How do I merge two containers efficiently with std::unordered_map for large datasets?

Merging two containers efficiently with std::unordered_map in C++ can significantly improve performance when dealing with large datasets. The std::unordered_map provides average time complexity of O(1) for insertions and lookups, making it an ideal choice for merging operations.

To merge two std::unordered_map containers, you can simply iterate through one map and insert the elements into the other. If there are duplicate keys, you can decide how to handle them, such as keeping the existing value or replacing it.

#include <iostream>
#include <unordered_map>

void mergeMaps(std::unordered_map<int, std::string>& map1, 
               const std::unordered_map<int, std::string>& map2) {
    for (const auto& pair : map2) {
        map1[pair.first] = pair.second; // or choose a different strategy for duplicates
    }
}

int main() {
    std::unordered_map<int, std::string> map1 = {
        {1, "one"},
        {2, "two"}
    };

    std::unordered_map<int, std::string> map2 = {
        {2, "two_updated"},
        {3, "three"}
    };

    mergeMaps(map1, map2);

    for (const auto& pair : map1) {
        std::cout << pair.first << ": " << pair.second << std::endl;
    }

    return 0;
}