Can't use enum class as unordered_map key

Question

I have a class containing an enum class.

class Shader {
public:
    enum class Type {
        Vertex   = GL_VERTEX_SHADER,
        Geometry = GL_GEOMETRY_SHA         


        

Answer 1

I use a functor object to calculate hash of enum class:

struct EnumClassHash
{
    template 
    std::size_t operator()(T t) const
    {
        return static_cast(t);
    }
};

Now you can use it as 3rd template-parameter of std::unordered_map:

enum class MyEnum {};

std::unordered_map myMap;

So you don't need to provide a specialization of std::hash, the template argument deduction does the job. Furthermore, you can use the word using and make your own unordered_map that use std::hash or EnumClassHash depending on the Key type:

template 
using HashType = typename std::conditional::value, EnumClassHash, std::hash>::type;

template 
using MyUnorderedMap = std::unordered_map>;

Now you can use MyUnorderedMap with enum class or another type:

MyUnorderedMap myMap2;
MyUnorderedMap myMap3;

Theoretically, HashType could use std::underlying_type and then the EnumClassHash will not be necessary. That could be something like this, but I haven't tried yet:

template 
using HashType = typename std::conditional::value, std::hash::type>, std::hash>::type;

If using std::underlying_type works, could be a very good proposal for the standard.