Understanding OOP

[nexus]

Ok, I'm understanding the basic concepts in OOP. In Objective C I know how to create a class, write methods for that class and basic message passing.

I guess my confusion lies within the semantics. When I create a class, I do the following:

@interface Fraction : NSObject //NSObject is the parent class to our Fraction class
{
	// delcare instance variables for our class
	int numerator;
	int denominator;
	int fractionType;
}

//method headers

- (void) myPrintMethod;
- (void) mySetNumeratorMethod:		(int) n;
- (void) mySetDenominatorMethod:	(int) d;

@end

So, when a class is created, we assign it to NSObject. I think that's what's throwing me. Our class is a child of the parent NSObject, so what is the difference between classes and objects?

[smasher]

> So, when a class is created, we assign it to NSObject

You mean your Fraction class is extending or subclassing NSObject.

> Our class is a child of the parent NSObject,

True.

> So what is the difference between classes and objects?

A class is a template or plan for creating objects; kind of like a blueprint. You might make a single blueprint but then build six houses from it. The blueprint is your class, and the houses are your objects (also called instances).

In your case you've created your Fraction class; now you might alloc and init two or two or a hundred objects (instances) of that class and put different data in each one.

[nexus]

Originally Posted by smasher

> So, when a class is created, we assign it to NSObject

You mean your Fraction class is extending or subclassing NSObject.

> Our class is a child of the parent NSObject,

True.

> So what is the difference between classes and objects?

A class is a template or plan for creating objects; kind of like a blueprint. You might make a single blueprint but then build six houses from it. The blueprint is your class, and the houses are your objects (also called instances).

In your case you've created your Fraction class; now you might alloc and init two or two or a hundred objects (instances) of that class and put different data in each one.

Smasher,

Thank you for clearing that up. I went over to this very popular online video learning center (not sure If i'm allowed to mention the name here) and I now have a solid understanding of classes, objects, messages.

Thanks again.