Calling function with varying number of parameters in Objective C

Question

Is it possible to call a function that does not specify the exact number of parameters required?

E.g. I wish to call:

-(void)genFunction:(NSString *)someID {}

and then later call it again but with a different number of parameters

-(void)genFunction:(NSString *)someID AndMore:(NSString *)anotherParam {}

I want to do this without having to write multiple functions for each case...

Best Regards Luben

To stay consistent with the common Objective-C coding practices, it is a good idea to name your methods with lowerCamelCase (no initial capital). — dreamlax
– dreamlax, Commented Mar 25, 2011 at 11:24

CRD · Accepted Answer · 2011-03-25 10:38:54Z

3

Yes.

You will find them in the standard framework. For example NSArray has a method:

- (id)initWithObjects:(id)firstObj, ...

the ... indicates that the method takes a variable number of arguments.

To write your own variadic methods you need to use the standard variadic functions of C, see stdarg in the documentation. The outline goes as follows:

+ (void) msgWithFormat:(NSString *)format, ...
{
    va_list args;
    va_start(args, format);
    // use the va_arg() function to access the arguments - see docs for stdarg
    va_end(args);
}

This is directly analogous to the C equivalent:

void DebugLog_Msg(const char *format, ...)
{
    va_list args;
    va_start(args, format);
    // use the va_arg() function to access the arguments - see docs for stdarg
    va_end(args);
}

answered Mar 25, 2011 at 10:38

CRD

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Comments

pablasso · Accepted Answer · 2011-03-25 08:59:24Z

2

No, in Objective C parameters (or more correctly, messages) are part of the method name, Genfunction: is not related to GenFunction:AndMore: they're totally different methods.

But you could always put the common functionality in a single method and call it from the others. e.g.

- (void)genMethodByID:(NSString *)newID {
    // Your totally awesome special case.
    [self genMethod];
}

- (void)genMethodByDate:(NSDate *)newDate {
    // Your totally awesome special case.
    [self genMethod];
}

- (void)genMethod {
    // Your totally awesome common code.
}

Or just send the parameters inside NSDictionary.

- (void)genMethodWithParameters:(NSDictionary *)newDictionary {
    NSLog(@"I can haz ID: %@", [newDictionary objectForKey:@"newID"]);
    NSLog(@"I can haz date: %@", [newDictionary objectForKey:@"newDate"]);
}

edited Mar 25, 2011 at 8:59

answered Mar 25, 2011 at 8:42

pablasso

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2 Comments

CRD Over a year ago

Sorry, but you can have variadic functions in Objective-C - see next answer.

pablasso Over a year ago

Well, to be fair, I was stating that you can't just mess up with the message, like he tried to do in his example, or you will end with a new method. The variadic is still a single message that never changes. But yeah, you're right +1.

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