Type

 

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)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

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-catch block
Namespaces
Namespace declaration	Namespace aliases
Types
Fundamental types Enumeration types Function types	Class/struct types Union types
Specifiers
decltype (C++11) auto (C++11) alignas (C++11)	const/volatile constexpr (C++11)
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 - Explicit conversions
static_cast - dynamic_cast
const_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++

 

Basic Concepts

Comments

ASCII

Punctuation

Names and identifiers

Types

Fundamental types

Objects

Scope

Object lifetime

Storage duration and linkage

Definitions and ODR

Name lookup

Qualified name lookup

Unqualified name lookup

The as-if rule

Undefined behavior

Memory model

Character sets and encodings

Phases of translation

The main function

Modules (C++20)

 

Objects, references, functions including function template specializations, and expressions have a property called type, which both restricts the operations that are permitted for those entities and provides semantic meaning to the otherwise generic sequences of bits.

Type classification

The C++ type system consists of the following types:

• fundamental types (see also std::is_fundamental):

• the type void (see also std::is_void);

the type std::nullptr_t (see also std::is_null_pointer);

(since C++11)

• arithmetic types (see also std::is_arithmetic):

• integral types / integer types (including cv-qualified versions, see also std::is_integral):

• the type bool;
• character types:

• narrow character types:

• ordinary character types: char, signed char, unsigned char

the type char8_t

(since C++20)

• wide character types: char16_t, char32_t, (since C++11)wchar_t;

• signed integer types:

• standard signed integer types: signed char, short, int, long, long long;

extended signed integer types (implementation-defined);

(since C++11)

• unsigned integer types:

• standard unsigned integer types: unsigned char, unsigned short, unsigned, unsigned long, unsigned long long;

extended unsigned integer types (each corresponds to an extended signed integer type, and vice versa);

(since C++11)

• floating-point types (see also std::is_floating_point):

• standard floating-point types: float, double, long double and their cv-qualified versions;

extended floating-point types (including cv-qualified versions): fixed width floating-point types; other implementation-defined extended floating-point types;

(since C++23)

• compound types (see also std::is_compound):

• reference types (see also std::is_reference):

• lvalue reference types (see also std::is_lvalue_reference):

• lvalue reference to object types;
• lvalue reference to function types;

rvalue reference types (see also std::is_rvalue_reference): rvalue reference to object types; rvalue reference to function types;

(since C++11)

• pointer types (see also std::is_pointer):

• pointer-to-object types;
• pointer-to-function types;

• pointer-to-member types (see also std::is_member_pointer):

• pointer-to-data-member types (see also std::is_member_object_pointer);
• pointer-to-member-function types (see also std::is_member_function_pointer);

• array types (see also std::is_array);
• function types (see also std::is_function);
• enumeration types (see also std::is_enum);

• unscoped enumeration types;

scoped enumeration types (see also std::is_scoped_enum);

(since C++11)

• class types:

• non-union types (see also std::is_class);
• union types (see also std::is_union).

For every non-cv-qualified type other than reference and function, the type system supports three additional cv-qualified versions of that type (const, volatile, and const volatile).

Types are grouped in various categories based on their properties:

• object types are (possibly cv-qualified) types that are not function types, reference types, or possibly cv-qualified void (see also std::is_object);
• scalar types are (possibly cv-qualified) object types that are not array types or class types (see also std::is_scalar);
• trivial types (see also std::is_trivial), POD types (see also std::is_pod), literal types (see also std::is_literal_type), and other categories listed in the type traits library or as named type requirements.

Constructing a complete object type such that the number of bytes in its object representation is not representable in the type std::size_t (i.e. the result type of sizeof operator) is ill-formed.

Type naming

A name can be declared to refer to a type by means of:

• class declaration;
• union declaration;
• enum declaration;
• typedef declaration;
• type alias declaration.

Types that do not have names often need to be referred to in C++ programs; the syntax for that is known as type-id. The syntax of the type-id that names type T is exactly the syntax of a declaration of a variable or function of type T, with the identifier omitted, except that decl-specifier-seq of the declaration grammar is constrained to type-specifier-seq, and that new types may be defined only if the type-id appears on the right-hand side of a non-template type alias declaration.

int* p;               // declaration of a pointer to int
static_cast<int*>(p); // type-id is "int*"
 
int a[3];   // declaration of an array of 3 int
new int[3]; // type-id is "int[3]" (called new-type-id)
 
int (*(*x[2])())[3];      // declaration of an array of 2 pointers to functions
                          // returning pointer to array of 3 int
new (int (*(*[2])())[3]); // type-id is "int (*(*[2])())[3]"
 
void f(int);                    // declaration of a function taking int and returning void
std::function<void(int)> x = f; // type template parameter is a type-id "void(int)"
std::function<auto(int) -> void> y = f; // same
 
std::vector<int> v;       // declaration of a vector of int
sizeof(std::vector<int>); // type-id is "std::vector<int>"
 
struct { int x; } b;         // creates a new type and declares an object b of that type
sizeof(struct { int x; });   // error: cannot define new types in a sizeof expression
using t = struct { int x; }; // creates a new type and declares t as an alias of that type
 
sizeof(static int); // error: storage class specifiers not part of type-specifier-seq
std::function<inline void(int)> f; // error: neither are function specifiers

The declarator part of the declaration grammar with the name removed is referred to as abstract-declarator.

