91 lines
4.5 KiB
HTML
91 lines
4.5 KiB
HTML
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<html lang="en">
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<head>
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<title>Virtual Base Classes - STABS</title>
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<meta name="description" content="STABS">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Cplusplus.html#Cplusplus" title="Cplusplus">
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<link rel="prev" href="Inheritance.html#Inheritance" title="Inheritance">
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<link rel="next" href="Static-Members.html#Static-Members" title="Static Members">
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<!--
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Copyright (C) 1992-2019 Free Software Foundation, Inc.
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Contributed by Cygnus Support. Written by Julia Menapace, Jim Kingdon,
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and David MacKenzie.
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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 no
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Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
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<div class="node">
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<a name="Virtual-Base-Classes"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Static-Members.html#Static-Members">Static Members</a>,
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Previous: <a rel="previous" accesskey="p" href="Inheritance.html#Inheritance">Inheritance</a>,
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Up: <a rel="up" accesskey="u" href="Cplusplus.html#Cplusplus">Cplusplus</a>
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<hr>
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</div>
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<h3 class="section">8.13 Virtual Base Classes</h3>
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<p>A derived class object consists of a concatenation in memory of the data
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areas defined by each base class, starting with the leftmost and ending
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with the rightmost in the list of base classes. The exception to this
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rule is for virtual inheritance. In the example above, class <code>D</code>
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inherits virtually from base class <code>B</code>. This means that an
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instance of a <code>D</code> object will not contain its own <code>B</code> part but
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merely a pointer to a <code>B</code> part, known as a virtual base pointer.
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<p>In a derived class stab, the base offset part of the derivation
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information, described above, shows how the base class parts are
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ordered. The base offset for a virtual base class is always given as 0.
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Notice that the base offset for <code>B</code> is given as 0 even though
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<code>B</code> is not the first base class. The first base class <code>A</code>
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starts at offset 0.
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<p>The field information part of the stab for class <code>D</code> describes the field
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which is the pointer to the virtual base class <code>B</code>. The vbase pointer
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name is ‘<samp><span class="samp">$vb</span></samp>’ followed by a type reference to the virtual base class.
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Since the type id for <code>B</code> in this example is 25, the vbase pointer name
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is ‘<samp><span class="samp">$vb25</span></samp>’.
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<!-- FIXME!! fake linebreaks below -->
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<pre class="smallexample"> .stabs "D:Tt31=s32!3,000,20;100,25;0264,28;$vb25:24,128;Ddat:1,
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160,32;A_virt::32=##1;:i;2A*-2147483647;20;;B_virt::32:i;
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2A*-2147483647;25;;C_virt::32:i;2A*-2147483647;28;;D_virt:
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:32:i;2A*-2147483646;31;;;~%20;",128,0,0,0
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</pre>
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<p>Following the name and a semicolon is a type reference describing the
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type of the virtual base class pointer, in this case 24. Type 24 was
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defined earlier as the type of the <code>B</code> class <code>this</code> pointer. The
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<code>this</code> pointer for a class is a pointer to the class type.
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<pre class="example"> .stabs "this:P24=*25=xsB:",64,0,0,8
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</pre>
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<p>Finally the field offset part of the vbase pointer field description
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shows that the vbase pointer is the first field in the <code>D</code> object,
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before any data fields defined by the class. The layout of a <code>D</code>
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class object is a follows, <code>Adat</code> at 0, the vtable pointer for
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<code>A</code> at 32, <code>Cdat</code> at 64, the vtable pointer for C at 96, the
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virtual base pointer for <code>B</code> at 128, and <code>Ddat</code> at 160.
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</body></html>
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