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std::is_placeholder

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< cpp‎ | utility‎ | functional

 
 
 
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Bind
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bind (C++11)
is_bind_expression (C++11)
is_placeholder (C++11)
_1, _2, _3, ... (C++11)
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Defined in header <functional>
template< class T >
struct is_placeholder;
(depuis C++11)
Si T est le type d'un espace réservé standard (_1, _2, _3, ...), puis ce modèle est dérivé de std::integral_constant<int,1>, std::integral_constant<int,2>, std::integral_constant<int,3>, respectivement. Si T n'est pas un type d'espace réservé standard, ce modèle est dérivé de 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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Le modèle peut être spécialisé pour tout type défini par l'utilisateur qui doit être traitée par std::bind comme s'il s'agissait d'un espace réservé pour les arguments non liés .
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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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Sommaire

Inherited from std::integral_constant

Member constants

value
[
statique
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static
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]
placeholder value or 0 for non-placeholder types
(constante membre statique publique)

Member functions

operator int
convertit l'objet en int, retourne value
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converts the object to int, returns value
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(fonction membre publique)

Member types

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

[modifier] Exemple

#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';
}

Résultat :

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

[modifier] Voir aussi

(C++11)
liaison de paramètres et application partielle d'un foncteur
(fonction générique) [edit]
placeholders (numéros de paramètre) pour les std::bind
(constante) [edit]