std::integral_constant

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 std::integral_constant is undefined.

Helper alias 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)

Specializations

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>`

Member types

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

Member constants

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

Member functions

operator value_type	returns the wrapped value (public member function)
operator() (C++14)	returns the wrapped value (public member function)

std::integral_constant::operator value_type

constexpr operator value_type() const noexcept;

Conversion function. Returns the wrapped value.

std::integral_constant::operator()

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.

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
};

Notes

Feature-test macro	Value	Std	Feature
`__cpp_lib_integral_constant_callable`	201304L	(C++14)	`std::integral_constant::operator()`
`__cpp_lib_bool_constant`	201505L	(C++17)	`std::bool_constant`

Example

#include <type_traits>
 
int main()
{
    typedef std::integral_constant<int, 2> two_t;
    typedef std::integral_constant<int, 4> four_t;
 
    static_assert(not std::is_same<two_t, four_t>::value,
                  "two_t and four_t are 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");
 
    static_assert(std::is_same<my_e_e2, my_e_e2>::value,
                  "my_e_e2 != my_e_e2");
}

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