C++ Hash function for string in unordered_map

Question

It seems as if C++ does not have a hash function for strings in the standard library. Is this true?

What is a working example of using a string as a key in an unordered_

Answer 1

If you have a CustomType and you want to plug into the STL infrastructure this is what you could do.

namespace std
{
//namespace tr1
//{
    // Specializations for unordered containers

    template <>
    struct hash : public unary_function
    {
        size_t operator()(const CustomType& value) const
        {
            return 0;
        }
    };

//} // namespace tr1

template <>
struct equal_to : public unary_function
{
    bool operator()(const CustomType& x, const CustomType& y) const
    {
        return false;
    }
};

} // namespace std

If you then want to create say a std::unordered_map the STL will find the hash and equal_to functions without you having to do anything more with the template. This is how I like to write my custom equality comparer that support unordered data structures.