MSVC doesn't expand __VA_ARGS__ correctly

Question

Consider this code:

#define F(x, ...) X = x and VA_ARGS = __VA_ARGS__
#define G(...) F(__VA_ARGS__)
F(1, 2, 3)
G(1, 2, 3)

The expected output is X = 1 and VA_ARGS = 2, 3 for both macros, and that's what I'm getting with GCC, however, MSVC expands this as:

X = 1 and VA_ARGS = 2, 3
X = 1, 2, 3 and VA_ARGS =

That is, __VA_ARGS__ is expanded as a single argument, instead of being broken down to multiple ones.

Any way around this?

Answer 1

Edit: This issue might be resolved by using /Zc:preprocessor or /experimental:preprocessor option in recent MSVC. For the details, please see here.

MSVC's preprocessor seems to behave quite differently from the standard specification.
Probably the following workaround will help:

#define EXPAND( x ) x
#define F(x, ...) X = x and VA_ARGS = __VA_ARGS__
#define G(...) EXPAND( F(__VA_ARGS__) )

Answer 2

I posted the following Microsoft support issue:

The following program gives compilation error because the precompiler expands __VA_ARGS__ incorrectly:

#include <stdio.h>

#define A2(a1, a2) ((a1)+(a2))

#define A_VA(...) A2(__VA_ARGS__)

int main(int argc, char *argv[])
{
    printf("%d\n", A_VA(1, 2));
    return 0;
}

The preprocessor expands the printf to: printf("%d\n", ((1, 2)+()));

instead of printf("%d\n", ((1)+(2)));

I received the following unsatisfying answer from a Microsoft compiler team developer:

Hi: The Visual C++ compiler is behaving correctly in this case. If you combine the rule that tokens that match the '...' at the inital macro invocation are combined to form a single entity (16.3/p12) with the rule that sub-macros are expanded before argument replacement (16.3.1/p1) then in this case the compiler believes that A2 is invoked with a single argument: hence the error message.

Answer 3

What version of MSVC are you using? You will need Visual C++ 2010.

__VA_ARGS__ was first introduced by C99. MSVC never attempted to support C99, so the support was not added.

Now, however, __VA_ARGS__ is included in the new C++ standard, C++2011 (previously known as C++0x), which Microsoft apparently plans to support, so it has been supported in recent versions of MSVC.

BTW, you will need to use a .cpp suffix to your source file to get this support. MSVC hasn't updated its C frontend for a long time.