Objective C繼承和語法

Question

我開始學習Objective-C的基礎知識，目前正在閱讀Kochan的（Progamming Objective C）。

我被困在繼承主題。

我有2個類和1個子類，XYPoint和矩形，正方形（正方形是矩形的子類）。

我有方法可以保留矩形對象的值X和Y座標。

，這裏是我的main.m文件

#import <Foundation/Foundation.h> 
#import "Rectangle.h" 
#import "Square.h" 
#import "XYPoint.h" 

int main(int argc, const char * argv[]) 
{ 

    @autoreleasepool { 

     Square *square1 = [[Square alloc]init]; 
     XYPoint *myPoint = [[XYPoint alloc]init]; 

     [myPoint setX:33 andY:33]; 

     [square1 setOrigin:myPoint]; 

     [myPoint setX:25 andY:25]; 
     NSLog(@"square1 origin is %i and %i",[[square1 origin]x],[[square1 origin]y]); 
     [myPoint setX:125 andY:125]; 

     NSLog(@"square1 origin is %i and %i",[[square1 origin]x],[[square1 origin]y]); 

     [myPoint setX:25 andY:25]; 
     NSLog(@"square1 origin is %i and %i",[[square1 origin]x],[[square1 origin]y]); 


    } 
    return 0; 
} 

和我Rectangle.m

-(void) setOrigin:(XYPoint *)pt{ 


     if (!origin) 
     origin = [[XYPoint alloc]init]; 

     origin.x = pt.x; 
     origin.y = pt.y; 

} 

-(XYPoint *)origin { 
    return origin; 
} 

和XYPoint.m文件從該計劃

- (void) setX:(int)xVal andY: (int) yVal{ 


    x = xVal; 
    y = yVal; 

} 

輸出是：

2014-01-23 15:25:36.368 Rectangle1[4356:303] square1 origin is 33 and 33 
2014-01-23 15:25:36.370 Rectangle1[4356:303] square1 origin is 33 and 33 
2014-01-23 15:25:36.370 Rectangle1[4356:303] square1 origin is 33 and 33 

NSLogged 3次。這種保留方法-(void) setOrigin:(XYPoint *)pt; 有助於保留矩形對象上的XY值，甚至通過setX和Y方法重新設置XY值。

，但是當我改變了保留方法爲這樣：

- （空）setOrigin：（的Xypoint *）PT {

 if (!origin) 
     origin = [[XYPoint alloc]init]; 

     origin = pt; 
} 

NSLogged輸出變成這樣：

2014-01-23 15:29:54.287 Rectangle1[4398:303] square1 origin is 25 and 25 
2014-01-23 15:29:54.288 Rectangle1[4398:303] square1 origin is 125 and 125 
2014-01-23 15:29:54.289 Rectangle1[4398:303] square1 origin is 25 and 25 

什麼這兩者之間的區別是什麼？這兩個具有相同的特徵指向x和y的相同座標嗎？

Answer 1

因爲你使用創建一個名爲myPoint原點如下：

[myPoint setX:33 andY:33]; 

然後你正在做的myPoint正方形1使用來源：

[square1 setOrigin:myPoint]; 

而且那麼你可以用下面的語句改變平方的座標：

[myPoint setX:25 andY:25]; 
[myPoint setX:125 andY:125]; 
[myPoint setX:25 andY:25]; 

當您更改值，然後嘗試使用打印原點

[[square1 origin]x]; 
[[square1 origin]y]; 

你會得到修改後的值，因爲當你說：

[origin x]; // it points to myPoint.x which u r changing all the time. 

和

[origin Y]; // it points to myPoint.y which u r changing all the time. 

最後你得到正確的輸出。請妥善分析代碼。

編輯：您已經定義了問題：

-(void) setOrigin:(XYPoint *)pt; 

，當你這樣做：

-(void) setOrigin:(XYPoint *)pt{ 
    if (!origin){ 
      origin = [[XYPoint alloc]init]; 

      origin = pt;// makes origin.X to point to pt.x and origin.Y to point to pt.y 
    } 
} 

你要做的僅僅是起源點PT（這是myPoint），即原點不過是指向pt的指針，所以當myPoint的X和Y值發生變化時。您的origin.x和origin.y更改。 這是一個淺拷貝。

但是，當我們做到這一點：

-(void) setOrigin:(XYPoint *)pt{ 
if (!origin){ 
origin = [[XYPoint alloc]init]; 
origin.x = pt.x;// copies value in pt.x to origin.x, so when pt.x changes origin.x doesn't change 
origin.y = pt.y;// copies value in pt.y to origin.y, so when pt.y changes origin.y doesn't change 
} 

    } 

我們acheiving一個深複製，這意味着每當mypoint.x和mypoint.y改變了我們的origin.x和origin.y沒有變化。