Type-id may be used in the following situations:

• to specify the target type in cast expressions;
• as arguments to sizeof, alignof, alignas, new, and typeid;
• on the right-hand side of a type alias declaration;
• as the trailing return type of a function declaration;
• as the default argument of a template type parameter;
• as the template argument for a template type parameter;

in dynamic exception specification.

(until C++17)

Type-id can be used with some modifications in the following situations:

• in the parameter list of a function (when the parameter name is omitted), type-id uses decl-specifier-seq instead of type-specifier-seq (in particular, some storage class specifiers are allowed);
• in the name of a user-defined conversion function, the abstract declarator cannot include function or array operators.

Elaborated type specifier

Elaborated type specifiers may be used to refer to a previously-declared class name (class, struct, or union) or to a previously-declared enum name even if the name was hidden by a non-type declaration. They may also be used to declare new class names.

See elaborated type specifier for details.

Static type

The type of an expression that results from the compile-time analysis of the program is known as the static type of the expression. The static type does not change while the program is executing.

Dynamic type

If some glvalue expression refers to a polymorphic object, the type of its most derived object is known as the dynamic type.

// given
struct B { virtual ~B() {} }; // polymorphic type
struct D : B {};               // polymorphic type
 
D d; // most-derived object
B* ptr = &d;
 
// the static type of (*ptr) is B
// the dynamic type of (*ptr) is D

For prvalue expressions, the dynamic type is always the same as the static type.

Incomplete type

The following types are incomplete types:

• the type void (possibly cv-qualified);
• incompletely-defined object types:
  • class type that has been declared (e.g. by forward declaration) but not defined;
  • array of unknown bound;
  • array of elements of incomplete type;
  • enumeration type from the point of declaration until its underlying type is determined.

All other types are complete.

Any of the following contexts requires type T to be complete:

• definition of or call to a function with return type T or argument type T;
• definition of an object of type T;
• declaration of a non-static class data member of type T;
• new-expression for an object of type T or an array whose element type is T;
• lvalue-to-rvalue conversion applied to a glvalue of type T;
• an implicit or explicit conversion to type T;
• a standard conversion, dynamic_cast, or static_cast to type T* or T&, except when converting from the null pointer constant or from a pointer to possibly cv-qualified void;
• class member access operator applied to an expression of type T;
• typeid, sizeof, or alignof operator applied to type T;
• arithmetic operator applied to a pointer to T;
• definition of a class with base class T;
• assignment to an lvalue of type T;
• a catch-clause for an exception of type T, T&, or T*.

(In general, when the size and layout of T must be known.)

If any of these situations occur in a translation unit, the definition of the type must appear in the same translation unit. Otherwise, it is not required.

An incompletely-defined object type can be completed:

• A class type (such as class X) might be regarded as incomplete at one point in a translation unit and regarded as complete later on; the type class X is the same type at both points:

struct X;            // declaration of X, no definition provided yet
extern X* xp;        // xp is a pointer to an incomplete type:
                     // the definition of X is not reachable
 
void foo()
{
    xp++;            // ill-formed: X is incomplete
}
 
struct X { int i; }; // definition of X
X x;                 // OK: the definition of X is reachable
 
void bar()
{
    xp = &x;         // OK: type is “pointer to X”
    xp++;            // OK: X is complete
}

• The declared type of an array object might be an array of incomplete class type and therefore incomplete; if the class type is completed later on in the translation unit, the array type becomes complete; the array type at those two points is the same type.
• The declared type of an array object might be an array of unknown bound and therefore be incomplete at one point in a translation unit and complete later on; the array types at those two points ("array of unknown bound of T" and "array of N T") are different types.

The type of a pointer or reference to array of unknown bound permanently points to or refers to an incomplete type. An array of unknown bound named by a typedef declaration permanently refers to an incomplete type. In either case, the array type cannot be completed:

extern int arr[];   // the type of arr is incomplete
typedef int UNKA[]; // UNKA is an incomplete type
 
UNKA* arrp;         // arrp is a pointer to an incomplete type
UNKA** arrpp;
 
void foo()
{
    arrp++;         // error: UNKA is an incomplete type
    arrpp++;        // OK: sizeof UNKA* is known
}
 
int arr[10];        // now the type of arr is complete
 
void bar()
{
    arrp = &arr;    // error: different types
    arrp++;         // error: UNKA cannot be completed
}

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 328	C++98	class members of incomplete type were not prohibited if an object of the class type was never created	non-static class data members need to be complete
CWG 977	C++98	the point when an enumeration type becomes complete in its definition was unclear	the type is complete once the underlying type is determined
CWG 1362	C++98	user-defined conversions to type T* or T& required T to be complete	not required
CWG 1464	C++98	object size might be not representable in std::size_t	such type is ill-formed
CWG 2006	C++98	cv-qualified void types were object type and complete type	excluded from both categories
CWG 2448	C++98	only cv-unqualified types could be integral and floating-point types	allowed cv-qualified types
CWG 2630	C++98	it was unclear whether a class is considered complete outside the translation unit where the definition of the class appears	the class is complete if its definition is reachable in this case
CWG 2643	C++98	the type of a pointer to array of unknown bound could not be completed (but it is already complete)	the pointed-to array type cannot be completed