👀 Views: 383 💬 Answers: 1 📅 Created: 2025-06-05

c++20 unordered_map custom_hash C++

I need help solving This might be a silly question, but I'm working with inconsistent behavior when using a `std::unordered_map` with a custom hash function in C++20... My custom hash function is supposed to handle a specific type of class, but when I insert and retrieve elements, I'm sometimes getting unexpected results. Here’s a simplified example of my setup: ```cpp #include <iostream> #include <unordered_map> #include <string> class MyClass { public: std::string name; int id; MyClass(std::string n, int i) : name(n), id(i) {} bool operator==(const MyClass& other) const { return name == other.name && id == other.id; } }; struct MyClassHash { std::size_t operator()(const MyClass& mc) const { return std::hash<std::string>()(mc.name) ^ std::hash<int>()(mc.id); } }; int main() { std::unordered_map<MyClass, int, MyClassHash> myMap; MyClass obj1("item1", 1); MyClass obj2("item1", 1); myMap[obj1] = 10; std::cout << "Value for obj2: " << myMap[obj2] << std::endl; // This may throw an behavior or return zero unexpectedly return 0; } ``` I expected `myMap[obj2]` to return `10`, but sometimes I get a segmentation fault or a default-constructed value instead. I’ve ensured that both `MyClass` and `MyClassHash` are correctly defined and the equality operator is implemented. I’ve tried debugging with breakpoints but the scenario seems to be intermittent and I need to pinpoint when it happens. Could this be related to how the hash function is combining the hashes or the way `std::unordered_map` handles collisions? Any insights would be greatly appreciated. Is there a better approach? I'm coming from a different tech stack and learning C++. Hoping someone can shed some light on this.