std::integral_constant — cppreference.com

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Déclaré dans l'en-tête `<type_traits>`
`template< class T, T v > struct integral_constant;`		(depuis C++11)

std::integral_constant encapsule une constante statique du type spécifié. Il s'agit de la classe de base pour les caractères de C + + .

Original:

std::integral_constant wraps a static constant of specified type. It is the base class for the C++ type traits.

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Deux spécialisations pour la bool type sont fournis:

Original:

Two specializations for the type bool are provided:

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Defined in header `<type_traits>`

Type d' Original: Type The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions.	Definition

`true_type`	`std::integral_constant<bool, true>`

`false_type`	`std::integral_constant<bool, false>`

Types de membres


Type d' Original: Type The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions.	Definition

`value_type`	`T`

`type`	`std::integral_constant<T,v>`

Constantes membres


Nom Original: Name The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions.	Value

`value`	statique expression constante de type avec `T` `v` valeur Original: static constant expression of type `T` with value `v` The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions.

Fonctions membres


	convertit l'objet en `value_type`, retourne `value` Original: converts the object to `value_type`, returns `value` The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions. (fonction membre publique)

Mise en œuvre possible

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; }
};

Exemple

#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");
}