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std::enable_if

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rel_ops::operator!=
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aligned_storage(C++11)
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enable_if(C++11)
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Digite constantes traço
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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;
(a partir do C++ 11)
Se B é true, std::enable_if tem um type membro pública typedef, igual a T, caso contrário, não há nenhum 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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Este metafunção é usado para remover condicionalmente funções e classes de resolução de sobrecarga com base em características de tipo e para fornecer sobrecargas de função separados e especializações para características de tipo diferente. std::enable_if pode ser usado como um argumento de função adicional (não aplicável a sobrecargas de operador), como um tipo de retorno (não aplicável a construtores e destruidores), ou como um modelo de classe ou parâmetro do modelo função.
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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Índice

[editar] Tipos de membro

Tipo
Original:
Type
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Definition
type
ou T ou não esse membro, de acordo com o valor de B
Original:
either T or no such member, depending on the value of B
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[editar] Possível implementação

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

[editar] Exemplo

#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

[editar] Veja também