Ivan Dossev Ivan Dossev - 4 days ago 3
Objective-C Question

Difference between NSMutableData's mutableBytes and bytes methods

Both return the same pointer. I know

- bytes
belongs to
NSData
, why does
NSMutableData
introduce
- mutableBytes
? Is it just for code clarity so it is more obvious you are accessing mutable data? Does it really matter which one is used?

NSMutableData* mydata = [[NSMutableData alloc] init];
[mydata appendData: [@"hello" dataUsingEncoding:NSUTF8StringEncoding]];
NSLog(@"%p", [mydata mutableBytes]);
NSLog(@"%p", [mydata bytes]);


Thanks.

Answer

There are a couple of reasons why NSMutableData might provide a separate mutableBytes method:

  • As you suggested in your question, using mutableBytes makes it clear to the reader that you want to change the data.

  • The bytes method returns a const void *. The mutableBytes method returns a void *. If you want to change the bytes, you need a void * with no const qualifier. The mutableBytes method eliminates the need to cast away the const qualifier.

In theory there could be a third reason: the -[NSData mutableCopy] method could return an NSMutableData that points to the same buffer as the original NSData, and only create a new, mutable copy of the buffer when you call mutableBytes. However, I don't think it's implemented this way based on my very limited testing.