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| <h3 class="section">8.4 Garbage Collection</h3> |
| |
| <p>This section is specific for the GNU Objective-C runtime. If you are |
| using a different runtime, you can skip it. |
| |
| <p>Support for garbage collection with the GNU runtime has been added by |
| using a powerful conservative garbage collector, known as the |
| Boehm-Demers-Weiser conservative garbage collector. |
| |
| <p>To enable the support for it you have to configure the compiler using |
| an additional argument, <samp><span class="option">--enable-objc-gc</span></samp><!-- /@w -->. This will |
| build the boehm-gc library, and build an additional runtime library |
| which has several enhancements to support the garbage collector. The |
| new library has a new name, <samp><span class="file">libobjc_gc.a</span></samp> to not conflict with |
| the non-garbage-collected library. |
| |
| <p>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. |
| |
| <p>There is a new runtime function <code>class_ivar_set_gcinvisible()</code> |
| which can be used to declare a so-called <dfn>weak pointer</dfn> |
| 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 <code>id</code>, |
| <code>Class</code> and <code>SEL</code>. |
| |
| <p>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: |
| |
| <pre class="smallexample"> |
| @interface WeakPointer : Object |
| { |
| const void* weakPointer; |
| } |
| |
| - initWithPointer:(const void*)p; |
| - (const void*)weakPointer; |
| @end |
| |
| |
| @implementation WeakPointer |
| |
| + (void)initialize |
| { |
| if (self == objc_lookUpClass ("WeakPointer")) |
| class_ivar_set_gcinvisible (self, "weakPointer", YES); |
| } |
| |
| - initWithPointer:(const void*)p |
| { |
| weakPointer = p; |
| return self; |
| } |
| |
| - (const void*)weakPointer |
| { |
| return weakPointer; |
| } |
| |
| @end |
| |
| </pre> |
| <p>Weak pointers are supported through a new type character specifier |
| represented by the ‘<samp><span class="samp">!</span></samp>’ character. The |
| <code>class_ivar_set_gcinvisible()</code> function adds or removes this |
| specifier to the string type description of the instance variable named |
| as argument. |
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