"You don't pay for what you don't use" does not mean "the compiler doesn't generate worse code than if you had written the same by hand". These 2 things are C+

"You don't pay for what you don't use" does not mean "the compiler doesn't generate worse code than if you had written the same by hand". These 2 things are C++ core principle, but they are 2 different things. "You don't pay for what you don't use" means that I you do not use a C++ feature, your runtime performance won't be affected by this feature. For example, non virtual functions are not slower because virtual functions exist. In our case here, if you do not use "std::unordered_map" and decide to implement your own unordered map, then it is as-if "std::unordered_map" never existed. You are not forced to use "std::unordered_map" and you own map won't be slower because it exists. "the compiler doesn't generate worse code than if you had written the same by hand", or rather "What you do use, you couldn’t hand code any better", means that if you decided to implement a C++ feature by hand in C++ or C, your implementation could only hope to match C++ implementation. For instance calling a virtual function in C++ will never be slower than a similar handwritten late dispatch implementation.