struct Error **error
On the other end of spectrum is using good old error codes, not reinventing exceptions on your own.
Therefore I would say this approach is bad for C and C++ but not bad in general.
There is a solid rationale behind it.
Not sure what you mean exactly but I don't think you need copies all over the place since this is C++ so instances can be moved and there's also copy elision. And should you use 'int fun(Error& error)' with error containing an std::string for the message then at least the principle of 'you need a pointer' is dealt with by std::string insetad of having to do it manually.
There is a solid rationale behind it.
Might be but from a usage point of view it seems harder to deal with than alternatives (see 'ErrorOr' in another comment, or passing 'Error& error' or so: these days raw pointers are frowned upon, imo with reason, and raw pointer to pointer doesn't make that any better). Suppose I write an
int fun(struct Error **error) {
if(somethingWentWrong) {
StoreErrorCode(error);
}
}