Namespace
Varianti

std::is_placeholder

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
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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
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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
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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>
template< class T >
struct is_placeholder;
(dal C++11)
Se T è il tipo di un segnaposto standard (_1, _2, _3,...), allora questo modello è derivato da std::integral_constant<int,1>, std::integral_constant<int,2>, std::integral_constant<int,3>, rispettivamente. Se T non è un tipo segnaposto standard, questo modello è derivato da 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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Il modello può essere specializzato per qualsiasi tipo definito dall'utente, che deve essere trattato da std::bind come se fosse un segnaposto per gli argomenti non legati.
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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Indice

Inherited from std::integral_constant

Member constants

value
[statico]
placeholder value or 0 for non-placeholder types
(pubblico membro statico costante)

Member functions

operator int
converte l'oggetto in int, restituisce value
Original:
converts the object to int, returns value
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(membro pubblico funzione)

Member types

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

[modifica] Esempio

#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

[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]
segnaposto per gli argomenti non legate in un'espressione std::bind
Original:
placeholders for the unbound arguments in a std::bind expression
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(costante) [edit]