How do you report errors in C++ firmware without exceptions?

The same disciplined approaches as good C, plus C++ niceties. The baseline is status-code returns, a function returns an enum class Error { Ok, Nack, Timeout, ... } and the caller checks it and handles locally (retry, log, fail safe). For functions that must return both a value and a possible error, use a result type: std::expected<T, E> (C++23) or a hand-rolled/library Result<T, E> that holds either a value or an error, so the caller is forced by the type to handle both cases before extracting the value. You can also use out-parameters with a status return (the C idiom). The key properties you preserve are that errors are explicit in the signature, checkable, and propagate without unwinding the stack non-deterministically. RAII still works alongside this, destructors clean up on the error return path just as they would during exception unwinding, so you keep leak-free cleanup without the exception machinery.