Question

I’m trying to use va_arg to make a generic factory function in my GUI library.
When passing va_arg twice in the same function they pass on the same value instead of two different:

GUIObject* factory(enumGUIType type, GUIObject* parent, ...){
   va_list vl;
   va_start(vl, parent);
   ...
   label->SetPosition(va_arg(vl, int), va_arg(vl, int));
   va_end(vl);
   return finalObjectPointer;
}

factory(LABEL, theParent, 100,200); // Results in position 200:200

What causes this unexpected behavior?

Answer 1

The compiler is not guaranteed to evaluate arguments in order. Add some additional local variables and do the two assignments in sequence.

See this other stack overflow posting.

int v1 = va_arg(vl, int);
int v2 = va_arg(vl, int);

label->SetPosition(v1, v2);

To get what you are observing: the exact same value twice — probably requires a compiler bug piled on top of the undefined order of evaluation situation, or some entertaining aspect of the particular macro expansion of va_arg in your environment.