Namespace
Varianti

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

Da cppreference.com.
< cpp‎ | utility‎ | functional

 
 
Utilità libreria
Tipo di supporto (basic types, RTTI, type traits)
Gestione della memoria dinamica
La gestione degli errori
Programma di utilità
Funzioni variadic
Data e ora
Funzione oggetti
initializer_list(C++11)
bitset
hash(C++11)
Gli operatori relazionali
Original:
Relational operators
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rel_ops::operator!=
rel_ops::operator>
rel_ops::operator<=
rel_ops::operator>=
Coppie e tuple
Original:
Pairs and tuples
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pair
tuple(C++11)
piecewise_construct_t(C++11)
piecewise_construct(C++11)
Swap, in avanti e spostare
Original:
Swap, forward and move
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swap
forward(C++11)
move(C++11)
move_if_noexcept(C++11)
declval(C++11)
 
Funzione oggetti
Funzione wrapper
Original:
Function wrappers
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function(C++11)
mem_fn(C++11)
bad_function_call(C++11)
Associare
Original:
Bind
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bind(C++11)
is_bind_expression(C++11)
is_placeholder(C++11)
_1, _2, _3, ...(C++11)
Wrapper di riferimento
Original:
Reference wrappers
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reference_wrapper(C++11)
ref
cref
(C++11)
(C++11)
Operatore wrapper
Original:
Operator wrappers
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Negatori
Original:
Negators
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Sconsigliata a leganti e adattatori
Original:
Deprecated binders and adaptors
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unary_function(deprecato)
binary_function(deprecato)
ptr_fun(deprecato)
pointer_to_unary_function(deprecato)
pointer_to_binary_function(deprecato)
mem_fun(deprecato)
mem_fun_t
mem_fun1_t
const_mem_fun_t
const_mem_fun1_t
(deprecato)
(deprecato)
(deprecato)
(deprecato)
mem_fun_ref(deprecato)
mem_fun_ref_t
mem_fun1_ref_t
const_mem_fun_ref_t
const_mem_fun1_ref_t
(deprecato)
(deprecato)
(deprecato)
(deprecato)
binder1st
binder2nd
(deprecato)
(deprecato)
bind1st
bind2nd
(deprecato)
(deprecato)
 
Defined in header <functional>
extern /*unspecified*/ _1;

extern /*unspecified*/ _2;
.
.

extern /*unspecified*/ _N;
Lo spazio dei nomi std::placeholders contiene gli oggetti segnaposto [_1, . . . _N] dove N è una implementazione definito numero massimo.
Original:
The std::placeholders namespace contains the placeholder objects [_1, . . . _N] where N is an implementation defined maximum number.
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Quando viene utilizzato come argomento in un'espressione std::bind, gli oggetti segnaposto sono memorizzati nell'oggetto funzione generata, e quando questo oggetto funzione viene richiamata con argomenti non legati, ogni _N segnaposto viene sostituito dal corrispondente argomento non legato 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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I tipi di oggetti segnaposto sono DefaultConstructible e CopyConstructible, la loro copia di default / spostamento costruttori non generano eccezioni, e per qualsiasi _N segnaposto, il std::is_placeholder<decltype(_N)> tipo è definito ed è derivato dal 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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[modifica] Esempio

Il codice seguente mostra la creazione di oggetti di funzione con un argomento segnaposto .
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

[modifica] Vedi anche

(C++11)
lega uno o più argomenti a un oggetto funzione
Original:
binds one or more arguments to a function object
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(funzione di modello) [edit]
indica che un oggetto è un segnaposto standard o può essere usato come uno
Original:
indicates that an object is a standard placeholder or can be used as one
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(classe template) [edit]