Classes and Instances

We come now to an extremely characteristic and profound feature of object-based programming. Just like in the real world, every object in the object-based programming world is of some type. This type, called a class, is the object-based analogy to the data type in C. Just as a simple variable in C might be an int or a float, an object in the object-based programming world might be a Dog (or an NSString). In the object-based programming world, the idea of this arrangement is to ensure that more than one individual object can be relied upon to act the same way.

There can, for example, be more than one dog. You might have a dog called Fido and I might have a dog called Rover. But both dogs know how to eat, come for a walk, and bark. In object-based programming, they know that because they both belong to the Dog class. The knowledge of how to eat, come for a walk, and bark is part of the Dog class. Your dog Fido and my dog Rover possess this knowledge solely by virtue of being Dog objects.

From the programmer’s point of view, what this means is simple: all the code you write is put into a class. All the methods you write will be part of some class or other. You don’t program an individual dog object: you program the Dog class.

But I just got through saying that an object-based program works through the sending of messages to individual objects. So even though the programmer does not write the code for an individual dog object, there still needs to be an individual ...

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