访问tr1 / unordered_map的固有类型哈希函数

Question

I'm messing around with the unordered_map class template, and I'd like to write a custom hasher for my class. The documentation for it mentions that default hashing functions are provided for the intrinsic types. So, if you declare:

std::tr1::unordered_map<std::string, int> foo;

You automatically get a hasher defined for you.

There's a good example on here on how to provide a functor if you want a custom hash function.

However, what if I have a complex class that has a std::string member that I would like to use as the key for insertion/deletion into the unordered_map ? I don't want to rewrite my own hasher. I want to leverage what was already written for the std::string type.

Answer 1

The default hash functor is provided by std::hash<T> which returns size_t .

So you can combine the hash from several members of the class eg by computing (std::hash<T>()(a) + prime * (std::hash<T>()(b) + prime * std::hash<T>()(c))) .