Why is std::hash a struct instead of a function?

Question

Standard library implements std::hash as a template struct that is specialized for different types. It is used like this:

#include 
#include          


        

Answer 1

A template function can not be partially specialized for types, while std::hash specialized for different types as a class template.

And, in this template class based way, you can do some meta programming such as accessing to return type and key type like below:

std::hash::argument_type
std::hash::result_type