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# std::integral_constant

From cppreference.com
< cpp‎ | types

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.

Two specializations for the type bool are provided:

Defined in header `<type_traits>`
Type | Definition
`true_type` | std::integral_constant<bool, true>
`false_type` | std::integral_constant<bool, false>

### [edit] Member types

Type | Definition
`value_type` | `T`
`type` | std::integral_constant<T,v>

### [edit] Member constants

Name | Value
`value` | static constant expression of type `T` with value `v`

### [edit] Member functions

operator value_type | converts the object to `value_type`, returns `value` (public member function) [edit]
operator() (C++14) | returns `value` (public member function) [edit]

### [edit] 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 { return value; }
        constexpr value_type operator()() const { return value; }
    };

### [edit] Example

Run this code

    #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 a same types"

        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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