It is possible to use variable length argument lists (also known as varargs and stdarg) in General/ObjectiveCee.

Here is an example:

• (void)reportError:(General/NSString)msg explanation:(General/NSString)explanation, … { va_list ap; /* points to each unamed arg in turn / General/NSString explanation2;

  va_start(ap, explanation); /* make ap point to 1st unnamed arg */ explanation2 = General/[[NSString alloc] initWithFormat:explanation arguments:ap];

  General/NSRunAlertPanel(msg, explanation2, @”OK”, nil, nil);

  [explanation2 release]; va_end(ap); /* clean up when done */ }

You can call this function with printf-style arguments:

[self reportError:@”Uh oh” explanation:@”I had %d errors”, 47];

…and it will do the right thing.

However, according to Matt Watson at General/AppleComputer, it is not possible to make such a call via General/DistributedObjects. In other words, varargs functions must be called on the same thread as the caller.

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Woah! I wouldn’t go that far. I don’t see why you can’t call a vararg function, such as printf from a secondary thread safely. It’s possible to write thread-safe programs w/o using DO. If my var arg’ed General/ObjectiveCee method is inherently thread safe, why bother with DO? I’d change that last sentence to “In other words, avoid vararg methods when using DO for IPC” – General/MikeTrent

OTOH, “must be called on the same thread as the caller” is not “must be called from the primary thread”. I think this is a non-argument.

*Agreed. I think that just means that you can just call a vararg function like any other C function. inter-thread communication doesn’t support varargs. Printf itself is thread safe, so it’s fine to call multi-threded. *

See also: General/UsingVariableNumbersOfArguments, ‘stdarg’ man page

Is there any way to make all of the arguments variable? As in something like this:

• (General/NSString *)stringFromFormatArguments … { va_list args; va_start(args, _cmd); General/NSString *formattedString = General/[[NSString alloc] initWithFormat:self arguments:args]; va_end(args);

  return [formattedString autorelease]; }

I’ve looked around a bit but there seems to be no way to do this… –General/JediKnil

C and the varargs implementation should support it, since you already have two implicit arguments to the function that’s created, self and _cmd. However, the General/ObjC compiler doesn’t seem to support the syntax, which is unfortunate.

Well, it doesn’t support it because, if you want to make a method that has arguments, you need to have a colon. Also, va_ only supports variable arguments if the first argument has a fixed type (see manpage); however, it should be OK to NOT have one such argument in an General/ObjectiveCee method because the runtime already supplies self and _cmd. The method would work if you did something like this, I believe:*

• (General/NSString*) stringFromFormatArguments:… { // note the colon

Except that, as I stated above, the General/ObjC compiler doesn’t support this syntax, so you can’t create such methods.