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.

If the program adds specializations for std::integral_constant, the behavior 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>`
Name	Definition
`true_type`	std::integral_constant<bool,true>
`false_type`	std::integral_constant<bool,false>

Member types

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

Member constants

Name	Value
value [static]	`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>

using two_t = std::integral_constant<int, 2>;
using four_t = std::integral_constant<int, 4>;

static_assert(not std::is_same_v<two_t, four_t>);
static_assert(two_t::value * 2 == four_t::value, "2*2 != 4");
static_assert(two_t() << 1 == four_t() >> 0, "2*2 != 4");

enum class E{ e1, e2 };
using c1 = std::integral_constant<E, E::e1>;
using c2 = std::integral_constant<E, E::e2>;
static_assert(c1::value != E::e2);
static_assert(c1() == E::e1);
static_assert(std::is_same_v<c2, c2>);

int main() {}