NSString property: copy or retain?

Question

Let's say I have a class called SomeClass with a string property name:

@interface SomeClass : NSObject
{
    NSString* name;
}

@property (nonatomic, retain) NSString* name;

@end

I understand that name may be assigned a NSMutableString in which case this may lead to errant behavior.

• For strings in general, is it always a good idea to use the "copy" attribute instead of "retain"?
• Is a "copied" property in any way less efficient than such a "retain-ed" property?

Answer 1

I would argue that you should never use copy unless you really mean to make a copy of the object.

The reason why is it might break developer expectations. Other developers are probably expecting to be working with the same pointer when assigning objects around in code, and would expect a change to a value to reflect to all other classes pointing at that value. This is pretty subjective though depending on the developers background. (background in high level language vs low level language)

/edited out memory part since its superficial

Answer 2

Copy should be used for NSString. If it's Mutable, then it gets copied. If it's not, then it just gets retained. Exactly the semantics that you want in an app (let the type do what's best).

Answer 3

For attributes whose type is an immutable value class that conforms to the NSCopying protocol, you almost always should specify copy in your @property declaration. Specifying retain is something you almost never want in such a situation.

Here's why you want to do that:

NSMutableString *someName = [NSMutableString stringWithString:@"Chris"];

Person *p = [[[Person alloc] init] autorelease];
p.name = someName;

[someName setString:@"Debajit"];

The current value of the Person.name property will be different depending on whether the property is declared retain or copy — it will be @"Debajit" if the property is marked retain, but @"Chris" if the property is marked copy.

Since in almost all cases you want to prevent mutating an object's attributes behind its back, you should mark the properties representing them copy. (And if you write the setter yourself instead of using @synthesize you should remember to actually use copy instead of retain in it.)