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Showing content from https://en.cppreference.com/w/cpp/algorithm/../types/integral_constant.html below:

std::integral_constant - cppreference.com

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.

[edit] 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) [edit] Specializations

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

Name Definition true_type std::integral_constant<bool, true> false_type std::integral_constant<bool, false> [edit] Member types Name Definition value_type T type std::integral_constant<T, v> [edit] Member constants Name Value

constexpr T value

[static]

v
(public static member constant) [edit] Member functions 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.

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.

[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] Notes [edit] 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() {}

[edit] See also

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