Why Can't I Use `std::pair` as the Key in an `std::unordered_map` and How Do I Fix It?

Why can't I compile an unordered_map with a pair as key?

The issue faced here is the absence of a suitable hash function for the key type. To resolve this, provide a custom hash function for the pair key. Here's an example:

#include <unordered_map>
#include <functional>
#include <string>
#include <utility>

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

        return h1 ^ h2;  // Simple example, for better results use boost.hash_combine
    }
};

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

With this custom hash function, you can now create an unordered_map without compilation errors. Note that the hash function provided here is simplistic for demonstration purposes. For real-world scenarios, consider using techniques like Boost's boost.hash_combine to enhance the quality of your hash function.

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