Variadic macro trick

Question

What's the trick to create a variadic macro FOO(a1, a2, a3,..., an) such that it expands to FOOn(a1, a2, a3,..., an) for values of n in whatever preselected bounded range you choose? That is, FOO(a) should expand to FOO1(a), FOO(a, b, c) to FOO3(a, b, c), etc. I know there's a standard trick but I can't seem to find it.

Please feel free to mark this question as a duplicate and close it if there's another question with the answer. I suspect there is but I couldn't find it.

this post has something that you might find usefull: http://stackoverflow.com/questions/3420459/c-preprocessor-macro-overloading — Mat, Mar 19 '11 at 23:00
see the answer here: http://stackoverflow.com/questions/2124339/c-preprocessor-va-args-number-of-arguments — Anycorn, Mar 19 '11 at 23:14
@aaa: The answers to that question do not answer this question. The question linked by @Mat is more similar, though I wouldn't consider it an exact duplicate. — James McNellis, Mar 20 '11 at 00:26

score 13 · Answer 1 · answered Mar 19 '11 at 23:54

13

#define VA_NARGS_IMPL(_1, _2, _3, _4, _5, N, ...) N
#define VA_NARGS(...) VA_NARGS_IMPL(__VA_ARGS__, 5, 4, 3, 2, 1)

#define FOO_IMPL2(count, ...) FOO ## count (__VA_ARGS__)
#define FOO_IMPL(count, ...) FOO_IMPL2(count, __VA_ARGS__) 
#define FOO(...) FOO_IMPL(VA_NARGS(__VA_ARGS__), __VA_ARGS__)

FOO(a)
FOO(a, b)
FOO(a, b, c)

The invocations are replaced by:

FOO1 (a)
FOO2 (a, b)
FOO3 (a, b, c)

answered Mar 19 '11 at 23:54

James McNellis

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Excellent answer, but how do you solve the zero-parameter case when calling `FOO()`, where it should expand to `FOO0 ()`, but your solution expand to `FOO1 ()`? I'd like to know how to do this in pure c99 (no GNU extension) – mchiasson Jun 05 '15 at 16:48
@mchiasson: To the best of my knowledge, there's no way to do that in standard C. You must pass at least one token to the `__VA_ARGS__`; you can't call such a macro with an empty argument list. – James McNellis Jun 05 '15 at 21:39

score 7 · Answer 2 · answered Aug 04 '11 at 04:18

Improving upon James answer to add some flexibility:

#define VA_NARGS_IMPL(_1, _2, _3, _4, _5, N, ...) N
#define VA_NARGS(...) VA_NARGS_IMPL(X,##__VA_ARGS__, 4, 3, 2, 1, 0)
#define VARARG_IMPL2(base, count, ...) base##count(__VA_ARGS__)
#define VARARG_IMPL(base, count, ...) VARARG_IMPL2(base, count, __VA_ARGS__) 
#define VARARG(base, ...) VARARG_IMPL(base, VA_NARGS(__VA_ARGS__), __VA_ARGS__)

#define MyMacro0() Also works without arguments.
#define MyMacro2(x,y) [x...y]
#define MyMacro(...) VARARG(MyMacro, __VA_ARGS__)

MyMacro()
MyMacro(a)
MyMacro(a, b)
MyMacro(a, b, c)

Output:

Also works without arguments.
MyMacro1(a)
[a...b]
MyMacro3(a, b, c)

Excellent answer, but `##__VA_ARGS__` only works with the GNU extension. Do you know how to solve this for c99? — mchiasson, Jun 05 '15 at 16:49

Mat · Accepted Answer · 2016-05-22T17:35:14.120

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This post Variadic macro to count number of arguments has what you're looking for I believe. Look at the first and second responses.

edited May 22 '16 at 17:35

answered Mar 19 '11 at 23:05

Mat

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Thanks, this link was just what I needed. It's much better than the potential-duplicates posted. – R.. GitHub STOP HELPING ICE Mar 20 '11 at 00:44

Variadic macro trick

3 Answers3

Linked