How do I use heterogeneous lookup with std::unordered_map?

In C++, heterogeneous lookup using `std::unordered_map` allows for more flexibility in key types by leveraging type erasure to access different types of keys using a consistent interface. This means you can easily look up or access values associated with keys of different types.

Here’s a simple example demonstrating heterogeneous lookup with `std::unordered_map`:


        #include <iostream>
        #include <unordered_map>
        #include <string>
        #include <variant>

        using KeyType = std::variant<int, std::string>;

        struct KeyHasher {
            std::size_t operator()(const KeyType& key) const {
                return std::visit([](const auto& k) { return std::hash<std::decay_t<decltype(k)>>{}(k); }, key);
            }
        };

        struct KeyEqual {
            bool operator()(const KeyType& lhs, const KeyType& rhs) const {
                return lhs == rhs;
            }
        };

        int main() {
            std::unordered_map<KeyType, std::string, KeyHasher, KeyEqual> my_map;

            // Insert values
            my_map[42] = "Answer to life";
            my_map[std::string("hello")] = "Greeting";

            // Access values
            std::cout << std::get<std::string>(my_map[std::string("hello")]) << std::endl; // Output: Greeting
            std::cout << my_map[42] << std::endl; // Output: Answer to life

            return 0;
        }

How do I use heterogeneous lookup with std::unordered_map?

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How do I use heterogeneous lookup with std::unordered_map?

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