std:: integral_constant

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Type trait constants
integral_constant bool_constant true_type false_type
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Compile-time rational arithmetic
Compile-time integer sequences
(C++14)
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
constexpr T 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

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

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