std::enable_if
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| Defined in header <type_traits>
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| template< bool B, class T = void > struct enable_if; |
(C + + 11以来) | |
もし
Bですtrue、std::enable_iftypeに等しいpublicメンバのtypedefTを持ち、そうでない場合には、typedefなメンバはありません. Original:
If
B is true, std::enable_if has a public member typedef type, equal to T; otherwise, there is no member typedef. The text has been machine-translated via Google Translate.
You can help to correct and verify the translation. Click here for instructions.
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このメタ関数は、条件付きで型の特徴に基づいて、オーバーロードの解決から関数やクラスを削除するには、別の型の特徴ごとに別々の関数オーバーロードと特殊化を提供するために使用されます。 std::enable_ifは、戻り値の型（コンストラクタとデストラクタには適用されません）として、あるいはクラステンプレートまたは関数テンプレートパラメータとして、追加の関数の引数（演算子のオーバーロードには適用されません）として使用することができます.
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.
The text has been machine-translated via Google Translate.
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目次 |
[編集] メンバータイプ
| タイプ
Original: Type The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions. |
Definition |
type
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Tの値に応じてBまたはそのような会員はいません、どちらかOriginal: either T or no such member, depending on the value of BThe text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions. |
[編集] 可能な実装
template<bool B, class T = void> struct enable_if {}; template<class T> struct enable_if<true, T> { typedef T type; }; |
[編集] 例
#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
