NSString property: copy or retain?

Debajit_2288585 0
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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?
objective-c cocoa cocoa-touch
10 Answers

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.)

user_7140

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).

user_3380

I try to follow this simple rule:

  • Do I want to hold on to the value of the object at the point in time when I am assigning it to my property? Use copy.

  • Do I want to hold on to the object and I don't care what its internal values currently are or will be in the future? Use strong (retain).

To illustrate: Do I want to hold on to the name "Lisa Miller" (copy) or to I want to hold on to the person Lisa Miller (strong)? Her name might later change to "Lisa Smith", but she will still be the same person.

user_1719330
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