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 specializations for the type bool are provided:

Defined in header <type_traits>

Type Definition

true_type

`std::integral_constant<bool, true>`	(until C++17)
`std::bool_constant<true>`	(since C++17)

false_type

`std::integral_constant<bool, false>`	(until C++17)
`std::bool_constant<false>`	(since C++17)

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 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;
    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");
}

Links:

http://en.cppreference.com/w/cpp/types/integral_constant

doc_CPP

2016-10-11 10:03:21

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