std::map Insertion with Custom Comparator optimization guide as Expected in C++20
Could someone explain I can't seem to get Could someone explain I'm working with a `std::map` and a custom key type in C++20, using a lambda function as a comparator. However, I am working with a question where the map doesn't seem to be ordering the keys correctly. Here's a simplified example of what I'm trying to achieve: ```cpp #include <iostream> #include <map> #include <string> struct Key { std::string value; }; auto customComparator = [](const Key& a, const Key& b) { return a.value.length() < b.value.length(); }; int main() { std::map<Key, int, decltype(customComparator)> myMap(customComparator); myMap[{"short"}] = 1; myMap[{"longer"}] = 2; myMap[{"tiny"}] = 3; for (const auto& pair : myMap) { std::cout << pair.first.value << " : " << pair.second << '\n'; } return 0; } ``` When I run this code, I expected the output to be sorted by the length of the string, but it appears to be unordered. The insertion order seems to dictate the order of the output instead. Additionally, I get a compilation warning stating that `Key` does not have a defined comparison operator. After some testing, I found that if I define the `operator<` for the `Key` struct instead of using the lambda, the map behaves as expected: ```cpp struct Key { std::string value; bool operator<(const Key& other) const { return value.length() < other.value.length(); } }; ``` This resolves the ordering scenario, but I would prefer to keep using my lambda function for clarity and flexibility. Is there a reason my lambda comparator isn't working as intended, or is this just a limitation of how `std::map` expects its comparator to be implemented? Any insights would be greatly appreciated. Any help would be greatly appreciated! I'm working with Cpp in a Docker container on Ubuntu 20.04. I'd really appreciate any guidance on this. My development environment is Windows 11. My team is using Cpp for this application. I'd love to hear your thoughts on this. I'm working on a desktop app that needs to handle this.