Variadic macros
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 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++.
#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.
