How to do a pointer to a function in a class

Question

I've been trying to do an ios app with:

a viewcontrolloer with some functions/methods

a new class with a pointer to these functions/methods so that class1.point2f () would work

// testclass header
@property void (*point2f)();


// viewcontroller header        
#import "testclass.h"

@interface ViewController : UIViewController
{
    testclass *test;
}

@property testclass *test;

@end


// viewcontroller implementation
void downx() 
{
    NSLog(@"hello");
};

- (void)viewDidLoad
{
    [super viewDidLoad];

    test.point2f = downx;

    test.point2f ();     // crashes only at this line

I would be very thankfull for answers or at least keywords I could look into further.

did you create an instance of the testclass like `self.test = [[testclass alloc] init];` — Ahmed Mohammed, Jul 15 '13 at 19:49
@picciano not even close. This is asking about storing a c-function pointer in an objc class, that is asking about selectors. — Kevin, Jul 15 '13 at 19:54
you're welcome. I didn't see you comment. So I posted an answer. — Ahmed Mohammed, Jul 15 '13 at 20:01

score 0 · Accepted Answer · answered Jul 15 '13 at 20:00

The problem is that you are trying to call a NULL function pointer. In objective-c, instance variables are initialized to nil (NULL). So the variable test is nil. It is not an error to access a property of a nil pointer but that does not do anything. Therefore, test.point2f = downx; does nothing and test is still nil. Every property of a nil pointer also returns nil (0, NO etc..). So test.point2f is actually nil. So effectively what you're doing is

((void (*)())NULL) ();    // dereferencing a NULL pointer

You need to instantiate a testclass object and assign it to test before accessing any of it's properties.

- (void)viewDidLoad
{
    [super viewDidLoad];

    self.test = [[testclass alloc] init];
    test.point2f = downx;
    test.point2f();     // won't crash anymore

    // rest of your code
}

How to do a pointer to a function in a class

1 Answers1