Objective-c – NSString property: copy or retain

cocoacocoa-touchobjective c

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?

Best Answer

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.)

Related Solutions

Objective-c – NSString @property, using copy instead of retain

Yes, it's so that it can make a unique copy without affecting the existing data. The synthesized setters essentially look like this:

// For @synthesize(nonatomic, retain) foo:
- (void) setFoo(NSFoo *theFoo)
{
    [theFoo retain];  // retain new value
    [foo release];    // release old value, if any
    foo = theFoo;     // assign new value
}

// For @synthesize(nonatomic, copy) foo:
- (void) setFoo(NSFoo *theFoo)
{
    NSFoo* newFoo = [theFoo copy];  // make copy
    [foo release];  // release old value, if any
    foo = newFoo;   // assign new value
}

Note the order of operations here is important - the new value must be retained/copied before the old value is released, in case of self-assignment. If you released first and then assigned the property to itself, you might deallocate the value by accident. Also note that if the old value is nil, sending it a release message is ok, since sending a message to a nil object is explicitly allowed and does nothing.

The choice of retaining versus copying just determines whether or not the object's property shares the same value with what you're setting it to. Consider the following code:

// suppose the 'foo' property is declared 'retain' and the 'bar' property is
// declared 'copy'
NSFoo *foo = ...;
NSBar *bar = ...;
someObject.foo = foo;
someObject.bar = bar;
[foo changeInternalState];  // someObject.foo also changes, since it's the same object
[bar changeInternalState];  // someObject.bar does NOT change, since it's a copy

Ios – How to convert an NSString value to NSData

NSString* str = @"teststring";
NSData* data = [str dataUsingEncoding:NSUTF8StringEncoding];

Best Answer

Related Solutions

Objective-c – NSString @property, using copy instead of retain

Ios – How to convert an NSString value to NSData

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