How Can I Provide a Default Hash Function for Custom Types in C Unordered Containers?

Specializing std::hash for Custom Types in Unordered Containers

In unordered containers like std::unordered_set and std::unordered_map, supporting custom key types requires providing both operator== and a hashing functor. While it's convenient to use a custom hash functor, it's also desirable to have a default hash for custom types similar to built-in types.

To achieve this, one can specialize std::hash for the custom type Key. However, this raises questions about its legality, compliance, and portability.

Legality

Specifying a specialization within the std namespace is permitted and, in fact, encouraged for cases like this.

Compliance

The correct way to specialize std::hash is to define a custom struct within the std namespace. The following code snippet demonstrates the correct syntax:

namespace std {
  template <> struct hash<Foo> {
    size_t operator()(const Foo &x) const {
      // Hashing logic for custom type
    }
  };
}

Portability

This method of specializing std::hash is portable across major compilers such as g and MSVC.

Additional Features

Apart from std::hash, one can also specialize other functors like std::less, std::equal_to, and std::swap for custom types to enhance functionality.

Conclusion

By specializing std::hash, one can provide a default hash function for custom types in unordered containers, making it more convenient to use them in standard library containers. This approach is both legal and portable while adhering to C 11 standards.

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