Implement a pure virtual method in Objective-C

Question

I want to go to there. Seriously though, how does one implement a pure virtual method in an "Apple" way? Do you use a Protocol with your base class and throw exceptions on those methods?

Answer 1

When you program in Objective-C you need to purge your mind of such things as virtual methods. You don't call methods on Objective-C objects, you send messages to them. Objects either respond to messages or they don't, but due to the dynamic binding, you can't tell this until run time.

Thus, you can declare a method on a base object and not not provide an implementation, no problem (except for the compiler warning), but you can't have the compiler flag up when you directly instantiate an object with such methods and it won't throw an error at runtime unless you actually send that message to the object.

The best way to create "virtual" base classes (in my opinion) is to declare the method and give it a stub implementation that throws a suitable exception.

Answer 2

In Objective-C, there is no pure virtual support as in C++.

A simulation would be that you declare a method in your interface but don't implement it in your .m file. Of course you'd get compiler warnings but IIRC you can turn those off. But you won't get warnings/errors if you don't overwrite them in the subclass, which you get in C++ (IIRC).

An alternative would be to implement them with just an NSAssert(NO, @"Subclasses need to overwrite this method"); body. Still, you'd only catch this at runtime, not compiletime.

Answer 3

Depending on what you're doing the delegate pattern may be more appropriate than a subclass, where the delegate is defined as id<YourDelegateProtocol>. The compiler will generate a warning if the required methods in the delegate protocol are not implemented.

Subclassing is generally avoided in Objective-C since objects cannot inherit from multiple superclasses but they can implement multiple protocols.

Answer 4

You should use the:

- (void)doesNotRecognizeSelector:(SEL)aSelector method. 

As noted by Apple, here: https://developer.apple.com/library/mac/#documentation/cocoa/reference/Foundation/Classes/NSObject_Class/Reference/Reference.html

Answer 5

You have a few options, but you're on the right track.

ObjC doesn't support this directly, forcing subclasses to implement a protocol is the best way to check it at compilation.

'Secretly' implementing the method in the base class and asserting is what I do to confirm the subclasser has subclassed correctly at runtime. Some people have mixed feelings about assertions, or must leave them active, so that's not always a good solution.

You can also force subclasses use a specific class constructor and initialization sequence, then verify they have implemented everything required before returning an instance, in case compiler warnings don't cut it.

But ObjC is missing some lang features which allow clients to shoot themselves in the foot, or workaround what they wish so... you shouldn't get too stuck on enforcing it.

note: Exceptions are very uncommon (and a bit unsafe, too) in ObjC.

Answer 6

A virtual method is a method whose behavior can be overridden within an inheriting class by a function with the same signature (i.e same name with same number of params and type of params).

Example:-

@implementation BaseClass

-(void)viewDidLoad
{
 [self virtualMethod:123];
}

-(void)virtualMethod:(int)param
{
 //implement this method in subclass
}

@end

////////////////////////////////////////////////////

@interface ChildClass:BaseClass
@end

@implementation ChildClass

-(void)virtualMethod:(int)param
{
 NSLog(@"There is no keyword "Virtual" in Objective C.");
}
@end

Output:-

"There is no keyword "Virtual" in Objective C."