Question

I have three classes which implement the same protocol, and have the same parent class which doesn’t implement the protocol. Normally I would have the protocol as pure virtual functions in the parent class but I couldn’t find an Objective-C way to do that.

I want to utilize polymorphism on these subclasses by declaring pure virtual functions in the superclass then have the children implement those functions. However the only Objective-C way I’ve found to do this is to have each child explicitly decide to implement a protocol, when I do it this way the superclass doesn’t know the children will implement that protocol so there are compile time warnings all over the place.

Some pseudo-code if that didn’t make sense:

@interface superclass: NSObject
{}

@interface child1: superclass<MyProtocol>
{}

@interface child2: superclass<MyProtocol>
{}

The consumer of these classes:

@class child1
@class child2
@class superclass

@interface SomeViewController: UIViewController
{
    child1 *oneView;
    child2 *otherView;
    superclass *currentView;
}

-(void) someMethod
{
    [currentView protocolFunction];
}

The only nice way I’ve found to do pure virtual functions in Objective-C is a hack by putting [self doesNotRecognizeSelector:_cmd]; in the parent class, but it isn’t ideal since it will cause runtime errors rather than compile time.

Answer 1

I was able to get the compiler to warn me correctly by making the superclass *currentView property look like this:

@property (nonatomic, retain) superclass<MyProtocol> *currentView;