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

Using std::map in C++ can be performance-sensitive, especially when the overhead from rehashing becomes a bottleneck in your applications. One effective way to avoid rehashing is to reserve enough space upfront or use alternative data structures that provide better performance for specific use cases.

For scenarios where key-value pairs are being inserted into a std::map frequently, it’s important to minimize the overhead associated with resizing the underlying data structure. Below are a couple of strategies to mitigate this issue.

Strategies to Avoid Rehashing Overhead

• Reserve Space Ahead of Time: While std::map does not have a `reserve` method like std::unordered_map, understanding the expected data range and managing inserts accordingly can help.
• Use std::unordered_map: When you need a hash table that allows faster average time complexity for inserts and lookups, consider switching to std::unordered_map.
• Use std::vector<:pair value>>: For sorted data, consider using a std::vector of pairs, followed by sorting the vector, if your dataset is static or infrequently modified.

Example Code

#include <iostream>
#include <map>

int main() {
    std::map myMap;
    
    // Inserting multiple elements
    for (int i = 0; i < 1000; ++i) {
        myMap[i] = "Value " + std::to_string(i);
    }

    // Accessing an element to check performance
    std::cout << myMap[500] << std::endl;

    return 0;
}