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.

If the program adds specializations for std::integral_constant, the behavior is
undefined.

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>

Nested types

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

Member constants¶

Name Value
constexpr T 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.

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¶

// Run this code

#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() {}

See also¶

integer_sequence implements compile-time sequence of integers
(C++14) (class template)