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Re: Garbage Collection

From: David Chisnall
Subject: Re: Garbage Collection
Date: Wed, 25 May 2011 13:04:37 +0100

On 25 May 2011, at 12:47, David Chisnall wrote:

> {lots of stuff, and then forgot to say half of what he actually meant say}

Apple's GC APIs are designed to be trivial to use, there are just a few things 
you need to do:

If you are allocating memory that contains pointers, use 
NSAllocateCollectable(size, NSScannedOption), not malloc().

All memory allocated by NSAllocateCollectable() will be GC'd automatically, so 
it does not require an explicit call to any function like free().  If you add 
NSCollectorDisabledOption, then the GC will be temporarily disabled for that 
allocation.  You can reenable it by calling NSMakeCollectable() on the pointer. 
 This is only intended for interoperability with CoreFoundation, so generally 
GNUstep code can ignore this.

Implement -finalize to do cleanup, not -dealloc (well, you can implement both, 
but -dealloc won't be called in non-GC mode).  This should free memory 
allocated with malloc() and close file descriptors, but little else.  For 
example, you don't need to unregister with notification centres here - they use 
__weak pointers.

Use __strong {type}* on pointers that will store GC'd references.  This is not 
actually required currently, but it may be in the future (it is on OS X, 
because they are a bit more aggressive about reducing the number of paths that 
the GC has to follow than I am).  

Use __weak id for zeroing weak references.  These will not prevent the 
referenced object from being finalized, and will become 0 when the object is 

You can skip -retain/-release/-autorelease messages, but the compile will 
ignore them if you compile in -fobjc-gc-only mode, so you can leave them in if 
you want to write code that works with GC or non-GC mode.

If you want to pass an object to some C code that is not GC-aware, you can use 
CFRetain() and CFRelease().  These increment and decrement a reference count 
for the object, so must be paired.  When an object's reference count is 
nonzero, it will not be eligible for collection.  Objects do not have a 
reference count associated with them by default.  

If you write code that only works in GC mode, then add this to prevent people 
compiling it in non-GC mode and wondering why it's broken:

#ifndef __OBJC_GC__
#error This code requires garbage collection


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