Thread: gcc va_arg does not support non-pod types

Originally Posted by Elkvis

I have a function that uses the va_list and its associated functionality, and it works great on vc++ with non-pod types, but gcc gives me warnings about it. is there any way, short of hacking the standard library, to make it accept std::string and other aggregate types?

For portability's sake, don't try to use varargs with non-POD types. You'll have to alter your function to take a pointer to the object instead of the object itself. Working with pointers is no problem.

EDIT: And in C++ there are usually preferable alternative to varargs in the first place. More detail about what you're doing might lead to a better solution.

Why would you want to do that? Passing non-PODs to varargs is undefined, full stop. VC++ would be a better compiler if it disallowed it too. I've got some code to maintain that liberally uses CString::Format and passes CString arguments cast to const char* as format arguments. It's a real hassle to make sure that every CString is cast correctly, since VC++ neither rejects the CString, nor is it capable of parsing and validating the format string as GCC does.

Also, hacking the standard library wouldn't help. You'd have to hack the compiler. But if it's only a warning, I suppose you can just ignore it, at your own peril.

Originally Posted by CornedBee

Also, hacking the standard library wouldn't help. You'd have to hack the compiler. But if it's only a warning, I suppose you can just ignore it, at your own peril.

Yeah -- I would expect that va_arg does a straight memcpy() from the stack into the provided space, breaking object copy semantics.