Difference between [[NSDate date] retain] and [[NSDate alloc] init]

Question

As both of the following serves the same purpose,

today = [[NSDate date] retain];    

and

today = [[NSDate alloc] init]; 
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Answer 1

There is effectively no difference between the two. In the first example, you're just retaining an autoreleased instance created by the convenience method, which would have done something like return [[[NSDate alloc] init] autorelease].

Answer 2

Most of the time, when a class has an autoreleased initializer - it looks like this:

return [[[NSDate alloc] init] autorelease];

So when you call [[NSDate date] retain];, you are effectively calling

[[[[NSDate alloc] init] autorelease] retain];

Which, if you ask me, is fairly pointless - I'd just stick to [[NSDate alloc] init]; for initializing objects.

The convinience method is there so you can quickly get an autoreleased object - not to be used in conjunction with retain. It will do the same, but I would say it's better just to call the standard initialiser if you want a retained object.

Answer 3

When you do this:

[NSDate date];

…a new NSDate is created which will automatically be released (not deallocated!) at the end of the event loop. You can, of course, retain it to keep it around longer.

When you do this:

[[NSDate alloc] init];

…a new NSDate is created which you should release when you’re done with it.

From a memory management standpoint, the main differece between [[NSDate date] retain] and the alternative is that this NSDate will be around at least until the end of the event loop. If you’re just creating a few objects, it doesn’t matter. But, if you create (and release) a lot of objects — say, while processing data in a loop — using the former pattern could cause the memory usage of your application to spike (and then drop suddenly at the end of the event loop). With the latter pattern, the object gets destroyed as soon as you release it.

Answer 4

[NSDate date] is a convenience constructor using which you can leave the headache of releasing the object to autorelease pool. Sending a retain message to the convenience constructor like [[NSDate date] retain] makes you the owner of the object and you are responsible for releasing it properly.

[[NSDate alloc] init] is the default initializer by which you become the owner of the object, which is almost equal to [[NSDate date] retain].

Answer 5

There is essentially no difference, except that the former puts the object in the autorelease pool unnecessarily.

If I want to retain the object after creating it, and an -init style method is available for the class, I almost always choose that over the convenience constructor plus -retain.