NAME

std::integral_constant - std::integral_constant

Synopsis

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

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 alias templates

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

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

Feature-test macro: __cpp_lib_bool_constant

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 constant of type T with value v
[static] (public static member constant)

Member functions

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

Feature-test macro: __cpp_lib_integral_constant_callable

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
};

Example

// Run this code

#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");
}