How to Effectively Combine Hash Values in C 0x?

Combining Hash Values in C 0x

C 0x introduces hash<>() to facilitate hashing in the standard library. However, it lacks a built-in hash_combine function similar to Boost's implementation. This raises the question of how to emulate such functionality in C 0x.

C 0x xor_combine vs. Boost's hash_combine

The Boost hash_combine function performs bitwise operations to efficiently combine multiple hash values into a single one. While C 0x provides xor_combine, it's not a direct replacement for hash_combine. xor_combine can only be used to combine two hash values, whereas hash_combine handles an arbitrary number of values.

Implementing hash_combine in C 0x

To implement hash_combine in C 0x, one can take inspiration from Boost's approach. Here's a straightforward implementation:

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 implementation mimics Boost's algorithm, using a combination of XOR operations, bit shifting, and constant addition to ensure thorough mixing of the hash values.

By leveraging this hash_combine function, you can conveniently combine multiple hash values into a single result using C 0x's powerful hashing capabilities.

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