Variadic macros

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More complex than object-like macros, a function-like macro definition declares the names of formal parameters within parentheses, separated by commas. An empty formal parameter list is legal: such a macro can be used to simulate a function that takes no arguments. C99 adds support for function-like macros with a variable number of arguments. XL C++ supports function-like macros with a variable number of arguments, as a language extension for compatibility with C and as part of C++11.

Variadic macro extensions

Variadic macro extensions refer to two extensions to C99 and C++03 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. Both extensions have been implemented to facilitate porting programs developed with GNU C and C++.

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 and the comma followed by ## precedes the variable argument identifier in the function macro definition.