165 lines
6.8 KiB
HTML
165 lines
6.8 KiB
HTML
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- Copyright (C) 1988-2016 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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<!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Using the GNU Compiler Collection (GCC): Garbage Collection</title>
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<meta name="description" content="Using the GNU Compiler Collection (GCC): Garbage Collection">
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<meta name="keywords" content="Using the GNU Compiler Collection (GCC): Garbage Collection">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Objective_002dC.html#Objective_002dC" rel="up" title="Objective-C">
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<link href="Constant-string-objects.html#Constant-string-objects" rel="next" title="Constant string objects">
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<link href="Method-signatures.html#Method-signatures" rel="prev" title="Method signatures">
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</style>
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</head>
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<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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<a name="Garbage-Collection"></a>
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<div class="header">
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<p>
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Next: <a href="Constant-string-objects.html#Constant-string-objects" accesskey="n" rel="next">Constant string objects</a>, Previous: <a href="Type-encoding.html#Type-encoding" accesskey="p" rel="prev">Type encoding</a>, Up: <a href="Objective_002dC.html#Objective_002dC" accesskey="u" rel="up">Objective-C</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="Garbage-Collection-1"></a>
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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>
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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>
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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="nolinebreak">--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>libobjc_gc.a</samp> to not conflict with
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the non-garbage-collected library.
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</p>
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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>
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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 <em>weak pointer</em>
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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>
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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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</p>
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<div class="smallexample">
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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></div>
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<p>Weak pointers are supported through a new type character specifier
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represented by the ‘<samp>!</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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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Constant-string-objects.html#Constant-string-objects" accesskey="n" rel="next">Constant string objects</a>, Previous: <a href="Type-encoding.html#Type-encoding" accesskey="p" rel="prev">Type encoding</a>, Up: <a href="Objective_002dC.html#Objective_002dC" accesskey="u" rel="up">Objective-C</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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</body>
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</html>
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