How to Use Pairs as Keys in Unordered Maps in C ?

Using Pairs as Keys in Unordered Maps

When attempting to declare an unordered_map where the keys are pairs (Vote = pair), many encounter an unexpected error:

Implicit instantiation of undefined template 'std::__1::hash, std::__1::basic_string > >'

This error arises because unordered_map relies on a hash function tailored to its key type. Since the default hash function is not applicable to pairs, you must provide a custom hash function.

For instance, consider the following custom hash function (pair_hash):

struct pair_hash {
    template <class T1, class T2>
    std::size_t operator() (const std::pair<T1, T2> &amp;p) const {
        auto h1 = std::hash<T1>{}(p.first);
        auto h2 = std::hash<T2>{}(p.second);
        return h1 ^ h2;
    }
};

This hash function combines the hash values of the pair's components using the XOR operator. To use it, modify the unordered_map declaration as follows:

using Vote = std::pair<std::string, std::string>;
using Unordered_map = std::unordered_map<Vote, int, pair_hash>;

This approach allows the creation of unordered maps with pairs as keys by providing a suitable hash function. However, it's worth noting that this simplistic implementation may not exhibit optimal hash properties and consider exploring Boost's hash_combine function or set hash_value for improved results.

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