How to Use Pairs as Keys in Unordered Maps?

Understanding Pair as Keys in Unordered Maps

When attempting to create an unordered map with a pair as the key, users may encounter a compilation error. This error arises from the lack of a suitable hash function for the custom key type, pair.

To resolve this issue, std::unordered_map requires users to provide a hash function specifically designed for the key type. This function is responsible for mapping the key to a unique identifier, which is crucial for fast and efficient retrieval.

Implementing a Custom Hash Function

A simple, albeit not optimal, hash function for a pair of std::hash-able types can be implemented as follows:

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; // Combine the hashes using a simple XOR
    }
};

Using the Custom Hash Function

Once the hash function is defined, it can be used with std::unordered_map to resolve the compilation error:

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

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

int main() {
    Unordered_map um;
}

Alternative Solutions

For broader support and better performance, consider utilizing libraries like Boost. Boost provides:

• boost::hash_combine: A more robust method for combining hashes
• set_hash_value function: Prepares hash functions for standard types like std::pair

By leveraging these options, users can ensure reliable hashing for their custom key types within std::unordered_map, enabling efficient and optimal code performance.

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