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std::enable_if

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< cpp‎ | types

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C++
 
Bibliothèque d'utilitaires
 
Soutien aux types fondamentaux
Les types de base
Original:
Basic types
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Types fondamentaux
Fixe les types entiers largeur (C++11)
Limites numériques
Original:
Numeric limits
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numeric_limits
C numérique limite interface
Informations sur le type d'exécution
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Runtime type information
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Caractères de type
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Type traits
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Catégories de type primaire
Original:
Primary type categories
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is_void (C++11)
is_array (C++11)
is_pointer (C++11)
is_enum (C++11)
is_union (C++11)
is_class (C++11)
is_function (C++11)
is_object (C++11)
is_scalar (C++11)
is_compound (C++11)
is_integral (C++11)
is_floating_point (C++11)
is_fundamental (C++11)
is_arithmetic (C++11)
is_reference (C++11)
is_lvalue_reference (C++11)
is_rvalue_reference (C++11)
is_member_pointer (C++11)
is_member_object_pointer (C++11)
is_member_function_pointer (C++11)
Les propriétés du type
Original:
Type properties
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is_const (C++11)
is_volatile (C++11)
is_pod (C++11)
is_empty (C++11)
is_polymorphic (C++11)
is_abstract (C++11)
Opérations prises en charge
Original:
Supported operations
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is_constructible
is_trivially_constructible
is_nothrow_constructible		 (C++11)
(C++11)
(C++11)
is_default_constructible
is_trivially_default_constructible
is_nothrow_default_constructible		 (C++11)
(C++11)
(C++11)
is_copy_constructible
is_trivially_copy_constructible
is_nothrow_copy_constructible		 (C++11)
(C++11)
(C++11)
is_move_constructible
is_trivially_move_constructible
is_nothrow_move_constructible		 (C++11)
(C++11)
(C++11)
is_assignable
is_trivially_assignable
is_nothrow_assignable		 (C++11)
(C++11)
(C++11)
is_copy_assignable
is_trivially_copy_assignable
is_nothrow_copy_assignable		 (C++11)
(C++11)
(C++11)
is_move_assignable
is_trivially_move_assignable
is_nothrow_move_assignable		 (C++11)
(C++11)
(C++11)
is_destructible
is_trivially_destructible
is_nothrow_destructible		 (C++11)
(C++11)
(C++11)
has_virtual_destructor (C++11)
Les relations et les requêtes de propriété
Original:
Relationships and property queries
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is_same (C++11)
is_base_of (C++11)
is_convertible (C++11)
alignment_of (C++11)
rank (C++11)
extent (C++11)
Modifications de type
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Type modifications
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remove_cv
remove_const
remove_volatile		 (C++11)
(C++11)
(C++11)
add_cv
add_const
add_volatile		 (C++11)
(C++11)
(C++11)
make_signed (C++11)
make_unsigned (C++11)
Transformations de type
Original:
Type transformations
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aligned_storage (C++11)
aligned_union (C++11)
decay (C++11)
enable_if (C++11)
conditional (C++11)
common_type (C++11)
underlying_type (C++11)
result_of (C++11)
Tapez constantes traits
Original:
Type trait constants
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integral_constant (C++11)
 
Déclaré dans l'en-tête <type_traits>
template< bool B, class T = void >
struct enable_if;
 (depuis C++11)
Si B est true, std::enable_if a une type membre du public typedef, égale à T, sinon, il n'y a aucun membre typedef .
Original:
If B is true, std::enable_if has a public member typedef type, equal to T; otherwise, there is no member typedef.
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Cette métafonction est utilisée de façon conditionnelle pour supprimer les fonctions et les classes de la résolution de surcharge basée sur les caractères morphologiques et de fournir des surcharges de fonction séparées et spécialisations pour les caractères de type différents. std::enable_if peut être utilisé comme un argument de fonction supplémentaire (non applicable aux surcharges d'opérateur), comme type de retour (ne s'applique pas aux constructeurs et destructeurs), ou comme un modèle de classe ou d'un paramètre de modèle de fonction .
Original:
This metafunction is used to conditionally remove functions and classes from overload resolution based on type traits and to provide separate function overloads and specializations for different type traits. std::enable_if can be used as an additional function argument (not applicable to operator overloads), as a return type (not applicable to constructors and destructors), or as a class template or function template parameter.
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Sommaire

[modifier] Types de membres

Type d'
Original:
Type
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 Definition
type
T supporte ou non tel élément, en fonction de la valeur de B
Original:
either T or no such member, depending on the value of B
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[modifier] Mise en œuvre possible

template<bool B, class T = void>
struct enable_if {};
 
template<class T>
struct enable_if<true, T> { typedef T type; };

[modifier] Exemple

#include <type_traits>
#include <iostream>
 
// foo1 overloads are enabled via the return type
template<class T>
typename std::enable_if<std::is_floating_point<T>::value, T>::type 
    foo1(T t) 
{
    std::cout << "foo1: float\n";
    return t;
}
 
template<class T>
typename std::enable_if<std::is_integral<T>::value, T>::type 
    foo1(T t) 
{
    std::cout << "foo1: int\n";
    return t;
}
 
// foo2 overload is enabled via a parameter
template<class T>
T foo2(T t, typename std::enable_if<std::is_integral<T>::value >::type* = 0) 
{
    return t;
}
 
// foo3 overload is enabled via a template parameter
template<class T ,
         class = typename std::enable_if<std::is_integral<T>::value>::type >
T foo3(T t) // note, function signature is unmodified
{
    return t;
}
 
// A is enabled via a template parameter
template<class T, class Enable = void>
class A; // undefined
 
template<class T>
class A<T, typename std::enable_if<std::is_floating_point<T>::value >::type> {
};
 
int main()
{
    foo1(1.2); // OK, calls the first version of foo1()
    foo1(10); // OK, calls the second version of foo1()
 
//  foo2(0.1); // compile-time error
    foo2(7); // OK
 
//  A<int> a1; // compile-time error
    A<double> a1; // OK
}

Résultat : 

foo1: float
foo1: int

[modifier] Voir aussi

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