Invalid argument count in a variadic macro - c++

Question

I am trying to create a macro that would iterate over a defined list of terms, and for each invoke another macro, possibly with additional argument list. Here is what I have got:
#define ITERATE_OVER_TERMS(MACRO, ...) \
MACRO(Term1, __VA_ARGS__) \
MACRO(Term2, __VA_ARGS__) \
MACRO(Term3, __VA_ARGS__) \
... and so on
However, when I was trying to use it with Visual Studio 2015, i get an error
warning C4003: not enough actual parameters for macro 'BODY'
where BODY is the name of the macro passed as the MACRO argument. While technically a warning, it shows that something has gone wrong with the expansion.
In an attempt to narrow down the error, I reduced my example to the following:
#include <iostream>
#define ITERATE(MACRO, ...) \
MACRO(1, __VA_ARGS__) MACRO(2, __VA_ARGS__)
#define BODY(IterationArg, Arg1, Arg2) \
std::cout << IterationArg << Arg1 << Arg2 << std::endl;
int main() {
ITERATE(BODY, 8, 9)
return 0;
}
It gives me the error as shown above, while I expected it to compile succesfully and produce the output
189
289
It does seem to compile with g++, but not Visual Studio.
What am I missing? Is there some walkaround for this to work?

Answer 1

The problem is that Visual Studio expands __VA_ARGS__ after they are being passed into a subsequent macro, and not before. This has caused problems in the past as well, for example here --
Why does this variadic argument count macro fail with VC++?
In your case, consider a simple change in the code:
#include <iostream>
#define ITERATE(MACRO, ...) \
MACRO(1, __VA_ARGS__) MACRO(2, __VA_ARGS__)
#define BODY(IterationArg, Arg1, Arg2) \
std::cout << #Arg1 << std::endl;
int main() {
ITERATE(BODY, 8, 9)
return 0;
}
The argument #Arg1 is stringified, showing us its contents in the output:
8, 9
8, 9
Not what we expected, huh?
A solution is the same as in the linked question: force an expansion through a dummy EXPAND macro:
#define EXPAND(x) x
#define ITERATE(MACRO, ...) \
EXPAND(MACRO(1, __VA_ARGS__)) EXPAND(MACRO(2, __VA_ARGS__))
This gives you the desired result both in VS and gcc.