Why this code provides specialization for **ALL** enums for std::hash template?

Question

I'm not a pro in C++ but somehow I provided a solution while porting my MSVS 2015 C++ code to MinGW 4.9.2 to specialize std::hash class to support all enums. It there's any C++ compiler developers or C++ pro programmers here, can you explain why this specialization works, although it is Undefined Behavior according the C++ standard they say?

Link for full code with samples on Gist

#include <unordered_set>
#include <functional>

#if (defined __GNUC__) && (__GNUC__ < 6)
// Adds support of std::hash<enum T> to libstdc++.
// GCC 6 provides it out of the box.
namespace std {
    template<typename T>
    struct hash {
        constexpr size_t operator()(typename std::enable_if<std::is_enum<T>::value, T>::type s) const noexcept {
            return static_cast<size_t>(s);
        }
    };
}
#endif

提供对的支持std::hash<enum T>意味着所有类似的类std::unordered_XXX都将支持任何一个enum作为键。

具有此std::hash定义的GCC 6.1.0 因错误而失败

error: redefinition of 'struct std::hash<_Tp>'

没有此std::hash定义的GCC 5.3.0 对于std :: unordered_set失败，出现以下错误：

In file included from /usr/local/gcc-5.3.0/include/c++/5.3.0/bits/hashtable.h:35:0,
                 from /usr/local/gcc-5.3.0/include/c++/5.3.0/unordered_set:47,
                 from prog.cc:1:
/usr/local/gcc-5.3.0/include/c++/5.3.0/bits/hashtable_policy.h: In instantiation of 'struct std::__detail::__is_noexcept_hash<Foo, std::hash<Foo> >':
/usr/local/gcc-5.3.0/include/c++/5.3.0/type_traits:137:12:   required from 'struct std::__and_<std::__is_fast_hash<std::hash<Foo> >, std::__detail::__is_noexcept_hash<Foo, std::hash<Foo> > >'
/usr/local/gcc-5.3.0/include/c++/5.3.0/type_traits:148:38:   required from 'struct std::__not_<std::__and_<std::__is_fast_hash<std::hash<Foo> >, std::__detail::__is_noexcept_hash<Foo, std::hash<Foo> > > >'
/usr/local/gcc-5.3.0/include/c++/5.3.0/bits/unordered_set.h:95:63:   required from 'class std::unordered_set<Foo>'
prog.cc:25:25:   required from here
/usr/local/gcc-5.3.0/include/c++/5.3.0/bits/hashtable_policy.h:85:34: error: no match for call to '(const std::hash<Foo>) (const Foo&)'
  noexcept(declval<const _Hash&>()(declval<const _Key&>()))>
                                  ^
In file included from /usr/local/gcc-5.3.0/include/c++/5.3.0/bits/move.h:57:0,
                 from /usr/local/gcc-5.3.0/include/c++/5.3.0/bits/stl_pair.h:59,
                 from /usr/local/gcc-5.3.0/include/c++/5.3.0/utility:70,
                 from /usr/local/gcc-5.3.0/include/c++/5.3.0/unordered_set:38,
                 from prog.cc:1:
/usr/local/gcc-5.3.0/include/c++/5.3.0/type_traits: In instantiation of 'struct std::__not_<std::__and_<std::__is_fast_hash<std::hash<Foo> >, std::__detail::__is_noexcept_hash<Foo, std::hash<Foo> > > >':
/usr/local/gcc-5.3.0/include/c++/5.3.0/bits/unordered_set.h:95:63:   required from 'class std::unordered_set<Foo>'
prog.cc:25:25:   required from here
/usr/local/gcc-5.3.0/include/c++/5.3.0/type_traits:148:38: error: 'value' is not a member of 'std::__and_<std::__is_fast_hash<std::hash<Foo> >, std::__detail::__is_noexcept_hash<Foo, std::hash<Foo> > >'
     : public integral_constant<bool, !_Pp::value>
...

Answer 1

这是一个格式错误的程序，不需要进行诊断，因为不允许为模板类型提供基本的专业知识std。

之所以起作用，是因为在某些实现中，提供这种基本专业化的目的是将所有内容 重定向，std::hash<T>而无需将显式专业化到您的哈希实现。然后，您的operator()收益只能与enums一起使用，但这不是它起作用的原因，也不能防止它使程序在不需要诊断的情况下不正确。

实际上，它可能是因为有人在SFINAE中启用std::hash了gcc 6.1中的空基本实现而失败了：不确定，这可能是某些C ++标准强制要求的，但是如果不能，这是实现改进的质量。

正确的方法是创建

struct enum_hash {
  template<typename T>
  constexpr
  typename std::enable_if<std::is_enum<T>::value,std::size_t>::type
  operator()(T s) const noexcept {
    return static_cast<std::size_t>(s);
  }
};

可以散列任何枚举的类型。

现在，通过,enum_hash以unordered_set<some_enum, enum_hash>这样的。