2.5.2 Macros With a Variable Number of Arguments

The C++ compiler accepts #define preprocessor directives of the following form.

#define identifier (...) replacement-list
#define identifier (identifier-list, ...) replacement-list

If the macro parameter list ends with an ellipsis, an invocation of the macro is allowed to have more arguments than there are macro parameters. The additional arguments are collected into a single string, including commas, that can be referenced by the name __VA_ARGS__ in the macro replacement list.

The following example demonstrates how to use a variable-argument-list macro.

#define debug(...) fprintf(stderr, __VA_ARGS__)
#define showlist(...) puts(#__VA_ARGS__)
#define report(test, ...) ((test)?puts(#test):\
                        printf(__VA_ARGS__))
debug(“Flag”);
debug(“X = %d\n”,x);
showlist(The first, second, and third items.);
report(x>y, “x is %d but y is %d”, x, y);

which results in the following:

fprintf(stderr, “Flag”);
fprintf(stderr, “X = %d\n”, x);
puts(“The first, second, and third items.”);
((x>y)?puts(“x>y”):printf(“x is %d but y is %d”, x, y));