iPhone - Creating variadic functions in objective c issue

Question

I referred this SO Answer for creating variadic function in objective C. I tested your code by passing arguments like below:

[self logMessage:@"string: %@\n number: %@\n image: %@",
 @"asdf",
 [NSNumber numberWithInt:23],
 [UIImage imageNamed:@"local.png"]];

and edited the code with NSLog();

- (void)logMessage:(NSString *)format, ... {

    va_list args;
    va_start(args, format);

    id arg = nil;
    int i = 1;
    NSLogv(format, args);
    while ((arg = va_arg(args,NSString *))) {

        NSLog(@"val: %d", i++);
        /// Do your thing with arg here
        //NSString *name = NSStringFromClass([arg class]);
        //NSLog(@"string: %@", name);
    }        
    va_end(args);
}

But the output as follows:

2012-09-28 19:34:45.271 SIMO[2384:c07] string: asdf
 number: 23
 image: <UIImage: 0x8151f80>
2012-09-28 19:34:45.273 SIMO[2384:c07] val: 1
2012-09-28 19:34:45.273 SIMO[2384:c07] val: 2
2012-09-28 19:34:45.274 SIMO[2384:c07] val: 3
2012-09-28 19:34:45.274 SIMO[2384:c07] val: 4
2012-09-28 19:34:45.274 SIMO[2384:c07] val: 5
2012-09-28 19:34:45.275 SIMO[2384:c07] val: 6
2012-09-28 19:34:45.275 SIMO[2384:c07] val: 7
2012-09-28 19:34:45.276 SIMO[2384:c07] val: 8

This tells that the argument is 8, but I passed only 3 (NSString, NSNumber, UIImage) I can't get the concept.. Could you clarify me Could anyone clarify this

Jeremy Roman · Accepted Answer · 2012-09-28 14:20:05Z

2

va_arg doesn't really know when the argument list ends. The approach you're using expects the argument list to be terminated by nil, like +[NSArray arrayWithObjects:].

So either change your invocation to add nil at the end of the argument list, or find some other way of knowing when the arguments are over (e.g. for a printf clone, you might know the number of format arguments from the format string itself).

answered Sep 28, 2012 at 14:20

Jeremy Roman

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