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std::enable_if

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Grundtypen
Original:
Basic types
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Grundtypen
Feste Breite Integer-Typen (C++11)
Numerische Grenzen
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Numeric limits
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numeric_limits
C numerische begrenzt Schnittstelle
Laufzeit-Typinformationen
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Runtime type information
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Exterieur
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Type traits
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Primären Typ Kategorien
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)
Typ Eigenschaften
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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)
Unterstützte Operationen
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)
Beziehungen und Eigentum Abfragen
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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)
Typ Modifikationen
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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)
Typ Transformationen
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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)
Typ-Trait-Konstanten
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;
(seit C++11)
Wenn B ist true, std::enable_if hat einen öffentlichen Member typedef type gleich T, andernfalls gibt es kein Mitglied 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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Diese metafunction verwendet wird, bedingt entfernen Funktionen und Klassen vor Überlastung Auflösung basierend auf Exterieur und separate Funktion Überlastungen und Spezialisierungen für verschiedene Exterieur bieten. std::enable_if kann als zusätzliche Funktion Argument (gilt nicht für überladene Operatoren) verwendet werden, als Rückgabetyp (gilt nicht für Konstruktoren und Destruktoren) oder als Klasse Vorlage oder Funktion Template-Parameter .
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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Inhaltsverzeichnis

[Bearbeiten] Mitglied Typen

Type
Original:
Type
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Definition
type
entweder keine oder solche T Element, je nach dem Wert des B
Original:
either T or no such member, depending on the value of B
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[Bearbeiten] Mögliche Implementierung

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

[Bearbeiten] Beispiel

#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

[Bearbeiten] Siehe auch