c++ 是否为引用指定了哈希函数？

Question

我的问题很快。

C++标准库是否为引用实现了哈希函数，如std::hash<T&>

Answer 1

不，不是。 [function.objects]中列举的专业是：

// Hash function specializations
template <> struct hash<bool>;
template <> struct hash<char>;
template <> struct hash<signed char>;
template <> struct hash<unsigned char>;
template <> struct hash<char16_t>;
template <> struct hash<char32_t>;
template <> struct hash<wchar_t>;
template <> struct hash<short>;
template <> struct hash<unsigned short>;
template <> struct hash<int>;
template <> struct hash<unsigned int>;
template <> struct hash<long>;
template <> struct hash<long long>;
template <> struct hash<unsigned long>;
template <> struct hash<unsigned long long>;

template <> struct hash<float>;
template <> struct hash<double>;
template <> struct hash<long double>;

template<class T> struct hash<T*>;

而且，这样的事情到底意味着什么？