[ACCEPTED]-Appending to __VA_ARGS__-variadic-macros
Yes, you can. The following supports up 12 to 4 arguments, but it can be trivially 11 expanded to support more:
#define MACRO(api, ...) \
bool ret = api(__VA_ARGS__ VA_COMMA(__VA_ARGS__) 456)
/*
* VA_COMMA() expands to nothing if given no arguments and a comma if
* given 1 to 4 arguments. Bad things happen if given more than 4
* arguments. Don't do it.
*/
#define VA_COMMA(...) GET_6TH_ARG(,##__VA_ARGS__,COMMA,COMMA,COMMA,COMMA,)
#define GET_6TH_ARG(a1,a2,a3,a4,a5,a6,...) a6
#define COMMA ,
/* EXAMPLES */
MACRO(foo) /* bool ret = foo( 456) */
MACRO(foo,1) /* bool ret = foo(1 , 456) */
MACRO(foo,1,2,3,4) /* bool ret = foo(1,2,3,4 , 456) */
/* uh oh, too many arguments: */
MACRO(foo,1,2,3,4,5) /* bool ret = foo(1,2,3,4,5 5 456) */
MACRO(foo,1,2,3,4,5,6) /* bool ret = foo(1,2,3,4,5,6 5 456) */
This same trick 10 is used to:
Explanation
VA_COMMA surrounds its arguments (__VA_ARGS__) with 9 six additional arguments: one empty argument 8 before (doesn't have to be empty—it's 7 thrown away) and four commas and an empty 6 argument after.
These six or more arguments 5 are passed to GET_6TH_ARG, which, as its name implies, expands 4 to the sixth argument. All other arguments 3 are discarded.
Thus, MACRO(foo) is expanded like this:
step 0: MACRO(foo)
step 1: bool ret = foo( VA_COMMA() 456)
step 2: bool ret = foo( GET_6TH_ARG(,COMMA,COMMA,COMMA,COMMA,) 456)
step 3: bool ret = foo( 456)
MACRO(foo,1) is 2 expanded like this:
step 0: MACRO(foo,1)
step 1: bool ret = foo(1 VA_COMMA(1) 456)
step 2: bool ret = foo(1 GET_6TH_ARG(,1,COMMA,COMMA,COMMA,COMMA,) 456)
step 3: bool ret = foo(1 COMMA 456)
step 4: bool ret = foo(1 , 456)
MACRO(foo,1,2) is expanded like this:
step 0: MACRO(foo,1,2)
step 1: bool ret = foo(1,2 VA_COMMA(1,2) 456)
step 2: bool ret = foo(1,2 GET_6TH_ARG(,1,2,COMMA,COMMA,COMMA,COMMA,) 456)
step 3: bool ret = foo(1,2 COMMA 456)
step 4: bool ret = foo(1,2 , 456)
MACRO(foo,1,2,3,4,5) is 1 expanded like this:
step 0: MACRO(foo,1,2,3,4,5)
step 1: bool ret = foo(1,2,3,4,5 VA_COMMA(1,2,3,4,5) 456)
step 2: bool ret = foo(1,2,3,4,5 GET_6TH_ARG(,1,2,3,4,5,COMMA,COMMA,COMMA,COMMA,) 456)
step 3: bool ret = foo(1,2,3,4,5 5 456)
No. The behaviour of ## which allows this 6 to work in the first case is a GCC extension 5 (C99 does not allow the variable argument 4 part to be empty), and it specifically applies 3 to the case with a comma on the left and 2 __VA_ARGS__ on the right. See e.g. http://gcc.gnu.org/onlinedocs/gcc-4.5.1/gcc/Variadic-Macros.html#Variadic-Macros (at the bottom 1 of the page).
Well, I think it is possible with something 2 like this:
#define NO_FIRST(first, ...) __VA_ARGS__
#define DO_APPEND_LAST(last, ...) NO_FIRST(__VA_ARGS__, last)
#define MACRO(api, ...) bool ret = api(DO_APPEND_LAST(456, dummy, ##__VA_ARGS__));
Haven't check it, however should 1 work in latest VS and gcc.
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