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POWhy can't I assign variable value directly?
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copied!<p>I am new Objective C. I am trying to understand and workout problems from Stephen Kochan's Programming in objective c 2.0. This problem is from chapter 8. </p> <p>What I don't understand is: why can't I assign variable values directly when I can get them. </p> <p>For example: </p> <p>This does not give me a error or warning. But it also does not set the value of X and Y. </p> <pre><code>go.origin.x = 300; go.origin.y = 100; </code></pre> <p>But, this works fine and gives me the intended result:</p> <pre><code>NSLog (@"print %i, %i", go.origin.x, go.origin.y); </code></pre> <p>Here is my XYPoint.h</p> <pre><code>#import <Foundation/Foundation.h> @interface XYPoint : NSObject { int _x; int _y; } @property int x; @property int y; @end </code></pre> <p>XYPoint.m</p> <pre><code>#import "XYPoint.h" @implementation XYPoint @synthesize x = _x; @synthesize y = _y; @end </code></pre> <p>GraphicObject.h</p> <pre><code>#import <Foundation/Foundation.h> #import "XYPoint.h" @interface GraphicObject : NSObject { XYPoint *_origin; } @property (nonatomic, retain) XYPoint *origin; -(void) setOrigin : (XYPoint *) pt; @end </code></pre> <p>GraphicObject.m</p> <pre><code>#import "GraphicObject.h" @implementation GraphicObject @synthesize origin = _origin; -(void) setOrigin : (XYPoint *) pt { _origin = [[XYPoint alloc] init]; _origin = pt; //_origin.x = pt.x; //_origin.y = pt.y; } @end </code></pre> <p>Also if don't want my origin.x and origin.y to be changed when I change my xypoint x and y, why do I have to do this:</p> <pre><code>_origin.x = pt.x; _origin.y = pt.y; </code></pre> <p>and why not just:</p> <pre><code>_origin = pt; </code></pre> <p>For example in the <strong>main</strong> file:</p> <pre><code>XYPoint *myPoint = [[XYPoint alloc] init]; go.origin = myPoint; myPoint.x = 250; myPoint.y = 50; NSLog(@"Origin values %i, %i", go.origin.x, go.origin.y); myPoint.x = 100; myPoint.y = 10; NSLog(@"Origin values %i, %i", go.origin.x, go.origin.y); </code></pre> <p>I understand that the values of origin don't change. But do I have to, in the setOrigin: method, assign it in this way:</p> <pre><code>_origin.x = pt.x </code></pre> <p>and not like this:</p> <pre><code>_origin = pt; </code></pre> <p>Thanks everyone. I know this is a long question. I really tried to understand and also looked in many places but I don't even know what to search for. What do you call this kind of assignment? </p>

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