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objective-ccopyretain

Objective-C copy and retain

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

Solution

  • 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.