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, char *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.