# Talking about the "depth copy" of ObjC and the "depth copy" of objc

Source: Internet
Author: User

Talking about the "depth copy" of ObjC and the "depth copy" of objc

The ObjC should have no deep copy concept, but I found this problem in actual use, that is, when object A and object B point to the same content in the memory at the same time, if object B changes the value, object A also changes the value accordingly. This leads to a problem, which is consistent with the idea of copying in depth in C ++. The general idea is this. Let's take a look at the code.

There are five files: Point. h Point. m Rectangle. h Rectangle. m and main. m.

Rectangle contains the Point object. Main is the main function.

`// Point.h#import <foundation/Foundation.h>@interface XYPoint: NSObject@property int x, y;-(void) setX: (int) xVal andY: (int) yVal;@end`

`// Point.m#import "Point.h"@implementation XYPoint@synthesize x, y;-(void) setX: (int) xVal andY: (int) yVal{  x = xVal;  y = yVal;}@end`

`// Rectangle.h#import <Foundation/Foundation.h>@class XYPoint;@interface Rectangle: NSObject@property int width, height;-(XYPoint*) origin;-(void) setOrigin: (XYPoint*) pt;-(void) setWidth: (int) w andHeight: (int) h;-(void) setDeepOrigin: (XYPoint*) pt;@end`

`// Rectangle.m#import "Rectangle.h"#import "Point.h"@implementation Rectangle{  XYPoint* origin;}@synthesize width, height;-(void) setWidth: (int) w andHeight: (int) h{  width = w;  height = h;}-(void) setOrigin: (XYPoint*) pt{  origin = pt;}-(void) setDeepOrigin: (XYPoint*) pt{  if (!origin)    origin = [[XYPoint alloc] init];  [origin setX: [pt x] andY: [pt y]];}-(XYPoint*) origin{  return origin;}@end`

`// Main. m # import "Point. h "# import" Rectangle. h "int main (int argc, char * argv []) {@ autoreleasepool {Rectangle * rec = [[Rectangle alloc] init]; XYPoint * ptr = [[XYPoint alloc] init]; [ptr setX: 100 andY: 200]; [rec setWidth: 5 andHeight: 8]; [rec setDeepOrigin: ptr]; NSLog (@ "Rectangle w = % I, h = % I", [rec width], [rec height]); // rec. width is also OK. NSLog (@ "Origin at (% I, % I)", [[rec origin] x], [rec origin] y]); [ptr setX: 999 andY: 999]; NSLog (@ "Deep copy Origin at (% I, % I)", [[rec origin] x], [[rec origin] y]); [rec setOrigin: ptr]; [ptr setX: 999 andY: 999]; NSLog (@ "Shallow copy Origin at (% I, % I )", [[rec origin] x], [[rec origin] y]);} return 0 ;}`

We can see the differences between the two copies.

Although ObjC does not have a clear concept of deep copy, the C ++ idea of deep copy can be transplanted to ObjC.

At the same time, it is worth noting that the concept of a C ++ combination (composition) is also used here, that is, Point is a member of Rectangle, in addition, after the @ class XYPoint is declared in the Rectangle interface, the header file of the Point does not need to be imported. By default, the system treats the Point as a class. However, the member functions and member variables of Point cannot be used, because the system only knows that it is a class, but does not know what its member functions and variables are, if you want to reference its member functions and variables, You need to import the header file of Point.

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