Different behavior in visual c++ versus gcc/clang while stringifying parameter which contains comma

Question

I'm using stringizing operator to convert parameter which may contains comma passed to a macro into string. As I know, some characters cannot be stringified – notably, the comma(,) because it is used to delimit parameters and the right parenthesis()) because it marks the end of the parameter. So I use a variadic macro to pass commas to the stringizing operator like this:

#include <stdio.h>

#define TEST 10, 20

#define MAKE_STRING(...)  #__VA_ARGS__
#define STRING(x)       MAKE_STRING(x)

int main()
{
    printf("%s\n", STRING(TEST) );
    return 0;
}

it works fine. But it occurs to me what would happen without variadic macro, so I modify the macro: #define MAKE_STRING(x) #x. It compiles fine unexpectedly in visual c++ 2008/2010, and output 10, 20 while gcc/clang give the compilation error as expected:

macro "MAKE_STRING" passed 2 arguments, but takes just 1

So my question: is the Visual c++ doing additional work or the behavior is undefined?

Answer 1

VS in general allows extra parameters in macros and then just drops them silently: STRING(10, 20, 30) - still works and prints 10. This is not the case here, but it pretty much means VS don't even have the error gcc threw at you.

It's not any additional work but "merely" a difference in substitution order.

Answer 2

I am not sure if this will answer your question but i hope this will help you solving your problem. When defining a string constant in C, you should include it in double quotes (for spaces). Also, the # macro wrap the variable name inside double quotes so, for example, #a become "a".

#include <stdio.h>

#define TEST "hello, world"
#define MAKE_STRING(x) #x

int main()
{
    int a;
    printf("%s\n", TEST);
    printf("%s\n", MAKE_STRING(a));
    return 0;
}

I compiled this code using gcc 4.7.1 and the output is:

hello, world
a

Answer 3

I dunno why this has upvotes, or an answer got downvoted (so the poster deleted it) but I don't know what you expect!

#__VA_ARGS__ makes no sense, suppose I have MACRO(a,b,c) do you want "a,b,c" as the string?

http://gcc.gnu.org/onlinedocs/cpp/Variadic-Macros.html#Variadic-Macros

Read, that became standard behaviour, variable length arguments in macros allow what they do in variable length arguments to functions. The pre-processor operates on text!

The only special case involving # is ##, which deletes a comma before the ## if there are no extra arguments (thus preventing a syntax error)

NOTE:

It is really important you read the MACRO(a,b,c) part and what do you expect, a string "a,b,c"? or "a, b, c" if you want the string "a, b, c" WRITE THE STRING "a, b, c"

Using the # operator is great for stuff like

#define REGISTER_THING(THING) core_of_program.register_thing(THING); printf("%s registered\n",#THING);