How can I create a custom hash function for an unordered map in C++11?

How can I create a custom hash function for an unordered map in C++11?

To create a custom hash function in C++11, you need to define a struct or a class that implements the operator().

Can you show me an example of such a hash function?

Sure! Here's a simple example:

struct CustomHash {
    std::size_t operator()(const std::string& key) const {
        return std::hash<std::string>()(key) ^ (key.size() << 1);
    }
};

This hash function combines the standard hash with the size of the key.

How do I use this custom hash function with an unordered map?

You can use it like this:

std::unordered_map<std::string, int, CustomHash> myMap;

This tells the unordered map to use your CustomHash for hashing keys.

What if I want to use a different type as a key?

You can create a similar hash function for any type by implementing the operator() for that type. Just ensure it returns a std::size_t.