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

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.

Answer 2

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>

Answer 3

In my case it was really distraction.

I had a type X for which I implemented hashing for const& X an utilized it somewhere with

std::unordered_map<const X, int> m_map;

Then I wanted to have another map which key are of the type X and did:

std::unordered_map<X, int> map_x;

Notice the LACK of const on the second case.

Answer 4

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

//} // namespace tr1

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

} // namespace std

If you then want to create say a std::unordered_map<CustomType> 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.

Answer 5

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;
}