How do I merge two containers efficiently with std::unordered_map for embedded targets?

#include <iostream>
#include <unordered_map>

int main() {
    // Create two unordered_maps
    std::unordered_map map1 = {
        {1, "One"},
        {2, "Two"},
        {3, "Three"}
    };
    
    std::unordered_map map2 = {
        {3, "Three Updated"},
        {4, "Four"},
        {5, "Five"}
    };

    // Merging map2 into map1
    for (const auto& pair : map2) {
        map1[pair.first] = pair.second; // This will update if exists, else insert
    }

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

    return 0;
}