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std::is_placeholder

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Ligar
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Bind
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bind(C++11)
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is_placeholder(C++11)
_1, _2, _3, ...(C++11)
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Defined in header <functional>
template< class T >
struct is_placeholder;
(a partir do C++ 11)
Se T é o tipo de um espaço reservado padrão (_1, _2, _3,...), então este modelo é derivado do std::integral_constant<int,1>, std::integral_constant<int,2>, std::integral_constant<int,3>, respectivamente. Se T não é um tipo de espaço reservado padrão, este modelo é derivado do std::integral_constant<int,0>.
Original:
If T is the type of a standard placeholder (_1, _2, _3, ...), then this template is derived from std::integral_constant<int,1>, std::integral_constant<int,2>, std::integral_constant<int,3>, respectively. If T is not a standard placeholder type, this template is derived from std::integral_constant<int,0>.
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O modelo pode ser especializado para qualquer tipo definido pelo usuário, que deve ser tratada por std::bind como se fosse um espaço reservado para os argumentos não ligados.
Original:
The template may be specialized for any user-defined type which should be treated by std::bind as if it was a placeholder for unbound arguments.
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Índice

Inherited from std::integral_constant

Member constants

value
[estática]
placeholder value or 0 for non-placeholder types
(membro estático público constante)

Member functions

operator int
converte o objeto em int, retorna value
Original:
converts the object to int, returns value
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(função pública membro)

Member types

Tipo
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Type
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Definition
value_type int
type std::integral_constant<int, value>

[editar] Exemplo

#include <iostream>
#include <type_traits>
#include <functional>
 
struct My_2 {
} my_2;
 
namespace std {
    template<>
    struct is_placeholder<My_2> : public integral_constant<int, 2> {};
}
 
int f(int n1, int n2)
{
    return n1+n2;
}
 
int main()
{
    std::cout << "Standard placeholder _5 is for the argument number "
              << std::is_placeholder<decltype(std::placeholders::_5)>::value
              << '\n';
 
    auto b = std::bind(f, my_2, 2);
    std::cout << "Adding 2 to 11 selected with a custom placeholder gives " 
              << b(10, 11) 
              << '\n';
}

Output:

Standard placeholder _5 is for the argument number 5
Adding 2 to 11 selected with a custom placeholder gives 13

[editar] Veja também

(C++11)
se liga um ou mais argumentos para um objeto de função
Original:
binds one or more arguments to a function object
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(modelo de função) [edit]
espaços reservados para os argumentos não ligados em uma expressão de std::bind
Original:
placeholders for the unbound arguments in a std::bind expression
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(constante) [edit]