Objective-C复制并保留

Question

When should I use copy instead of using retain? I didn't quite get it.

Answer 1

You would use copy when you want to guarantee the state of the object.

NSMutableString *mutString = [NSMutableString stringWithString:@"ABC"];
NSString *b = [mutString retain];
[mutString appendString:@"Test"];

At this point b was just messed up by the 3rd line there.

NSMutableString *mutString = [NSMutableString stringWithString:@"ABC"];
NSString *b = [mutString copy];
[mutString appendString:@"Test"];

In this case b is the original string, and isn't modified by the 3rd line.

This applies for all mutable types.

Answer 2

retain : It is done on the created object, and it just increase the reference count.

copy -- It creates a new object and when new object is created retain count will be 1. Hope this may help you.

Answer 3

Copy is useful when you do not want the value that you receive to get changed without you knowing. For example if you have a property that is an NSString and you rely on that string not changing once it is set then you need to use copy. Otherwise someone can pass you an NSMutableString and change the value which will in turn change the underlying value of your NSString . The same thing goes with an NSArray and NSMutableArray except copy on an array just copies all the pointer references to a new array but will prevent entries from being removed and added.