Espaços nominais
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std::placeholders::_1, std::placeholders::_2, ..., std::placeholders::_N

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initializer_list(C++11)
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Os operadores relacionais
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rel_ops::operator!=
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Bind
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Negators
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Obsoleta ligantes e adaptadores
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Defined in header <functional>
extern /*unspecified*/ _1;

extern /*unspecified*/ _2;
.
.

extern /*unspecified*/ _N;
O namespace std::placeholders contém os objetos de espaço reservado [_1, . . . _N] onde N é uma implementação número máximo definido.
Original:
The std::placeholders namespace contains the placeholder objects [_1, . . . _N] where N is an implementation defined maximum number.
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Quando usado como um argumento em uma expressão std::bind, os objetos de espaço reservado são armazenados na função de objeto gerado, e quando o objeto função é chamada com argumentos não ligados, cada _N espaço reservado é substituído pelo argumento não ligado correspondente Nth.
Original:
When used as an argument in a std::bind expression, the placeholder objects are stored in the generated function object, and when that function object is invoked with unbound arguments, each placeholder _N is replaced by the corresponding Nth unbound argument.
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Os tipos de objetos de espaço reservado são DefaultConstructible e CopyConstructible, sua cópia default / movimento construtores não lançar exceções, e para qualquer _N espaço reservado, o std::is_placeholder<decltype(_N)> tipo está definido e é derivada de std::integral_constant<int, N>.
Original:
The types of the placeholder objects are DefaultConstructible and CopyConstructible, their default copy/move constructors do not throw exceptions, and for any placeholder _N, the type std::is_placeholder<decltype(_N)> is defined and is derived from std::integral_constant<int, N>.
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[editar] Exemplo

O código a seguir mostra a criação de objetos de função com um argumento de espaço reservado .
Original:
The following code shows the creation of function objects with a placeholder argument.
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#include <functional>
#include <string>
#include <iostream>
 
void goodbye(const std::string& s)
{
    std::cout << "Goodbye " << s << '\n';
}
 
class Object {
public:
    void hello(const std::string& s)
    {
        std::cout << "Hello " << s << '\n';
    }
};
 
int main(int argc, char* argv[])
{
    typedef std::function<void(const std::string&)> ExampleFunction;
    Object instance;
    std::string str("World");
    ExampleFunction f = std::bind(&Object::hello, &instance, 
                                  std::placeholders::_1);
 
    // equivalent to instance.hello(str)
    f(str);
    f = std::bind(&goodbye, std::placeholders::_1);
 
    // equivalent to goodbye(str)
    f(str);    
    return 0;
}

Output:

Hello World
Goodbye World

[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]
indica que um objecto é um marcador padrão ou podem ser utilizados como um
Original:
indicates that an object is a standard placeholder or can be used as one
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(modelo de classe) [edit]