Why am I "not supposed to use exceptions"? I don't understand why exception should be used when errors occur occasionally, or when they occur 50,000/sec. They are a control flow method for when errors occur.
If I'm writing a string_to_int function today I would return an optional<int> (or some Either variant). But if exceptions were cheap I would use them. If I'm catching an exception in the context of immediate caller and the compiler inlines the calle, it ought to be able to optimise the exceptional path away. But it doesn't happen.
no, not any errors, exceptional errors. Input data coming from "outside" being invalid is not exceptional. The filesystem suddenly becoming inaccessible during a copying or caching operation, the OS not being able to join a thread, your logging backend becoming unable to log because the disk is full, or a regular expression being incorrect are what comes to mind when I think of exceptional situations (unless your program allows users to input regexps). If your program runs on a normal computer in normal conditions, no exceptions should ever be thrown.