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From: Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups: comp.lang.c
Subject: Re: is it possible to have functions with 0, 1, or 2 args?
Date: Sun, 11 Aug 2024 22:15:52 -0700
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Mark Summerfield <mark@qtrac.eu> writes:

> Given
>
> ```
> // vaargs.h
> #pragma once
>
> #define NARGS(...)  NARGS_(__VA_ARGS__, 5, 4, 3, 2, 1, 0)
> #define NARGS_(_5, _4, _3, _2, _1, N, ...)  N
>
> #define CONC(A, B) CONC_(A, B)
> #define CONC_(A, B) A##B
>
> // va_test.h
> #include "vaargs.h"
>
> #define va_test(...)  CONC(va_test, NARGS(__VA_ARGS__))(__VA_ARGS__)
> int va_test0();
> int va_test1(int);
> int va_test2(int, int);
>
> // va_test.c
>
> #include "va_test.h"
>
> int va_test0() { return va_test2(3, 11); }
> int va_test1(int a) { return va_test2(3, a); }
> int va_test2(int a, int b) { return a + b; }
>
> // va_tests.c
> #include "va_test.h"
> #include <stdbool.h>
> #include <stdio.h>
>
> int main() {
>     int i = va_test();
>     if (i != 14) {
>         fprintf(stderr, "FAIL: va_test() expecte 14 go %d\n", i);
>     }
>     i = va_test(5);
>     if (i != 8) {
>         fprintf(stderr, "FAIL: va_test() expecte 8 go %d\n", i);
>     }
>     i = va_test(2, 9);
>     if (i != 11) {
>         fprintf(stderr, "FAIL: va_test() expecte 11 go %d\n", i);
>     }
> }
> ```
>
> This does *not* work for the `va_test()` call.  But if I supply 1 or 2 args
> it works great.
>
> The rational is that I'd like to create a function `new_thing()` which
> takes an optional size, e.g.,
>
> ```
> thing new_thing0() { return new_thing1(10); }
> // above uses default size of 10
> thing new_thing1(int size) { ... }
> ```

The short answer is, it is possible, from C99 onwards, to define a
macro va_test(...) that does what you want.  It isn't easy, but
it is possible.

If it's really important to define a macro that behaves like what
you describe, and you can't find a suitable workaround, feel free
to ask again and I can try to find and work through materials I
have somewhere, to give an explanation of how to do what you want.
Please note that I am making no guarantees about whether I can
accomplish that, only that I can try to do so.