One of C++'s most glaring problems is that everybody only ever uses some subset of some version of the language, but you still need to know basically the entire massive, monstrous language because not everybody uses the same subset and you need to be prepared for anything. On top of that, you're lucky if your employer or potential recipient of contributions (e.g., FOSS project) or whatever has actually codified the subset they want to you to use: most of the time, you need to infer the allowable subset from the existing code.