GetTheSizeOfAnObject

Just want to know how to get the memory footprint of an object.

The code below prints “size of a NSObject: 4”

NSLog (@”size of a NSObject: %i”, sizeof (NSObject));

Even according to Foundation/NSObject.h it is 4 bytes:

@interface NSObject { Class isa; } …

but I know it can’t be (i.e. what about the retain count and other stuff which is private???)

Is there any way to determine exactly the footprint of any object? Thx

– peacha

Because all you are really checking with the function sizeof is the size of the pointer to the object. So 4 bytes is correct for that. The size of the object itself has its size checked using a different method. – DavidKopec

NSObject is not a pointer, it is the object itself. id is a pointer. – KritTer

KritTer is right. If you want a proof:

@interface MyObject : NSObject { id anID; } @end

void showMe () { NSLog (@”size of MyObject: %i”, sizeof (MyObject)); }

gives “size of MyObject: 8”

Back to the question: I haven’t found find any class methods or whatever giving the size of an object… Any help?

– peacha

Alas, even the objc runtime agrees with sizeof. The following:

NSLog(@"NSObject size: %d",
      ((struct objc_class *)[NSObject class])->instance_size);
NSLog(@"NSObject ivars: %d",
      ((struct objc_class *)[NSObject class])->ivars->ivar_count);

gives “NSObject size: 4” and “NSObject ivars: 1”. (“ivars” = instance variables”)

I assume the retain count is kept elsewhere, perhaps by the NSAutoreleasePool class.

–KritTer

So it seems like I can trust sizeof… Thx – peacha

I don’t know for sure how it’s implemented by Apple, but I can tell you how it’s implemented in GNUstep. Here is a small part of the NSObject code :

This file is part of the GNUstep Base Library.
Released under the GPL v2 */
(id)retain { NSIncrementExtraRefCount(self); return self; }

void NSIncrementExtraRefCount(id anObject) { if (allocationLock != 0) { objc_mutex_lock(allocationLock); ((obj)anObject)[-1].retained++; objc_mutex_unlock (allocationLock); } else { ((obj)anObject)[-1].retained++; } }

So the retain count is kept 4 bytes before the actual position of the pointer. Why is it done like this ?

Partly to support garbage collecting. It lets you add garbage collecting without changing the interface of any class, you would just need to change the alloc/retain/release/dealloc methods of NSObject.

GNUstep does support garbage collecting for the base library (the equivalent of the FoundationKit), but I never tried ‘cause it doesn’t work for the gui part.

–PierreYvesRivaille

CocoaDev