How do I avoid rehashing overhead with std::unordered_map in multithreaded code?

In multithreaded applications, using `std::unordered_map` can be efficient for fast lookups, but it comes with the overhead of rehashing when the number of elements exceeds the current bucket count. To mitigate this overhead, it is essential to appropriately preallocate space and manage concurrency effectively.

Here are some strategies to avoid rehashing overhead:

Reserve Space: Use the `reserve()` method to preallocate enough space for expected elements to minimize rehashing.
Thread Safety: Since `std::unordered_map` is not thread-safe, consider using mutexes or thread-safe wrappers around data access.
Separate Maps: Use separate instances of `unordered_map` for different threads to avoid contention and allow each thread to manage its own data.

Here's a simple example that demonstrates the use of `std::unordered_map` in a multithreaded context:


        #include <iostream>
        #include <unordered_map>
        #include <thread>
        #include <mutex>

        std::unordered_map<int, int> myMap;
        std::mutex mapMutex;

        void insertData(int start, int end) {
            for (int i = start; i < end; ++i) {
                std::lock_guard<std::mutex> lock(mapMutex);
                myMap[i] = i * 10; // Insert data
            }
        }

        int main() {
            myMap.reserve(100); // Preallocate space for 100 elements
            std::thread t1(insertData, 0, 50);
            std::thread t2(insertData, 50, 100);

            t1.join();
            t2.join();

            for (const auto& pair : myMap) {
                std::cout << pair.first << " : " << pair.second << std::endl;
            }
            return 0;
        }

How do I avoid rehashing overhead with std::unordered_map in multithreaded code?

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How do I avoid rehashing overhead with std::unordered_map in multithreaded code?

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