For Objective-C ... Pointer to method

Question

I want to setup a Method dispatch table and I am wondering if it is possible to create pointer to a method in Objective-C (like pointer to function in C). I tried to use some obj-c runtime functions to dynamically switch methods but the problem is it will affect all instances.

As I am very new to obj-c, an illustrated example would be highly appreciated. thanks fellows.

Answer 1

Objective-C methods are called selectors, and are represented by the SEL datatype. If your object inherits from NSObject, you can tell it to perform a selector (i.e. call a method) like thus:

SEL selector = @selector(doSomething:);
[obj performSelector:selector withObject:argument];

This assumes you have a method defined such as:

-(void)doSomething:(MyObject*)arg;

Selectors are assigned to SEL datatypes through the @selector keyword, which takes the name of the method you would like to keep. The name of the method is the method name stripped of all arguments. For example:

-(void)doSomething:(MyObject*)arg withParams:(MyParams*)params

Would be referenced as @selector(doSomething:withParams:).

Answer 2

Yes! In Objective-C, function pointers are called selectors. If you have a method defined like this:

- (void)myFunctionWithObject:(NSObject*)obj otherObject:(NSNumber*)obj2
{
}

The selector is declared like this:

@selector(myFunctionWithObject:otherObject:)

To perform a selector on an object, you can use:

[object performSelector:@selector(myFunction)];

or

[object performSelector:@selector(myFunctionTakingParameter:) withObject: o];

The selector data type is particularly useful for threads and timers, where you can dispatch a thread and provide it a selector to the message you'd like it to invoke. If you need to create an array of selectors (or a dispatch table), or if you need to invoke selectors with multiple parameters, you can use the NSInvocation class. It provides a wrapper for a selector and allows you to specify actual arguments.

You should keep in mind that Objective-C is already based on a fully dynamic method dispatch table. It sounds like maintaining function pointers using selectors will work fine for you if you just need a reference to a function, though.