为啥NSString的属性要用copy而不用retain

   之前学习生活中,知道NSString的属性要用copy而不用retain,但是不知道为啥,这两天我研究了一下,然后终于明白了.

具体原因是因为用copy比用retain安全,当是NSString的时候,其实用copy和retain都行,当用NSMutableString,那么就要用copy,NSMutableString的值不会被修改,而用retain的时候,NSMutableString的值会被修改,具体情况,可以看下面的代码:

#import <Foundation/Foundation.h>

//协议有两种方式,第一是以ing结尾形式,第二,以delegate结尾形式
@interface person : NSObject<NSCopying>
@property (nonatomic,copy)NSString * name;

    person * p  = [[person alloc]init];
    
    NSMutableString * name = [[NSMutableString alloc]initWithString:@"hello"];
    p.name = name;
    
    [name appendString:@" word"];
    NSLog(@"%@",p.name);

2014-07-05 17:08:44.170 DepthCopy[1399:303] hello
Program ended with exit code: 0

再看下面用retain

#import <Foundation/Foundation.h>

//协议有两种方式,第一是以ing结尾形式,第二,以delegate结尾形式
@interface person : NSObject<NSCopying>

@property (nonatomic,retain)NSString * name;

    person * p  = [[person alloc]init];
    
    NSMutableString * name = [[NSMutableString alloc]initWithString:@"hello"];
    p.name = name;
    
    [name appendString:@" word"];
    NSLog(@"%@",p.name);

2014-07-05 17:13:19.531 DepthCopy[1412:303] hello word
Program ended with exit code: 0

所以,由以上代码,可以看出copy比retain安全,也就能明白为啥NSString的属性要用copy而不用retain了;

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为啥NSString的属性要用copy而不用retain

原文：http://blog.csdn.net/zuoyou1314/article/details/37056647