Modern C++ Programming Cookbook

How to do it...

In order to use your custom types with the unordered associative containers, you must perform the following steps:

1. Specialize the std::hash class template for your custom type; the specialization must be done in the std namespace.
2. Define synonyms for the argument and result type.
3. Implement the call operator so that it takes a constant reference to your type and returns a hash value.

To compute a good hash value, you should do the following:

1. Start with an initial value that should be a prime number (for example, 17).
2. For each field that is used to determine whether two instances of the class are equal, adjust the hash value according to the following formula:

        hashValue = hashValue * prime + hashFunc(field);

3. You can use ...