Differences in return object pointer?

Question

Can someone explain the difference between these two, the first one is taken from allowing xcode to automatically generate the declaration, the last one is taken from an example in "Cocoa Programming" by Aaron Hillegass.

- (NSString*)planetName {
    return [[planetName retain] autorelease];
}

.

- (NSString*)planetName {
    return planetName;
}

I am just curious whats going on, my understanding was that the method is returning a pointer to either nil or an existing string object. I don't understand the reason for retaining and then adding to the autorelease pool?

Answer 1

In both cases, yes, they are both returning a pointer to either nil or the string object.

The difference is that the first code block handles memory management, the second does not. The second code block is assuming you are managing planetName somewhere else in your class instance, whereas in the first code block Apple is being as conservative as possible in keeping that memory from leaking. By putting the memory in the current autorelease pool it will be destroyed with the pool.

My recommendation would be to stick with the latter case and to be a little wiser about managing your own object instances than what XCode is auto-generating for you.

Answer 2

Consider:

NSString *planetName = [myPlanet planetName];
[myPlanet setPlanetName: @"Bob"];
[planetName length];

Without [[planetName retain] autorelease], the above will very likely crash.

retain/autorelease puts the object into the current thread's autorelease pool. That effectively guarantees that the object will remain valid until the pool is drained, which is typically after the current event -- user event, timer firing, etc... -- is done processing.

(1) Use @property and @synthesize. It generates correct getter/setters for you.

(2) Read the Cocoa Memory Management guide. It answers all of these questions quite well.

http://developer.apple.com/mac/library/documentation/Cocoa/Conceptual/MemoryMgmt/MemoryMgmt.html