NSString属性声明中的copy和retain区别

规范上NSString做属性都是写成copy的，理论上应该是复制了字符串而不是单纯的增加引用计数，其实问题只会出现在把NSMutableString赋值给NSString的时候。

首先做以下几个变量声明：

@property (retain, nonatomic) NSString *retainStr;

@property (copy, nonatomic)   NSString *copyStr;

 

@property (retain, nonatomic) NSMutableString *retainMStr;

@property (copy, nonatomic)   NSMutableString *copyMStr;

实现代码如下：

    NSMutableString *mStr = [NSMutableString string];

    [mStr setString:@"我没变"];

    

    self.retainStr   = mStr;

    self.copyStr     = mStr;

    self.retainMStr = mStr;

    self.copyMStr   = mStr;

    

    NSLog(@"retainStr:%@",  self.retainStr);

    NSLog(@"copyStr:%@",    self.copyStr);

    NSLog(@"retainMStr:%@", self.retainMStr);

    NSLog(@"copyMStr:%@",   self.copyMStr);

    NSLog(@" ");

    

    [mStr setString:@"我变了"];

    

    NSLog(@"retainStr:%@",  self.retainStr);

    NSLog(@"copyStr:%@",    self.copyStr);

    NSLog(@"retainMStr:%@", self.retainMStr);

    NSLog(@"copyMStr:%@",   self.copyMStr);

    NSLog(@" ");

 

    NSString *str = @"我来了";//[[NSString alloc] initWithFormat:@"我来了"];//两种方式都一样。

    self.retainStr  = str;

    self.copyStr    = str;

    self.retainMStr = [str mutableCopy];

    self.copyMStr   = [str mutableCopy];

 

    NSLog(@"retainStr:%@",  self.retainStr);

    NSLog(@"copyStr:%@",    self.copyStr);

    NSLog(@"retainMStr:%@", self.retainMStr);

    NSLog(@"copyMStr:%@",   self.copyMStr);

    NSLog(@" ");

    

    str =@"我走了";//[[NSStringalloc] initWithFormat:@"我走了"];//两种方式都一样

 

    NSLog(@"retainStr:%@",  self.retainStr);

    NSLog(@"copyStr:%@",    self.copyStr);

    NSLog(@"retainMStr:%@", self.retainMStr);

    NSLog(@"copyMStr:%@",   self.copyMStr);

    NSLog(@" ");

 

打印结果如下：

2012-10-12 13:53:14.858 StringDemo[1515:11303] retainStr:我没变

2012-10-12 13:53:14.860 StringDemo[1515:11303] copyStr:我没变

2012-10-12 13:53:14.861 StringDemo[1515:11303] retainMStr:我没变

2012-10-12 13:53:14.862 StringDemo[1515:11303] copyMStr:我没变

2012-10-12 13:53:14.863 StringDemo[1515:11303] 

2012-10-12 13:53:14.864 StringDemo[1515:11303] retainStr:我变了

2012-10-12 13:53:14.865 StringDemo[1515:11303] copyStr:我没变

2012-10-12 13:53:14.866 StringDemo[1515:11303] retainMStr:我变了

2012-10-12 13:53:14.867 StringDemo[1515:11303] copyMStr:我没变

2012-10-12 13:53:14.868 StringDemo[1515:11303] 

2012-10-12 13:53:14.869 StringDemo[1515:11303] retainStr:我来了

2012-10-12 13:53:14.869 StringDemo[1515:11303] copyStr:我来了

2012-10-12 13:53:14.870 StringDemo[1515:11303] retainMStr:我来了

2012-10-12 13:53:14.871 StringDemo[1515:11303] copyMStr:我来了

2012-10-12 13:53:14.872 StringDemo[1515:11303] 

2012-10-12 13:53:14.873 StringDemo[1515:11303] retainStr:我来了

2012-10-12 13:53:14.874 StringDemo[1515:11303] copyStr:我来了

2012-10-12 13:53:14.875 StringDemo[1515:11303] retainMStr:我来了

2012-10-12 13:53:14.876 StringDemo[1515:11303] copyMStr:我来了