Question

I have a NSString and I want to write its value to a NSMutableString. Is this valid:

NSString *temp = [NSString stringWithString:@"test"];
NSMutableString *mutable = temp;

I ask because although this seems doable, I would think that this would assign both temp and mutable to the same address space. I have this question a lot when passing values into a method or returning values from a method. Sometimes I see other people do this or create the mutable string with stringWithString or initWithString. Thanks

Answer 1

You can use the mutableCopy method, which creates a mutable copy of the receiver and applies to any class which adopts the NSMutableCopying protocol (of which NSString is one of them):

NSString *temp = [NSString stringWithString:@"test"];
NSMutableString *mutable = [temp mutableCopy];

This will create a mutable copy of the string, as a new string instance. In this case it doesn’t apply, as temp is an autoreleased string, but you would otherwise need to release the old string that you have made a copy of, if you no longer need it.

Since mutableCopy contains “copy”, then you need to memory-manage the new string (you take ownership of it according to the Apple Object Ownership Policy).

The method that you have used simply assigns mutable as a pointer to the previously instantiated NSString.