Comma inside macro

[Vincent Rivière]

Hello.

I experience a problem when calling a macro with an expression containing a
comma.

$ cat bug.c
#define M(x, y) x,y
#define A(x) x
#define B(x) A(x)

B(M(a,b))

$ gcc -E bug.c
....
bug.c:5:9: error: macro "A" passed 2 arguments, but takes just 1

GCC (all versions) fails with that error
HP-UX cc fails with similar error
Visual C++ (all versions) accepts that code and produces the obvious result:
a,b

Which compiler is right ?
How can I make GCC act as Visual C++ ?

 

[raof01]

Hello Vincent,

You need parentheses:
#define M(x, y) ((x), (y))

#define M(x, y) x,y

raof01
mailto:[email protected]
"Thoughts are but dreams till their effects be tried." -- William Shakespeare

 

[Vincent Rivière]

You need parentheses:
#define M(x, y) ((x), (y))

Of course, in the real world, parentheses would be mandatory, but I
removed them in order to keep the example as simple as possible.

I added them and... it works ! No more preprocessor error !

I my real world problem, the comma went from a template argument list,
such as:
#define M(x, y) f<x,y>() // fails

I've just tried to add additional parentheses around the macro
definition, and it works !
#define M(x, y) (f<x,y>()) // works

I always thunk that extra parentheses were here only for avoiding
priority problems in the expanded expression. I was wrong !

They you for your answer !