Question

In Objective-C I see

[object retain];

What does sending a retain message to an object mean and why would I use it?

Answer 1

Basically it is used to take ‘ownership’ on an object, i.e. by calling retain, the caller takes the responsibility of handling memory management of that object.

Two very common usages of the top of my hat are:

1- you initiate an object with auto memory managed methods, but want it to hang around some time : someObject = [[someArray objectAtIndex:someIndex] retain], without retain the object will be autoreleased at some time you don’t control.

2- you initiate an object by passing somePointer to it, you do your memory management and call release on somePointer, and now somePointer will hang around until the newly initiated object releases it, the object calls retain on somePointer and now owns it.

-(id) initWithSomePointer:(NSObject *)somePointer_{

if(self = [super init])

somePointer = [somePointer_ retain];

return self;

}

..

..

[somePointer release];