Using comma as macro name in C or C++

Question

I'd like to do something like:

#define , 
#define MAX 10,000,000
// ...
#undef ,

is there any trick to do so?

EDIT: I know about the ' digit separator in C++14. I'm looking for a trick to do the same for uncompliant compilers.
EDIT2: Please consider Variadic Macros.

Answer 1

Warning, black magic ahead.

Macros can indeed be used, albeit with a preset number of arguments. This number can be arbitrary, but each must be written by hand:

#include <stdio.h>
#include <stdlib.h>

#define MERGE_EXPAND( a , b )     a##b
#define MERGE( a , b )            MERGE_EXPAND( a , b )

#define COUNT_PICK( a , b , c , pick , ... )  pick

#define COUNT( ... )    COUNT_PICK( __VA_ARGS__ , 3 , 2 , 1 , 0 )

#define JOIN_1( a )           a
#define JOIN_2( a , b )       a##b
#define JOIN_3( a , b , c )   a##b##c

#define JOIN( ... ) MERGE( JOIN_ , COUNT( __VA_ARGS__ ) )( __VA_ARGS__ )

int main( void )
{
    printf( "%d\n" , JOIN( 12345 ) ) ;
    printf( "%d\n" , JOIN( 100,44 ) ) ;
    printf( "%d\n" , JOIN( -10,44,9999 ) ) ;  

    return EXIT_SUCCESS ;
}

The macro COUNT count the number of arguments passed to it. This is done by passing arguments to the helper macro COUNT_PICK, and adding additional argument which are consecutive numbers in reverse order. The number of original arguments passed to COUNT then manipulates the arguments of COUNT_PICK, so that one of the numbers is chosen.

That chosen number is then merged wtih JOIN, resulting in either JOIN_1, JOIN_2, or JOIN_3. The chosen macro is then used with original arguments and simply merges them into a single integer literal.

This example can be expanded by manually defining more JOIN_X macros where X is a consecutive number. Simultaneously the macros COUNT and COUNT_PICK, must be altered as well.

As an additional benefit, passing invalid arguments, like:

JOIN( 10,+44 );
JOIN( 10,-44 );
JOIN( 10,*44 );
JOIN( 10,/44 );
JOIN( /10,44 );
//etc...

will yield a compile time warning, but still allows for arguments that will result in a valid integer constant.

To be used with a Microsoft compiler, tested with SVC14 (Microsoft Visual Studio 2015 Update 3), the code must be amended. Macros COUNT_PICK and MERGE must be wrapped with an additional expand macro:

#define EXPAND(...)   __VA_ARGS__