< cpp‎ | types
Utilities library
General utilities
Relational operators (deprecated in C++20)
Integer comparison functions
Swap and type operations

Common vocabulary types

Elementary string conversions

Type support
Basic types
Fundamental types
Fixed width integer types (C++11)
Numeric limits
C numeric limits interface
Runtime type information
Type traits
Type categories
Type properties

(C++11)(until C++20)
(C++11)(deprecated in C++20)
Type trait constants
Constant evaluation context
Supported operations
Relationships and property queries
Type modifications
Type transformations

(C++11)(until C++20)(C++17)
Defined in header <type_traits>
template< class T, T v >
struct integral_constant;
(since C++11)

std::integral_constant wraps a static constant of specified type. It is the base class for the C++ type traits.

The behavior of a program that adds specializations for integral_constant is undefined.


Helper templates

A helper alias template std::bool_constant is defined for the common case where T is bool.

template <bool B>
using bool_constant = integral_constant<bool, B>;
(since C++17)

Two typedefs for the common case where T is bool are provided:

Defined in header <type_traits>
Type Definition
true_type std::integral_constant<bool, true>
false_type std::integral_constant<bool, false>

[edit] Member types

Type Definition
value_type T
type std::integral_constant<T,v>

[edit] Member constants

Name Value
constexpr T value
static constant of type T with value v
(public static member constant)

[edit] Member functions

operator value_type
returns the wrapped value
(public member function) [edit]
returns the wrapped value
(public member function) [edit]

std::integral_constant::operator value_type

constexpr operator value_type() const noexcept;

Conversion function. Returns the wrapped value.


constexpr value_type operator()() const noexcept;
(since C++14)

Returns the wrapped value. This function enables std::integral_constant to serve as a source of compile-time function objects.

[edit] Possible implementation

template<class T, T v>
struct integral_constant {
    static constexpr T value = v;
    using value_type = T;
    using type = integral_constant; // using injected-class-name
    constexpr operator value_type() const noexcept { return value; }
    constexpr value_type operator()() const noexcept { return value; } // since c++14

[edit] Example

#include <type_traits>
int main() 
    typedef std::integral_constant<int, 2> two_t;
    typedef std::integral_constant<int, 4> four_t;
//  static_assert(std::is_same<two_t, four_t>::value,
//                "two_t and four_t are not equal!"); 
//  error: static assertion failed: "two_t and four_t are not equal!"
    static_assert(two_t::value*2 == four_t::value,
       "2*2 != 4"
    enum class my_e { e1, e2 };
    typedef std::integral_constant<my_e, my_e::e1> my_e_e1;
    typedef std::integral_constant<my_e, my_e::e2> my_e_e2;
    static_assert(my_e_e1() == my_e::e1);
//  static_assert(my_e_e1::value == my_e::e2,
//               "my_e_e1::value != my_e::e2");
//  error: static assertion failed: "my_e_e1::value != my_e::e2"
    static_assert(std::is_same<my_e_e2, my_e_e2>::value,
                  "my_e_e2 != my_e_e2");