How Can I Efficiently Combine Hash Values in C 0x?

Combining Hash Values in C 0x

C 0x introduces the hash<...>() function, providing a standardized approach to computing hash values for various data types. However, it initially lacked a hash_combine function similar to the one found in Boost.

To address this, the Boost developers suggest a simple implementation that mirrors their own:

template <class T>
inline void hash_combine(std::size_t& seed, const T& v) {
    std::hash<T> hasher;
    seed ^= hasher(v) + 0x9e3779b9 + (seed << 6) + (seed >> 2);
}

This function effectively combines the hash value of an input value v with the existing seed. The addition, XOR operations, and shifting help distribute the bits of the hash values evenly, resulting in a more robust and even hash distribution.

By incorporating this function into your C 0x code, you can seamlessly combine hash values and benefit from the benefits of standardized hash value computation provided by the language.

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