Variadic macro extensions refer to two extensions to C99 and Standard C++ related to macros with variable number of arguments. One extension is a mechanism for renaming the variable argument identifier from __VA_ARGS__ to a user-defined identifier. The other extension provides a way to remove the dangling comma in a variadic macro when no variable arguments are specified.
The following examples demonstrate the use of an identifier in place of __VA_ARGS__. The first definition of the macro debug exemplifies the usual usage of __VA_ARGS__. The second definition shows the use of the identifier args in place of __VA_ARGS__.
#define debug1(format, ...) printf(format, ## __VA_ARGS__)
#define debug2(format, args ...) printf(format, ## args)| Invocation | Result of macro expansion |
|---|---|
| debug1("Hello %s\n","World"); | printf("Hello %s\n","World"); |
| debug2("Hello %s\n","World"); | printf("Hello %s\n","World"); |
The preprocessor removes the trailing comma if the variable arguments to a function macro are omitted or empty and the comma followed by ## precedes the variable argument identifier in the function macro definition.