NSString，NSMutableString使用retain, copy理解

一直以为copy就是深拷贝，会产生一个新的对象，指针和内容都是新的，retain只是引用计数+1。

今天测试代码的时候发现一个问题，再经仔细一测，才发现不是这么回事.

 int main(int argc, charchar *argv[])
 {
     NSString *str = @"google";
     NSString *strRetain = [str retain];
     NSString *strCopy = [str copy];
     NSMutableString *strMutCopy = [str mutableCopy];

     NSLog(@"str        = %p,    retainCount = %d", str, [str retainCount]);
     NSLog(@"strRetain  = %p,    retainCount = %d", strRetain, [strRetain retainCount]);
     NSLog(@"strCopy    = %p,    retainCount = %d", strCopy, [strCopy retainCount]);
     NSLog(@"strMutCopy = %p,     retainCount = %d", strMutCopy, [strMutCopy retainCount]);

 }

output:

 

 观察打印信息可以发现，不管是retain还是copy,对于NSString来说都是浅拷贝，还是指向的之前的内存区域。retainCount=-1是因为str为字符串常量，系统会用UINT_MAX来标记，系统不收回，也不做引用计数。

NSString 应用 retain 还是 copy？

1。对NSString应用retain，效率无疑是最好的

2。用copy最安全，因为NSString 为 NSMutableString 的基类，如果将NSMutableString 以retain的形式赋值给NSString后，后续修改NSMutableString会导致NSString内容的变化，这通常不是我们希望的，所以用copy最安全。

参考文档 NSCopying Protocol Reference

Your options for implementing this protocol are as follows:

• Implement NSCopying using alloc and init... in classes that don’t inherit copyWithZone:.
• Implement NSCopying by invoking the superclass’s copyWithZone: when NSCopying behavior is inherited. If the superclass implementation might use the NSCopyObject function, make explicit assignments to pointer instance variables for retained objects.
• Implement NSCopying by retaining the original instead of creating a new copy when the class and its contents are immutable.