How to Extend the Standard Hash Function for Custom Types in C Unordered Containers?

Extending Standard Hash for Custom Types in Unordered Containers

In order to utilize user-defined types as keys within unordered containers such as std::unordered_set and std::unordered_map, it is necessary to implement the equality operator (operator==) and a hash functor. A more convenient approach would be to create a default hash function specifically for the custom type, analogous to the built-in hash functions provided for standard types.

Upon examining various resources, it appears that specializing std::hash::operator() for a user-defined type X is indeed feasible. However, the following questions arise:

1. Is it permissible to add such a specialization to the std namespace?
2. Which version of std::hash::operator(), if any, is compliant with the C 11 standard?
3. Is there a platform-independent approach to achieve this?

Extending the Standard Namespace

The C standard explicitly permits and encourages the addition of specializations to the std namespace, provided that one of the types involved is user-defined. Therefore, the first question can be answered affirmatively.

C 11 Compliant Specializations

The correct and preferred method of specializing the hash function in C 11 is to define a structure within the std namespace as follows:

namespace std {
  template <>
  struct hash<Foo> {
    size_t operator()(const Foo & x) const {
      // Implementation of the hash function for type Foo
    }
  };
}

This syntax ensures compliance with the C 11 standard and allows for the specification of custom hash functions for user-defined types.

Portable Implementation

Unfortunately, there is no platform-independent method to specialize std::hash::operator() in C . Implementing a custom hash function within the std namespace requires either a compiler-specific implementation or the use of a third-party library.

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