Can I Use Pairs as Keys in C Unordered Maps, and How?

Can I use Pairs as Keys in Unordered Maps?

In C , you can encounter a compilation error when using an unordered_map with pair as its key. The issue arises from utilizing a custom type as the key, without providing an appropriate hash function for it.

Understanding the Error

The error you encounter is because unordered_map relies on a hashing mechanism to efficiently locate key-value pairs. However, it lacks a default hash function for complex types like pairs. The error message indicates that the compiler cannot implicitly instantiate an undefined hash function for your pair key type.

Solution: Providing a Hash Function

To resolve this issue, you need to define a hash function that calculates a unique hash value for your Vote pair type. This function should accept a pair as an argument and output a hash value.

Here's an example hash function implementation:

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;
    }
};

With this hash function, you can modify your code to create an unordered_map that uses pairs as keys:

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

Now, you can use this unordered_map as intended, avoiding the compilation error.

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