Sending messages to objects?

I have done this successfully many times before, but this time it just won't work.

The code in my .h file:

@interface Imag开发者_StackOverflowes : NSView {

}

- (void) draw;

@end

The .m file (of another object):

- (IBAction) go: (id) sender; {
[Images draw:nil];

}

When I try to compile this, it says the following:

'*' may not respond to '*

Images may not respond to '+draw'

This has me quite confused. Shouldn't this be working?

Please Note: Yes, I have seen the other questions about messages, but none of those answers was even partially helpful for me. After reading them, I was even more confused.

Your draw method is an instance method: it can only be called on instances of the Images class. In your go: method you're trying to call it as a class method—if this is what you want, change it to:

@interface Images : NSView
+ (void)draw;
@end

I think a review of some of the basic concepts of object-oriented programming is in order; namely the difference between a class and an object or instance.

A class, in the general sense, is a collection of data and the functions which act upon it. A class defines the interface that one uses to access and manipulate data that is logically grouped together, and serves as a blueprint for creating objects or instances. See http://en.wikipedia.org/wiki/Class_(computer_programming)

Instances of a class (objects) are the typical things you manipulate in an object-oriented program, and they are created from the class "blueprint" and follow the behavior as specified by the class.

A typical example would be a fruit- take apples for example. An imaginary Apple class would represent all apples in the general sense and would model properties such as color and size and actions such as wash and eat. An instance would represent one, single physical apple- a Granny Smith or Pippin or whatever variety.

Just as it doesn't make sense to wash or eat apples in the general sense of the word (the concept of apples, not the agglomeration), typically it doesn't make sense to tell a class what to do. You tell objects (individual apples) what to do.

The code you present above defines the class Images. The "-" in front of -(void)draw indicates that the draw method only exists for specific objects. It is an instance method in typical OO parlance.

Of course, in Obj-C it is also possible to send a message to a class without requiring an object. This is denoted by a "+" in front of the method name, as other answers indicate. This is called a static method and it typically used to control some shared behavior or aspect of all objects of that particular class.

The problem with your code is that you are declaring -(void)draw as an instance method but calling it as a static method. Which way you want to do things is up to you, and it's difficult to determine from your code what the intent of the Images class is.