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  1. <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">

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  16. <title>Virtual Base Classes (STABS)</title>

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  61. <a name="Virtual-Base-Classes"></a>

  62. <div class="header">

  63. <p>

  64. Next: <a href="Static-Members.html#Static-Members" accesskey="n" rel="next">Static Members</a>, Previous: <a href="Inheritance.html#Inheritance" accesskey="p" rel="prev">Inheritance</a>, Up: <a href="Cplusplus.html#Cplusplus" accesskey="u" rel="up">Cplusplus</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Types-Index.html#Symbol-Types-Index" title="Index" rel="index">Index</a>]</p>

  65. </div>

  66. <hr>

  67. <a name="Virtual-Base-Classes-1"></a>

  68. <h3 class="section">8.13 Virtual Base Classes</h3>

  69. 

  70. <p>A derived class object consists of a concatenation in memory of the data

  71. areas defined by each base class, starting with the leftmost and ending

  72. with the rightmost in the list of base classes.  The exception to this

  73. rule is for virtual inheritance.  In the example above, class <code>D</code>

  74. inherits virtually from base class <code>B</code>.  This means that an

  75. instance of a <code>D</code> object will not contain its own <code>B</code> part but

  76. merely a pointer to a <code>B</code> part, known as a virtual base pointer.

  77. </p>

  78. <p>In a derived class stab, the base offset part of the derivation

  79. information, described above, shows how the base class parts are

  80. ordered.  The base offset for a virtual base class is always given as 0.

  81. Notice that the base offset for <code>B</code> is given as 0 even though

  82. <code>B</code> is not the first base class.  The first base class <code>A</code>

  83. starts at offset 0.

  84. </p>

  85. <p>The field information part of the stab for class <code>D</code> describes the field

  86. which is the pointer to the virtual base class <code>B</code>. The vbase pointer

  87. name is &lsquo;<samp>$vb</samp>&rsquo; followed by a type reference to the virtual base class.

  88. Since the type id for <code>B</code> in this example is 25, the vbase pointer name

  89. is &lsquo;<samp>$vb25</samp>&rsquo;.

  90. </p>

  91. <div class="smallexample">

  92. <pre class="smallexample">.stabs &quot;D:Tt31=s32!3,000,20;100,25;0264,28;$vb25:24,128;Ddat:1,

  93.        160,32;A_virt::32=##1;:i;2A*-2147483647;20;;B_virt::32:i;

  94.        2A*-2147483647;25;;C_virt::32:i;2A*-2147483647;28;;D_virt:

  95.        :32:i;2A*-2147483646;31;;;~%20;&quot;,128,0,0,0

  96. </pre></div>

  97. 

  98. <p>Following the name and a semicolon is a type reference describing the

  99. type of the virtual base class pointer, in this case 24.  Type 24 was

  100. defined earlier as the type of the <code>B</code> class <code>this</code> pointer.  The

  101. <code>this</code> pointer for a class is a pointer to the class type.

  102. </p>

  103. <div class="example">

  104. <pre class="example">.stabs &quot;this:P24=*25=xsB:&quot;,64,0,0,8

  105. </pre></div>

  106. 

  107. <p>Finally the field offset part of the vbase pointer field description

  108. shows that the vbase pointer is the first field in the <code>D</code> object,

  109. before any data fields defined by the class.  The layout of a <code>D</code>

  110. class object is a follows, <code>Adat</code> at 0, the vtable pointer for

  111. <code>A</code> at 32, <code>Cdat</code> at 64, the vtable pointer for C at 96, the

  112. virtual base pointer for <code>B</code> at 128, and <code>Ddat</code> at 160.

  113. </p>

  114. 

  115. <hr>

  116. <div class="header">

  117. <p>

  118. Next: <a href="Static-Members.html#Static-Members" accesskey="n" rel="next">Static Members</a>, Previous: <a href="Inheritance.html#Inheritance" accesskey="p" rel="prev">Inheritance</a>, Up: <a href="Cplusplus.html#Cplusplus" accesskey="u" rel="up">Cplusplus</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Types-Index.html#Symbol-Types-Index" title="Index" rel="index">Index</a>]</p>

  119. </div>

  120. 

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