/*
This might be language abuse. Variadic macros, designated
initializers, structs, and compound literals makes it possible to
have named and optional parameters. Tested in gcc 4.5 and clang 1.1.
*/
#include <stdio.h>
#define foo(...) foo_((struct foo){__VA_ARGS__})
struct foo { const char *name; int number; };
void foo_(struct foo foo)
{
printf("%s: %d\n", foo.name ?: "untitled", foo.number);
}
int main(int argc, char **argv)
{
foo("three", 3);
foo("hello");
foo(.name = "zero");
foo(.number = argc, .name = "argc",);
foo(.number = 42);
return 0;
}
/* Output
three: 3
hello: 0
zero: 0
argc: 1
untitled: 42
*/
on July 21st, 2010 at 08:10 AM
Absolutely brilliant trick !!!
on August 8th, 2010 at 02:25 AM
I’ve discovered it some time ago, and published on StackOverflow: http://stackoverflow.com/questions/2325514/adding-default-arguments-to-variadic-macro/2325737#2325737
Please note that accessing struct members like ”.number” is GCC extension, but the rest is C99.
on November 5th, 2010 at 12:53 PM
Could you provide command to compile this code with gcc 4.5?
This code was not compilable with gcc-4.5 unlill replace .name = “zero” to name: “zero”
But {number: 42} and {number: argc, name:”argc”} still cause “sorries”: “sorry, unimplemented: non-trivial designated initializers not supported”
on November 15th, 2010 at 11:45 AM
I used just simply “gcc” or “gcc -std=c99”. The only warning I get with “gcc -std=c99 -pedantic” is for abbreviating the ternary operator, which isn’t part of the trick.
$ gcc -std=c99 -pedantic what.c what.c: In function ‘foo_’: what.c:15:32: warning: ISO C forbids omitting the middle term of a ?: expression
$ gcc -v Using built-in specs. COLLECT_GCC=gcc COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-unknown-linux-gnu/4.5.1/lto-wrapper Target: x86_64-unknown-linux-gnu Configured with: ../configure—prefix=/usr—enable-languages=c,c+,fortran,objc,obj-c+,ada—enable-shared—enable-threads=posix—enable-__cxa_atexit—enable-clocale=gnu—enable-gnu-unique-object—enable-lto—enable-plugin—disable-multilib—disable-libstdcxx-pch—with-system-zlib—with-ppl—with-cloog—libdir=/usr/lib—libexecdir=/usr/lib—mandir=/usr/share/man—infodir=/usr/share/info Thread model: posix gcc version 4.5.1 (GCC)
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