Question

I need few clarification in iPhone memory management.

Here is an example for setters;
1).

-(void)setValue:(NSString*)input{
[value autorelease];
value = [input retain];
}

In this example, why we have to use autorelease?
Can we use like as follows?

if(value)
[value release];
value = [input retain];

In the first example, Why we should not release the memory for input
2).
If I use following statement; what is the retain count for value

NSString *value;
value = @"welcome";

After the above statement, just I am trying to set one more value. Then what will happen?

eg:
value = @"For testing";

3).
What is the different between 2) and 3)?

NSString *value;
value = [input1 retain];
...
...
value = [input2 retain];// Now what is the retain count for value

4).
If I use following statement, why the app is getting crash?

NSString *value = [[[NSString alloc] init] autorelease];
...
...

Thanks in advance..

Answer 1

If “input” is the exact same object as “value” then calling [value release] could dealloc the object. So you must retain the new input value, release the old value, then assign the new value to the ivar:

[input retain];
[value release];
value = input;

After each of 2) and 3), the NSString *value points to a literal NSString object, the retain count will be 1 in each case, and releasing it is probably not a good idea

After this code:

value = [input2 retain];

value is an alias to the input2 object. The thing to realize is that objects have retain counts, variables do not.

As for your last case,

NSString *value = [[[NSString alloc] init] autorelease];

It creates an autoreleased empty string. If you reference that object again once the autorelease actually happens, you may get a crash because you’ll be referring to an object that doesn’t exist any more.