objective-c - NSArray *array = otherarray;是深拷贝还是浅拷贝?该怎么理解

Question

---

NSArray *array = [[NSArray alloc] initWithObjects:@23, @"re", nil] ;
NSArray *arr2 = array;
NSLog(@"%lu",(unsigned long)[array retainCount]);
NSLog(@"%lu",(unsigned long)[arr2 retainCount]);

结果是 1,1

NSArray *array = [[NSArray alloc] initWithObjects:@23, @"re", nil] ;
NSArray *arr2 = [array copy];
NSLog(@"%lu",(unsigned long)[array retainCount]);
NSLog(@"%lu",(unsigned long)[arr2 retainCount]);

结果是 2,2

NSArray *array = [[NSArray alloc] initWithObjects:@23, @"re", nil] ;
NSArray *arr2 = [array mutbaleCopy];
NSLog(@"%lu",(unsigned long)[array retainCount]);
NSLog(@"%lu",(unsigned long)[arr2 retainCount]);

结果是1,1

所以应该怎么理解???

Answer 1

NSArray *arr2 = array;

The above code is neither a deep copy nor a shallow copy. It is just a pointer assignment. They point to the same memory.

---

NSArray *arr2 = [array copy];

The above code is theoretically array 创建了一份浅拷贝，可实际上并没有发生任何拷贝，仅仅做了一次 retain.

This is because NSArray 是一个不可被修改的只读数组，它在实现 NSCopying protocol 的时候很机(tou)智(lan)的仅仅做了一次 retain 而没有创建任何新对象，所以造成 array 和 arr2 the reference counts all become 2, which is somewhat counterintuitive.

Note that at this time array 和 arr2 points to the same memory, which can be clearly seen by printing the pointer address.

---

NSArray *arr2 = [array mutableCopy];

The above code, mutableCopy 返回的是一个 NSMutableArray，这时候 NSArray 没什么巧可取，就是老老实实的创建了一个新的 NSMutableArray 对象，array 指向 NSArray，arr2 指向新创建的 NSMutableArray, so the reference count is all 1.

---

Back to deep copy and shallow copy, for a container, if the objects in the container only increase the reference count when copying, then it is a shallow copy; otherwise, if a new object is created for each object, then it is a deep copy.

The questioner should use NSMutableArray 来尝试才能得到清晰正确的结果，NSArray This kind of immutable container cannot see the desired effect.

By the way, NSMutableArray 的 copy is a shallow copy.

Answer 2

As an aside, breakpoints, LLDB debugging or printing and observing memory addresses can help us better understand the mechanism here.