自我与超级之间的区别

Question

just had a noob question. I'm trying to understand the difference between calling self and super. Now I understand inheritance and other fundamental OOP concepts, but the idea of self and super is still not clear to me. I'll illustrate my question with an example.

So the the below code performs a segue when the phone is tilted upside-down. I understand that "Scene2ViewController" is a subclass of "UIViewController" and so "Scene2ViewController" inherits all of UIViewController's methods. And so below I'm calling the method performSegueWithIdentifier with the receiver of the message being self. Now when I change "self" to "super" the code still executes the same way. Isn't calling super the same as calling self? If someone could explain this to me it would be appreciated, thanks.

//Scene2ViewController.m
- (BOOL)shouldAutorotateToInterfaceOrientation:  (UIInterfaceOrientation)interfaceOrientation
{
if (interfaceOrientation == UIInterfaceOrientationPortraitUpsideDown) {
    [self performSegueWithIdentifier:@"SegueToScene1" sender:self];
}

return (interfaceOrientation == 
        UIInterfaceOrientationPortrait);
}

Answer 1

self and super actually both point to the same object. super is a keyword that tells the compiler to generate instructions that start the search for a method definition in the super class rather than in the current class.

@interface A : NSObject {}
- (void)foo;
@end

@implementation A
- (void)foo {
    NSLog(@"A's foo!");
}
@end

@interface B : A
@end

@implementation B
- (void)foo {
    NSLog(@"B's foo!");
}
@end

//...somewhere in a method of class B...
[self foo];    // prints "B's foo" in the console
[super foo];    // prints "A's foo" in the console

If we assume, per the comment, that the last lines are somewhere in a method of B, then self points to some instance of B. super also points to that same instance of B. But when you use self to call foo , the search for an implementation of foo starts with class B. When you use super , the search for a foo starts with B's superclass, A.

super is especially handy when you want to preserve the inherited behavior, but add something on. So, we could have B's implementation of foo call A's version using [super foo] . Without super there'd be no way to call the inherited method, and calling foo from the overridden method would result in infinite recursion.

Answer 2

When you call a method of self (or rather send a message to self in Objective-C terms) the runtime will search for an implementation of that method in the inheritance hierarchy, starting with self , going up to NSObject . So if self implemented that method, it will be executed. If not, the super class will be checked and so on.

Sending the message to super is very similar, with the exception that the runtime will start looking for an implementation in super and skip self .

Answer 3

Well sometimes, in a subclass, you might override a function that was already defined in the parent class. Frequently this happens in the init function. So if you need to call the parent class's init function you call super. If you need the subclass' function you call self. If only the parent has the function declared then Self and super act the same. But if only the subclass has the declaration then you cannot call the function from super.