How do I reserve capacity ahead of time with std::unordered_map in multithreaded code?

#include <unordered_map>
#include <vector>
#include <thread>
#include <iostream>

void populateMap(std::unordered_map& myMap) {
    // Simulating workload
    for (int i = 0; i < 100; ++i) {
        myMap[i] = i * i; // Squaring the numbers as a simple example
    }
}

int main() {
    // Create an unordered_map with an initial size
    std::unordered_map myMap;
    myMap.reserve(100); // Reserve capacity for 100 elements

    // Creating multiple threads to populate the map
    std::vector<std::thread> threads;
    for (int i = 0; i < 5; ++i) {
        threads.emplace_back(populateMap, std::ref(myMap));
    }

    // Join all threads
    for (auto& t : threads) {
        t.join();
    }

    // Outputting the contents of the map
    for (const auto& pair : myMap) {
        std::cout << pair.first << ": " << pair.second << std::endl;
    }

    return 0;
}