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133 lines
5.3 KiB
HTML
133 lines
5.3 KiB
HTML
4 years ago
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<html lang="en">
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<head>
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<title>Garbage Collection - Using the GNU Compiler Collection (GCC)</title>
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<meta http-equiv="Content-Type" content="text/html">
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<meta name="description" content="Using the GNU Compiler Collection (GCC)">
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<meta name="generator" content="makeinfo 4.13">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Objective_002dC.html#Objective_002dC" title="Objective-C">
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<link rel="prev" href="Type-encoding.html#Type-encoding" title="Type encoding">
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<link rel="next" href="Constant-string-objects.html#Constant-string-objects" title="Constant string objects">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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<!--
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Copyright (C) 1988-2015 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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Invariant Sections being ``Funding Free Software'', the Front-Cover
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Texts being (a) (see below), and with the Back-Cover Texts being (b)
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(see below). A copy of the license is included in the section entitled
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``GNU Free Documentation License''.
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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(b) The FSF's Back-Cover Text is:
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You have freedom to copy and modify this GNU Manual, like GNU
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software. Copies published by the Free Software Foundation raise
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funds for GNU development.-->
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<style type="text/css"><!--
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--></style>
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</head>
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<body>
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<div class="node">
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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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<!-- ========================================================================= -->
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</body></html>
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