Variable arguments in a Macro

Question

I have a function which takes variable arguments, something like the following

int log_data (LOG_TYPE eType, ...)
{
    /** some logging related stuff here **/
}

In the header file, I use something like

#ifdef LOGGING_ENABLED
int log_data (int nType, ...);
#else
#define log_data(_x_, ...)
#endif

Basically, the idea is to SWITCH debugging ON & OFF ~~~

• Problem: The above logic works perfectly fine in Linux & gcc, but errors outs during compilation in Windows VC++.

Answer 1

Older versions of VC++ do not support variable arguments in macros.

You can use this trick to get around it:

#ifdef LOGGING_ENABLED
#define log_data log_data_impl
#else
#define log_data
#endif

// usage:
log_data(level, ...)

UPDATE - Another possible workaround:

#ifdef LOGGING_ENABLED
#define log_data(P) log_data_impl P // no braces around P!
#else
#define log_data(P)
#endif

// usage: we have to use two braces
log_data((level, ...));

Answer 2

Variadic macros are relatively new, for example: this reference says that "Support for variadic macros was introduced in Visual C++ 2005." You might have an older version.

Edit: you are declaring log_data if you want debugging, and not if you don't. This means that you have the definition of the function in an #ifdef as well. As long as you're doing this, a solution would be:

int log_data (int nType, ...)
{
#ifdef LOGGING_ENABLED
     /* the code to do logging */
#else
     return 0; /* or whatever */
#endif
}

and in your header file, declare the function as usual:

int log_data (int nType, ...);

This has a disadvantage that the function call exists even when you're not logging, but the advantage is that it will work without preprocessor support for varargs.