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133 lines
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<title>Garbage Collection - Using the GNU Compiler Collection (GCC)</title>
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<a name="Garbage-Collection"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Constant-string-objects.html#Constant-string-objects">Constant string objects</a>,
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Previous: <a rel="previous" accesskey="p" href="Type-encoding.html#Type-encoding">Type encoding</a>,
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Up: <a rel="up" accesskey="u" href="Objective_002dC.html#Objective_002dC">Objective-C</a>
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<hr>
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</div>
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<h3 class="section">8.4 Garbage Collection</h3>
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<p>This section is specific for the GNU Objective-C runtime. If you are
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using a different runtime, you can skip it.
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<p>Support for garbage collection with the GNU runtime has been added by
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using a powerful conservative garbage collector, known as the
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Boehm-Demers-Weiser conservative garbage collector.
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<p>To enable the support for it you have to configure the compiler using
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an additional argument, <samp><span class="option">--enable-objc-gc</span></samp><!-- /@w -->. This will
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build the boehm-gc library, and build an additional runtime library
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which has several enhancements to support the garbage collector. The
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new library has a new name, <samp><span class="file">libobjc_gc.a</span></samp> to not conflict with
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the non-garbage-collected library.
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<p>When the garbage collector is used, the objects are allocated using the
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so-called typed memory allocation mechanism available in the
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Boehm-Demers-Weiser collector. This mode requires precise information on
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where pointers are located inside objects. This information is computed
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once per class, immediately after the class has been initialized.
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<p>There is a new runtime function <code>class_ivar_set_gcinvisible()</code>
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which can be used to declare a so-called <dfn>weak pointer</dfn>
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reference. Such a pointer is basically hidden for the garbage collector;
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this can be useful in certain situations, especially when you want to
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keep track of the allocated objects, yet allow them to be
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collected. This kind of pointers can only be members of objects, you
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cannot declare a global pointer as a weak reference. Every type which is
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a pointer type can be declared a weak pointer, including <code>id</code>,
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<code>Class</code> and <code>SEL</code>.
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<p>Here is an example of how to use this feature. Suppose you want to
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implement a class whose instances hold a weak pointer reference; the
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following class does this:
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<pre class="smallexample">
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@interface WeakPointer : Object
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{
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const void* weakPointer;
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}
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- initWithPointer:(const void*)p;
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- (const void*)weakPointer;
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@end
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@implementation WeakPointer
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+ (void)initialize
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{
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if (self == objc_lookUpClass ("WeakPointer"))
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class_ivar_set_gcinvisible (self, "weakPointer", YES);
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}
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- initWithPointer:(const void*)p
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{
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weakPointer = p;
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return self;
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}
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- (const void*)weakPointer
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{
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return weakPointer;
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}
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@end
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</pre>
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<p>Weak pointers are supported through a new type character specifier
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represented by the ‘<samp><span class="samp">!</span></samp>’ character. The
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<code>class_ivar_set_gcinvisible()</code> function adds or removes this
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specifier to the string type description of the instance variable named
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as argument.
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