Call instance method from class method

by

There is not a way to do this. It simply doesn’t work with object-orientation.

Classes are kinds of things. That’s it. They simply describe what that kind of thing does.

An example might be that you have a ‘Dog’ class. You would have instance methods that would define how a dog wags its tail or how it eats. You might have a class method to purchase a dog.

My pet dog Fido is an instance of class dog. I can send Fido messages, telling him to wag his tail and eat his food. I cannot, however, ask the class ‘Dog’ to wag its tail; whose tail would wag? Would it be Fido’s, or would it be my neighbor’s dog?

When you send a message to the class, you don’t have a ‘self’ variable to work with. There’s nothing that could tell itself to wag its own tail. Class messages are mostly used for creating instances of the class or getting at other general information.

Edit: To clarify, that last paragraph is an oversimplification. There is a ‘self’ variable in class methods as bbum describes – it’s the computer’s reference to the description of the class. That said, I don’t think I’ve ever had an occasion to use ‘self’ in a class method.

More Related Contents:

  1. How does an underscore in front of a variable in a cocoa objective-c class work?
  2. @class vs. #import
  3. NSString property: copy or retain?
  4. Having trouble adding objects to NSMutableArray in Objective C
  5. How to print out the method name and line number and conditionally disable NSLog?
  6. How to Check if an NSDate occurs between two other NSDates
  7. How do I declare class-level properties in Objective-C?
  8. Deep copying an NSArray
  9. setValue:forUndefinedKey: this class is not key value coding-compliant for the key [duplicate]
  10. @synthesize vs @dynamic, what are the differences?
  11. How to find out who called a method?
  12. Managing multiple asynchronous NSURLConnection connections
  13. self.variable and variable difference [duplicate]
  14. Is there a way to suppress warnings in Xcode?
  15. Sorting two NSArrays together side by side
  16. Difference between self.var and simply var
  17. If “a == b” is false when comparing two NSString objects
  18. Avoiding “NSArray was mutated while being enumerated”
  19. With ARC, what’s better: alloc or autorelease initializers?
  20. 2D arrays using NSMutableArray
  21. Removing duplicates from NSMutableArray
  22. NSPrivateQueueConcurrencyType Not saving properly
  23. Properties and Instance Variables in Objective-C
  24. Looping using NSRange
  25. What’s the difference between [NSNull null] and nil?
  26. Class method equivalent of -respondsToSelector:
  27. +[NSString stringWithString:] — what’s the point?
  28. Why is my transformable Core Data attribute not using my custom NSValueTransformer?
  29. Objective C – How do I use initWithCoder method?
  30. How to get the width of an NSString?

Leave a Comment