Is the 1st argument of an Objective C variadic function mandatory?

Question

Here is an example of a variadic function in Obj C.

// This method takes an object and a variable number of args
- (voi         


        

Answer 1

It doesn't have to be anything really. There are two hidden arguments to every Objective-C method, self, and _cmd (in that order). self is self-explanatory (haha), but a lesser-known one is _cmd, which is simply the selector that was used to invoke the current method. This makes it possible to use variadic arguments with Objective-C methods seemingly without using an initial argument like you do with a standard variadic C function.

- (void) someMethod:...
{
    va_list va;

    va_start(va, _cmd);

    // process all args with va_arg

    va_end(va);
}

Then you can call the method like this:

 [someObj someMethod:1, 2, 3];

Answer 2

The Objective-C way to implement variadic args is the same as in standard C. So you can pass in non-Objective-C object arguments.
Personally I'd use the first non-hidden arg to pass in the length of the following variadic list (for non-Objective-C objects - otherwise I'd use nil-termination)

- (void)appendIntegers:(NSInteger)count, ...
{
    va_list arguments;
    //the start of our variadic arguments is after the mandatory first argument    
    va_start(arguments, count);
    for(NSUInteger i = 0; i < count; ++i)
    {
        //to add the non-object type variadic arg, wrap it in a NSNumber object
        [list addObject:[NSNumber numberWithInteger:va_arg(arguments, NSInteger)]];
    }
    va_end(arguments);
    NSLog(@"%@", list);
}

- (void)applicationDidFinishLaunching:(NSNotification *)aNotification
{
    list = [NSMutableArray array];
    [self appendIntegers:3 /* count */, 1, 2, 3];
}

Answer 3

No, it doesn’t have to be an object. You can write a variadic function taking floats, for example:

- (void) doSomethingWithFloats: (float) float1, ...;