How do I iterate safely under modification with std::map in performance-sensitive code?

Iterating safely over a std::map while it is being modified can be challenging, especially in performance-sensitive code. The traditional approach of changing elements during iteration can lead to undefined behavior. Instead, utilizing an iterating method that preserves the state of the map or using a copy can be a safe way to manage modifications.

One effective method is to keep track of the elements that need to be removed or modified and then apply those changes after completing the iteration. Another option is to use a separate container to gather keys or values that need to be processed after the main iteration.

The following example outlines a safe way to iterate over an std::map and modify it without causing segmentation faults or other issues:


        #include 
        #include 
        #include 

        int main() {
            std::map myMap = {
                {1, "one"},
                {2, "two"},
                {3, "three"},
            };

            std::vector keysToRemove;

            // Safe iteration
            for (auto it = myMap.begin(); it != myMap.end(); ++it) {
                if (it->first % 2 == 0) {
                    keysToRemove.push_back(it->first); // Mark for removal
                }
            }

            // Now modify the map safely after the iteration
            for (const int& key : keysToRemove) {
                myMap.erase(key);
            }

            // Output the remaining elements
            for (const auto& pair : myMap) {
                std::cout << pair.first << ": " << pair.second << std::endl;
            }

            return 0;
        }

How do I iterate safely under modification with std::map in performance-sensitive code?

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How do I iterate safely under modification with std::map in performance-sensitive code?

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