CODEWITHSUNDEEP
objective-cmemory-management
Andrey Chernukha
Andrey Chernukha

Reputation: 21808

Why doesn't it crash?

Why this piece of code works:

NSNumber *a = [[NSNumber alloc] initWithInt:5];
[a release];

NSLog(@"%i", [a intValue]);// it shows 5. why???

The message intValue is sent to the deallocated object. its reference count must be 0. What's going on?

Upvotes: 2

Views: 88

Answers (3)

lukasz
lukasz

Reputation: 771

I think that NSNumber has a special implementation, just like NSString.

[NSNumber alloc] does not actually allocate memory, but rather returns a generic pointer. The allocation would be handled by one of the init methods, but here again initWithInt:5 is such a common situation, that instead of creating a new object a pointer to a default one is returned. And this one cannot be released anyway.

// do it once
NSNumber* five1 = [NSNumber alloc];
NSLog(@"%p", five1);
five1 = [five1 initWithInt:5];
NSLog(@"%p", five1);

// and once more
NSNumber* five2 = [NSNumber alloc];
NSLog(@"%p", five2);
five2 = [five2 initWithInt:5];
NSLog(@"%p", five2);

In fact, there should be no problem with sending a release message more than once:

[five1 release];
[five1 release];
[five1 release];

Upvotes: 2

benjymous
benjymous

Reputation: 2122

Releasing the pointer just tells the OS memory manager that the memory is no longer in use - the block of memory that the object sat in will not actually be changed.

However, since this is now "dead" memory, it's likely that a later allocation will re-use that block of memory.

If you experiment by putting additional allocations in between your release and the log value, you should be able to see this happening.

Upvotes: 2

Albert Mata
Albert Mata

Reputation: 127

I guess it won't work if you set 'a' to nil, but as long as it points to some place in the heap and there is a 5 integer there, you get that 5. But if you do something between the release and the NSLog you may get a different result (any kind of trash).

Upvotes: 0

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