C ++ SFINAE示例？ [英] C++ SFINAE examples?

问题描述

我想进入更多的模板元程序设计。我知道SFINAE代表替代故障不是一个错误。

I want to get into more template meta-programming. I know that SFINAE stands for "substitution failure is not an error." But can someone show me a good use for SFINAE?

推荐答案

下面是一个例子（从这里）：

template<typename T>
class IsClassT {
  private:
    typedef char One;
    typedef struct { char a[2]; } Two;
    template<typename C> static One test(int C::*);
    // Will be chosen if T is anything except a class.
    template<typename C> static Two test(...);
  public:
    enum { Yes = sizeof(IsClassT<T>::test<T>(0)) == 1 };
    enum { No = !Yes };
};

当 IsClassT< int> :: Yes 因为int不是一个类，所以它不能有一个成员指针，因此不能转换为 int int :: * 。如果SFINAE不存在，那么你会得到一个编译器错误，像'0不能转换为非类型int的成员指针。相反，它只是使用返回Two的 ... 形式，因此求值为false，int不是类类型。

When IsClassT<int>::Yes is evaluated, 0 cannot be converted to int int::* because int is not a class, so it can't have a member pointer. If SFINAE didn't exist, then you would get a compiler error, something like '0 cannot be converted to member pointer for non-class type int'. Instead, it just uses the ... form which returns Two, and thus evaluates to false, int is not a class type.

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