[学习笔记]stringByReplacingOccurrencesOfString返回autorelease的新对象

今天写代码的时候，用stringByReplacingOccurrencesOfString后出现崩溃，代码如下：

NSString
* s = [[NSString alloc] initWithFormat:@"1s"];

s
= [s stringByReplacingOccurrencesOfString:@"s"
withString:@"x"];

NSLog(@"%@",s);

[s
release];

我当时就奇怪了，明明是alloc之后再release，到底哪里出错了？

于是猜测stringByReplacingOccurrencesOfString也许返回的是另外一个指针，也就是说指针s的值变了，不再指向它原来alloc的内存单元了。

实验代码如下:

NSString
* s = [[NSString alloc] initWithFormat:@"1s"];

NSLog(@"%x",s);

s
= [s stringByReplacingOccurrencesOfString:@"s"
withString:@"x"];

NSLog(@"%x",s);

显示结果:

4e4b430

4e4bd40

两个地址真的不一样。然后前面的[s release];会出现崩溃，猜测返回的对象应该是autorelease的。下面三段实验代码:

第一段代码

NSString
*s = [[NSString alloc] initWithFormat:@"1s"];

NSString
*x = [s stringByReplacingOccurrencesOfString:@"s"
withString:@"x"];

NSLog(@"S:%@,%x,retainCount:%d",s,s,[s
retainCount]);

NSLog(@"X:%@,%x,retainCount:%d",x,x,[x
retainCount]);

控制台打印结果:

S:1s,4b42fc0,retainCount:1

X:1x,4b42970,retainCount:1

由结果可以看到，s和x的retainCount都是1。下面加个autorelease,再看看效果。

第二段代码

NSString
*s = [[NSString alloc] initWithFormat:@"1s"];

NSString
*x = [[s stringByReplacingOccurrencesOfString:@"s"
withString:@"x"] autorelease];

NSLog(@"S:%@,%x,retainCount:%d",s,s,[s
retainCount]);

NSLog(@"X:%@,%x,retainCount:%d",x,x,[x
retainCount]);

控制台打印结果会提示出错:

error
for object 0x4b5ab60: double free

结果显示释放了两次。

第三段代码

NSString
*s = [[NSString alloc] initWithFormat:@"1s"];

NSString
*x = [s stringByReplacingOccurrencesOfString:@"s"
withString:@"x"];

NSLog(@"S:%@,%x,retainCount:%d",s,s,[s
retainCount]);

NSLog(@"X:%@,%x,retainCount:%d",x,x,[x
retainCount]);

[x
release];

运行后直接就崩溃了,设断点DeBug提示:Program received signal: “EXC_BAD_ACCESS”，说明有对象提早释放了。

因此，由上面三段代码应该可以看出stringByReplacingOccurrencesOfString函数返回的新对象是autorelease的。