stringByReplacingOccurrencesOfString

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。