Life of instance variables?

[Albert Chun-Chieh Huang]

Dear all,
    My question is about instance variable. Let me explain in details:
Node and MyNode is two classes, their relationship is 
MyNode : Node : NSObject, where MyNode is a subclass of Node.
In my code, there is only instances of MyNode, for Node is an abstract
class. 

@interface MyNode : Node
{
   some instance variables
}
- init;
@end

@interface Node : NSObject
{
   int state;   // Initial value as 14
}
- init;
- (void)receivePacket: (id)aPacket;
- (void)setNodeState: (int)aState;
@end

@implementation Node
- (void)setNodeState: (int)aState
{
   state = aState;
}
@end

I create the instance of MyNode in another class, say Simulator, by:
aNode = [[MyNode alloc] init];
[aNode setNodeState: 1];

In the class Channel:
[aNode receivePacket: aPacket];  // aNode is an instance of MyNode

So I assume that when I call [aNode receivePacket: aPacket], state is
not possible to be 14 because the initial value won't be set again!
I'm sure that I didn't set it to 14 in other places. And now, I add
some NSLog in -[Node receivePacket:], I found that whenever I call
[aNode receivePacket: aPacket], "state" value is 14 not 1. Why did
this append? 

I don't know why it happens. I studied this in "Object-Oriented
Programming and the Objective-C Language" manual from NeXT Developer's
Library, and I couldn't find the reason. The instance variable "state"
should be "inherited" from Node. And when I call 
[aNode receivePacket: aPacket], the run-time system should find the
superclass of MyNode because MyNode doesn't have implementation of
receivePacket: method. Sorry, I'm really confused. Could anyone be 
kind of telling me why? Thanks in advance. 

Best Regards,
Albert

-- 
Chun-Chieh Huang, aka Albert   | E-mail: jjhuang AT cm.nctu.edu.tw
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