

Move assignment operator(C++11)

Destructor

Inheritance

Base and derived classes

Empty base optimization (EBO)

Virtual member functions

Pure virtual functions and abstract classes

override specifier(C++11)

final specifier(C++11)

 

A class is a user-defined type.

A class type is defined by class-specifier, which appears in decl-specifier-seq of the declaration syntax. See class declaration for the syntax of the class specifier.

A class can have the following kinds of members:

1) data members:

a)non-static data members, including bit-fields.

b)static data members

2) member functions:

a)non-static member functions

b)static member functions

3) nested types:

a)nested classes and enumerations defined within the class definition

b) aliases of existing types, defined with typedefor type alias(since C++11)declarations

c) the name of the class within its own definition acts as a public member type alias of itself for the purpose of lookup (except when used to name a constructor): this is known as injected-class-name

4)enumerators from all unscoped enumerations defined within the class, or introduced by using-declarations or using-enum-declarations(since C++20)

5)member templates (variable templates, (since C++14)class templates or function templates) may appear in the body of any non-local class/struct/union.

All members are defined at once in the class definition, they cannot be added to an already-defined class (unlike the members of namespaces)

A member of a class T cannot use T as its name if the member is

• a static data member,
• a member function,
• a member type,
• a member template,
• an enumerator of an enumeration (unless the enumeration is scoped)(since C++11), or
• a member of a member anonymous union.

However, a non-static data member may use the name T as long as there are no user-declared constructors.

A class with at least one declared or inherited virtual member function is polymorphic. Objects of this type are polymorphic objects and have runtime type information stored as part of the object representation, which may be queried with dynamic_cast and typeid. Virtual member functions participate in dynamic binding.

A class with at least one declared or inherited pure virtual member function is an abstract class. Objects of this type cannot be created.

A class with a constexpr constructor is a LiteralType: objects of this type can be manipulated by constexpr functions at compile time.

(since C++11)

Properties of classes

Trivially copyable class A trivially copyable class is a class that has at least one eligible copy constructor, move constructor, copy assignment operator, or move assignment operator, each eligible copy constructor is trivial each eligible move constructor is trivial each eligible copy assignment operator is trivial each eligible move assignment operator is trivial, and has a non-deleted trivial destructor. Trivial class A trivial class is a class that is trivially copyable, and has one or more eligible default constructors such that each is trivial. (deprecated in C++26) Standard-layout class A standard-layout class is a class that has no non-static data members of type non-standard-layout class (or array of such types) or reference, has no virtual functions and no virtual base classes, has the same access control for all non-static data members, has no non-standard-layout base classes, only one class in the hierarchy has non-static data members, and Informally, none of the base classes has the same type as the first non-static data member. Or, formally: given the class as S, has no element of the set M(S) of types as a base class, where M(X) for a type X is defined as: If X is a non-union class type with no (possibly inherited) non-static data members, the set M(X) is empty. If X is a non-union class type whose first non-static data member has type X0 (where said member may be an anonymous union), the set M(X) consists of X0 and the elements of M(X0). If X is a union type, the set M(X) is the union of all M(Ui) and the set containing all Ui, where each Ui is the type of the ith non-static data member of X. If X is an array type with element type Xe, the set M(X) consists of Xe and the elements of M(Xe). If X is a non-class, non-array type, the set M(X) is empty. A standard-layout struct is a standard-layout class defined with the class keyword struct or the class keyword class. A standard-layout union is a standard-layout class defined with the class keyword union.

(since C++11)

Implicit-lifetime class

An implicit-lifetime class is a class that

• is an aggregate whose destructor is not user-declared(until C++11)user-provided(since C++11), or
• has at least one trivial eligible constructor and a trivial, non-deleted destructor.

Notes: the implicit-lifetime property is clarified by defect report P0593R6.

POD class A POD class is a class that is an aggregate, has no user-declared copy assignment operator, has no user-declared destructor, and has no non-static data members of type non-POD class (or array of such types) or reference. (until C++11) is a trivial class, is a standard-layout class, and has no non-static data members of type non-POD class (or array of such types). (since C++11) A POD struct is a non-union POD class. A POD union is a union that is a POD class.

(deprecated in C++20)

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
CWG 148	C++98	POD classes could not contain pointers to member, which are themselves POD (scalar) types	restriction removed
CWG 383	C++98	copy assignment operators or destructors could be user-declared in POD classes if they are not defined	not allowed
CWG 1363	C++11	a class that has both trivial default constructors and non-trivial default constructors at the same time could be trivial	it is non-trivial
CWG 1496	C++11	a class that only has constructors that are all defined as deleted could be trivial	it is non-trivial
CWG 1672	C++11	a class could be a standard-layout class if it has multiple empty base classes	it is not a standard-layout class
CWG 1734	C++11	a trivially copyable class could not have non-trivial deleted copy/move constructors/assignment operators	can be trivial if deleted
CWG 1813	C++11	a class was never a standard-layout class if it has a base class that inherits a non-static data member	it can be a standard-layout class
CWG 1881	C++11	for a standard-layout class and its base classes, unnamed bit-fields might be declared in a different class declaring the data members	all non-static data members and bit-fields need to be first declared in the same class
CWG 1909	C++98	a member template could have the same name as its class	prohibited
CWG 2120	C++11	the definition of M(X) in determining a standard- layout class did not consider the case of a class whose first member is an array	addressed this case in the definition of M(X)
CWG 2605	C++98	an implicit-lifetime class could have a user-provided destructor	prohibited