Question

The following code using UIGestureRecognizer:

UIGestureRecognizer *gestureRecog = [[UIGestureRecognizer alloc] 
                                         initWithTarget:self 
                                         action:@selector(handletap:)];

[self.view addGestureRecognizer:gestureRecog];

can actually compile and run. I thought abstract class cannot be instantiated?

Answer 1

Abstract classes are not a language feature in Objective-C (unlike Java, for example), so it isn’t something the compiler could enforce.

When a class is marked as abstract in the documentation, it is just a hint how it is intended to be used, but neither the runtime, nor the compiler will actually prevent you from instantiating it directly.

The section on abstract classes in the Objective-C Programming Language Guide actually states that NSView is an example of an abstract class that you may sometimes use without subclassing, so the concept as such is not as strict as in other languages/frameworks.