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std::integral_constant

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C++
 
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(C++11)
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is_move_assignable
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Type trait constants
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integral_constant(C++11)
 
Defined in header <type_traits>
template< class T, T v >
struct integral_constant;
 (ya que C + +11)
std::integral_constant envuelve una constante estática del tipo especificado. Es la clase base para las C + + .. rasgos de tipo
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std::integral_constant wraps a static constant of specified type. It is the base class for the C++ type traits.
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Dos especializaciones para el bool tipo están disponibles:
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Two specializations for the type bool are provided:
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Defined in header <type_traits>
Tipo
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Type
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 Definition
true_type std::integral_constant<bool, true>
false_type std::integral_constant<bool, false>

Contenido

[editar] Tipos de miembros

Tipo
Original:
Type
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 Definition
value_type T
type std::integral_constant<T,v>

[editar] Constantes de miembros

Nombre
Original:
Name
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 Value
value
expresión estática constante de T tipo con v valor
Original:
static constant expression of type T with value v
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[editar] Las funciones miembro

operator value_type
convierte el objeto a value_type, devuelve value
Original:
converts the object to value_type, returns value
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(miembro público función)

[editar] Posible implementación

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; }
};

[editar] Ejemplo

#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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