Are Variadic macros nonstandard?

Question

For debugbuilds, I usually use Clang, as it formats warnings and errors better, and makes it a little easier to track them down, and fix them.

But recently after adding a Macro with variadic arguments, Clang told me the following (from a dummy project):

main.cpp:5:20: warning: named variadic macros are a GNU extension [-Wvariadic-macros]
#define stuff3(args...)  stuff_i(args)

I know that macroname(args...) compiles fine in a wide range of compilers, including Visualstudio, Sunstudio, and of course GCC. But just to make sure that clang is right, I tried two other ways of expanding the variadic arguments:

Number 1:

#define stuff1(...)  stuff_i(...)

Number 2:

#define stuff2(...)  stuff_i(__VA_ARGS__)

On both I receive this message:

main.cpp:3:16: warning: variadic macros were introduced in C99 [-Wvariadic-macros]

... Which makes me wonder if Variadic macros are actually part of the standard of C++ (and of course I know that the Preprocessor is interpreted independently)?

Answer 1

Quote Wikipedia:

Variable-argument macros were introduced in 1999 in the ISO/IEC 9899:1999 (C99) revision of the C language standard, and in 2011 in ISO/IEC 14882:2011 (C++11) revision of the C++ language standard.

So it's standard from C99 and C++11 onwards, but a GNU extension in C++03.

Answer 2

As of C++11, variadic macros are now included in standard C++. Section 16.3 of the C++11 standard specifies variadic macros such that they are compatible with variadic macros from C99 (the second form in the question).

Here is an example of a standard-conforming variadic macro definition in C++:

#define foo(x, y, ...)    bar(x, y, __VA_ARGS__)

Answer 3

In the form of your example "Number 2", they are standard in C99, and generally a C++ compiler's preprocessor is the same for C and C++ compilation.

They are also supported Microsoft VC++ despite its otherwise stubborn resistance to C99 compliance. So between that and GCC there are few reasons to avoid using them. Even on most embedded systems compilers I use they are supported.

Avoid the "Number 1" form however, that is firmly GCC specific, and no doubt deprecated.

Answer 4

Standard says in 16.3 Macro replacement:

The identifier _ _ VA_ARGS _ _ shall occur only in the replacement-list of a function-like macro that uses the ellipsis notation in the parameters.