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(gcc-300.info)Garbage Collection

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Garbage Collection
==================

   Support for a new memory management policy has been added by using a
powerful conservative garbage collector, known as the
Boehm-Demers-Weiser conservative garbage collector.  It is available
from `http://www.hpl.hp.com/personal/Hans_Boehm/gc/'.

   To enable the support for it you have to configure the compiler
using an additional argument, `--enable-objc-gc'.  You need to have
garbage collector installed before building the compiler.  This will
build an additional runtime library which has several enhancements to
support the garbage collector.  The new library has a new name,
`libobjc_gc.a' to not conflict with the non-garbage-collected library.

   When the garbage collector is used, the objects are allocated using
the so-called typed memory allocation mechanism available in the
Boehm-Demers-Weiser collector.  This mode requires precise information
on where pointers are located inside objects.  This information is
computed once per class, immediately after the class has been
initialized.

   There is a new runtime function `class_ivar_set_gcinvisible()' which
can be used to declare a so-called "weak pointer" reference.  Such a
pointer is basically hidden for the garbage collector; this can be
useful in certain situations, especially when you want to keep track of
the allocated objects, yet allow them to be collected.  This kind of
pointers can only be members of objects, you cannot declare a global
pointer as a weak reference.  Every type which is a pointer type can be
declared a weak pointer, including `id', `Class' and `SEL'.

   Here is an example of how to use this feature.  Suppose you want to
implement a class whose instances hold a weak pointer reference; the
following class does this:


     @interface WeakPointer : Object
     {
         const void* weakPointer;
     }
     
     - initWithPointer:(const void*)p;
     - (const void*)weakPointer;
     @end
     
     
     @implementation WeakPointer
     
     + (void)initialize
     {
       class_ivar_set_gcinvisible (self, "weakPointer", YES);
     }
     
     - initWithPointer:(const void*)p
     {
       weakPointer = p;
       return self;
     }
     
     - (const void*)weakPointer
     {
       return weakPointer;
     }
     
     @end

   Weak pointers are supported through a new type character specifier
represented by the `!' character.  The `class_ivar_set_gcinvisible()'
function adds or removes this specifier to the string type description
of the instance variable named as argument.