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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.

Helper 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)

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]
 static constant of type T with value 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;

Conversion function. Returns the wrapped value.

Exceptions

noexcept specification:
noexcept

std::integral_constant::operator() 

constexpr value_type operator()() const;
 (since C++14)

Returns the wrapped value. This function enables std::integral_constant to serve as a source of compile-time function objects.

Exceptions

noexcept specification:
noexcept

Possible implementation

template<class T, T v>
struct integral_constant {
    static constexpr T value = v;
    typedef T value_type;
    typedef integral_constant type; // using injected-class-name
    constexpr operator value_type() const noexcept { return value; }
    constexpr value_type operator()() const noexcept { return value; } //since c++14
};

Example

#include <iostream>
#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::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");
}

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