std::hash

在思考使用const char*来作为std::unordered_map的时候，遇到了hash的问题，然而在cppreference std::hash 中有这样的描述:

There is no specialization for C strings. std::hash produces a hash of the value of the pointer (the memory address), it does not examine the contents of any character array.

因此，我只能怪customize hash for const char*。

std::hash in C++ standard library的概述

在 stackoverflow std::hash value on char* value and not on memory address? 的回答中的一段话较好地概述了std::hash in C++ standard library:

Sadly enough, the current C++ standard library doesn't provide general purpose hash algorithms disentangled(拜托) from object-specific hash solutions. (But there is some hope this could change in the future.)

显然，这段话告诉了我们: 目前C++ standard library的std::hash是object-specific hash function，并没有提供general purpose hash algorithm。

Change in the future: open-std Types Don't Know

NOTE: 在 stackoverflow std::hash value on char* value and not on memory address? 的回答中"But there is some hope this could change in the future" some hope 所链接的是 open-std Types Don't Know 提案，本文就是基于这篇文章:

This paper proposes a new hashing infrastructure that completely decouples hashing algorithms from individual types that need to be hashed. This decoupling divides the hashing computation among 3 different programmers who need not coordinate with each other.