problem regarding inheritance concept in objective c

Question

hi all i am new in objective c and want to get know about inheritance ... I am aware of all concepts but confused withe the methods that programmer used to inherit a class....

Let i explain my problem.... suppose i have two classes class A and class B... and i want to make B child of A....

for that sometimes programmer used #import "class A" and sometimes used '@' sign... which one of them should be used.... and why ? is there any difference b/w their usage....

an another question is ":" sign which we write after class declaration. ex @interface class_A : class_name

...because in past i was a student of java and c# also....and there inheritance is also done in similar manner.... but in objective c (i am currently working for i phone) does it stands for same....?

thanks for your time and sorry for bad english..but please answer

Answer 1

There is a difference between those terms, and I can see where your confusion is.

The #import is used to load definitions of a class's h file. This is, in a way, similar to C#'s using keyword, but in Objective-C we need to specify everything in the class level, not in the namespace level -- there's no concept of namespace level encapsulation in Objective-C.

The @class keyword is used whenever you need to declare that an object is valid -- but if you're going to use the internals of that object you will eventually need to add an #import of the class anyway. There's a great answer here on the difference between @class and #import.

As with C# and Java, inheritance is achieved by using the : operator in your h file. So in your declaration of Class B, it should go like:

@interface Class_B : Class_A

Hope this clears everything up.

update for your comment:

Let's say I want to inherit class A into class B, and use class C as a variable somewhere. You'll need the ff to make it work:

#import "Class_A.h"

@class Class_C;

@interface Class_B : Class_A {
    Class_C *myvariable
}

Now, lets say somewhere inside your file you need to access a Class_C member e.g., myvariable.Property1, that's the time you turn @class Class_C into #import "Class_C.h".

I don't think declaring it like this:

@class Class_A;

@interface Class_B : Class_A

would work... you'll still need an #import "Class_A.h" somewhere which makes the @class declaration somewhat redundant.