How do I avoid rehashing overhead with std::unordered_map in performance-sensitive code?

To avoid rehashing overhead with std::unordered_map in performance-sensitive C++ code, you can reserve space in advance based on an estimated number of elements. This helps maintain the performance of the unordered map by reducing the number of times it has to resize its internal data structures.

Additionally, you can control the load factor of the unordered map to fine-tune performance.

#include <unordered_map> #include <iostream> int main() { // Reserve space for 1000 elements to avoid rehashing std::unordered_map<int, std::string> myMap; myMap.reserve(1000); for (int i = 0; i < 1000; ++i) { myMap[i] = "Value " + std::to_string(i); } std::cout << "Unordered map size: " << myMap.size() << std::endl; return 0; }

C++ std::unordered_map rehashing performance load factor reserve

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