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I am new to Objective-C and want to know about inheritance. I am aware of the concepts but confused with the methods that programmer uses to inherit a class.

I have two classes: class A and class B, and want to make B a child of A.

Sometimes a programmer uses #import "class A" and sometimes uses the @ sign. Which one of them should be used, and why? Is there any difference between their uses?

Another question I have is about the ":" sign which we write after class declaration, for example @interface class_A : class_name

In past I was a student of Java and C#, and their inheritance is similar to each other. But is Objective-C (I am currently working for iPhone) the same?

the Tin Man
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Ranjeet Sajwan
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2 Answers2

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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.

Community
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Jon Limjap
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  • great answer (and the link also) ...it means if we use only @class1 in header file....inheritance will work 100% (i mean access all methods and variables)?. or i need to do some more...(i mean declare #include in .m file) ....voted but waiting to accept.... – Ranjeet Sajwan Sep 27 '10 at 07:30
  • @ranjeet sajwan: if you want class B to inherit from class A, at the point where class B's interface is declared the compiler must already be aware of class A's interface. The main reason for this is it needs to know about class A's instance variables in order to know how big to make class B's instances. Thus to inherit, you must #import the superclass interface declaration. – JeremyP Sep 27 '10 at 07:49
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if we do this 

@interface Class_B : Class_A

mean we are inheriting the Class_A into Class_B, in Class_B we can access all the variables of class_A.

if we are doing this

#import ....
@class Class_A
@interface Class_B

here we saying that we are using the Class_A in our program, but if we want to use the Class_A variables in Class_B we have to #import Class_A in .m file(make a object and use it's function and variables).

Anshuman Mishra
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