Comment by fluoridation

I think your examples don't help your argument.

>std::regex_error and std::format_error are very much of interest to the callers of the APIs that throw them (e.g., because the patterns user-provided and the callers expect potential failures) but that doesn't mean they shouldn't be exceptions

Most commonly, regexes and format string are passed from program data, not user input, so in the majority of cases I don't think it's correct to say that errors are an expected situation. Programmer error is definitely not something that can be handled at the call site, so it makes for a prime candidate to be signaled by an exception.

>Or, for example, the only entity ever really interested in an std::out_of_range exception is the caller (say, catching it around a call like std::accumulate(i, j, [&](const auto& key) { return container.at(key); })).

Why would you write exception handling code around that call instead of ensuring before the call that the indices are valid? If you did ensure that and an exception was still thrown, that's the kind of situation that the caller is not able to resolve, so why would it be interested in the exception? What could it possibly do to recover once it's been proven that the vector was changed concurrently?

>If C++ exceptions were as cheap to throw as to avoid, then that would tip the balance significantly.

No, I don't agree. Sorry for not making my position clearer. What I said previously assumes that you don't have any additional constraints beyond designing the best error handling possible. If you have technical reasons to avoid exceptions (they're too expensive, you're about to cross an FFI boundary, etc.) then that's a reason to stay away from them. I think you should use exceptions up to as much as I said. To use your metaphor, my argument is the floor under the plate of reasons to use exceptions, and the balance can only tip in the other direction.