How do you store a function pointer in an ObjectiveC class?

Question

Some of Apple's obj-c API's still use C functions, e.g.:

-(NSArray * ) sortedArrayUsingFunction: (NSInteger (*)(id, id, void *))comparator context:(void *)context

...which is great, except I'm struggling to see how you can store fn-pointers inside ObjC classes.

e.g. to share the same "sort" function in different parts of your program. Say you have different data, in different contexts/classes, but you want the same sort in both places (for consistency).

I'm sure this is simple, but either my C is too rusty, or there's some gotcha. I tried sticking a plain variable inside the header file:

NSInteger (*)(id, id, void *) myComparator;

...and all I get is a compiler error:

Expected identifier or '(' before ')' token

Answer 1

Function pointers are a little weird, because the name goes inside the type definition.

If you want to pass around a function pointer for a method like this:

- (NSArray *)sortedArrayUsingFunction:(NSInteger (*)(id, id, void *))comparator context:(void *)context;

You would write a function like this:

NSInteger myComparisonFunction(id left, id right, void *context) {
    // do stuff...
}

And a typedef like this:

typedef NSInteger (ComparisonFunc *)(id, id, void *);

Then in your class you can declare an instance variable like this:

ComparisonFunc compFunc;

And a property like this:

@property (nonatomic) ComparisonFunc compFunc;

Then to set the property you can call:

myObject.compFunc = myComparisonFunction;

And within myObject you can use it like this:

sortedArray = [array sortedArrayUsingFunction:compFunc context:NULL];

Answer 2

Is it really necessary to store the pointer? Why not just include the .h with the function declaration, then pass in the reference to the function?

Answer 3

Instead of doing:

NSInteger (*)(id, id, void *) myComparator;

Use this instead:

NSInteger (* myComparator)(id, id, void *);

(This is just like block syntax, except blocks use ^ instead of *)

Answer 4

You can define the function pointer as a type (with typedef) and then use it in your class definitions. For example

In a common header:

typedef NSInteger (*COMPARATOR)(id, id, void *);

In the first class:

@interface MyClass : NSObject {
    NSObject *anotherField;
    COMPARATOR thecomparator;
}

- (COMPARATOR)comparator;

- (void)setComparator:(COMPARATOR) cmp;

@end

And in the second class:

@interface MyOtherClass : NSObject {
    NSObject *afield;
    COMPARATOR thecomparator;
}

- (COMPARATOR)comparator;

- (void)setComparator:(COMPARATOR) cmp;

@end

The type COMPARATOR is then used as any other type.

Edit: I add some methods to show how to pass and retrieve function pointer.