Namespace
Varianti

std::enable_if

Da cppreference.com.
< cpp‎ | types

 
 
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)
 
Tipo di supporto
Tipi base
Original:
Basic types
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Tipi fondamentali
Larghezza fissa interi tipi (C++11)
Limiti numerici
Original:
Numeric limits
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numeric_limits
C numerico limita interfaccia
Informazioni sul tipo di runtime
Original:
Runtime type information
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Tipo tratti
Original:
Type traits
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Categorie di tipo primario
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)
Proprietà del tipo
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)
Operazioni supportate
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)
I rapporti e le query di proprietà
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)
Tipo modifiche
Original:
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)
Tipo di trasformazioni
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)
Digitare costanti tratto
Original:
Type trait constants
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integral_constant(C++11)
 
Defined in header <type_traits>
template< bool B, class T = void >
struct enable_if;
(dal C++11)
Se B è true, std::enable_if ha un type membro pubblico typedef, pari a T, altrimenti, non vi è nessun membro 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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Questo MetaFunction viene utilizzato per rimuovere in modo condizionale funzioni e le classi di risoluzione sovraccarico sulla base di caratteri morfologici e per fornire overload della funzione separati e specializzazioni per i tratti di tipo diverso. std::enable_if può essere usato come un argomento di funzione aggiuntiva (non applicabile ai overload dell'operatore), come un tipo di ritorno (non applicabile ai costruttori e distruttori), o come un modello di classe o un modello di parametro di funzione.
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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Indice

[modifica] Membri tipi

Tipo
Original:
Type
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Definition
type
T sia o no tale membro, a seconda del valore di B
Original:
either T or no such member, depending on the value of B
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[modifica] Possibile implementazione

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

[modifica] Esempio

#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
}

Output:

foo1: float
foo1: int

[modifica] Vedi anche