C++ Hash function for string in unordered_map

C++ STL provides template specializations of std::hash for the various string classes. You could just specify std::string as key type for std::unordered_map:

#include <string>
#include <unordered_map>

int main()
{
    std::unordered_map<std::string, int> map;
    map["string"] = 10;
    return 0;
}

I ran into this today (actually with wstring, not string, but it's the same deal): using wstring as a key in an unordered_map generates an error about no hash function being available for that type.

The solution for me was to add:

#include <string>

Believe it or not, without the #include directive I still had the wstring type available but apparently NOT the ancillary functions like the hash. Simply adding the include above fixed it.

Actually, there is std::hash<std::string>

But there it is how you can use another hash function:

struct StringHasher {
    size_t operator()(const std::string& t) const {
          //calculate hash here.
    }
}

unordered_map<std::string, ValueType, StringHasher>