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A copy of the license is included in the section entitled "GNU 12Free Documentation License". --> 13<!-- Created by GNU Texinfo 5.1, http://www.gnu.org/software/texinfo/ --> 14<head> 15<title>STABS: Virtual Base Classes</title> 16 17<meta name="description" content="STABS: Virtual Base Classes"> 18<meta name="keywords" content="STABS: Virtual Base Classes"> 19<meta name="resource-type" content="document"> 20<meta name="distribution" content="global"> 21<meta name="Generator" content="makeinfo"> 22<meta http-equiv="Content-Type" content="text/html; charset=utf-8"> 23<link href="index.html#Top" rel="start" title="Top"> 24<link href="Symbol-Types-Index.html#Symbol-Types-Index" rel="index" title="Symbol Types Index"> 25<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> 26<link href="Cplusplus.html#Cplusplus" rel="up" title="Cplusplus"> 27<link href="Static-Members.html#Static-Members" rel="next" title="Static Members"> 28<link href="Inheritance.html#Inheritance" rel="previous" title="Inheritance"> 29<style type="text/css"> 30<!-- 31a.summary-letter {text-decoration: none} 32blockquote.smallquotation {font-size: smaller} 33div.display {margin-left: 3.2em} 34div.example {margin-left: 3.2em} 35div.indentedblock {margin-left: 3.2em} 36div.lisp {margin-left: 3.2em} 37div.smalldisplay {margin-left: 3.2em} 38div.smallexample {margin-left: 3.2em} 39div.smallindentedblock {margin-left: 3.2em; font-size: smaller} 40div.smalllisp {margin-left: 3.2em} 41kbd {font-style:oblique} 42pre.display {font-family: inherit} 43pre.format {font-family: inherit} 44pre.menu-comment {font-family: serif} 45pre.menu-preformatted {font-family: serif} 46pre.smalldisplay {font-family: inherit; font-size: smaller} 47pre.smallexample {font-size: smaller} 48pre.smallformat {font-family: inherit; font-size: smaller} 49pre.smalllisp {font-size: smaller} 50span.nocodebreak {white-space:nowrap} 51span.nolinebreak {white-space:nowrap} 52span.roman {font-family:serif; font-weight:normal} 53span.sansserif {font-family:sans-serif; font-weight:normal} 54ul.no-bullet {list-style: none} 55--> 56</style> 57 58 59</head> 60 61<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000"> 62<a name="Virtual-Base-Classes"></a> 63<div class="header"> 64<p> 65Next: <a href="Static-Members.html#Static-Members" accesskey="n" rel="next">Static Members</a>, Previous: <a href="Inheritance.html#Inheritance" accesskey="p" rel="previous">Inheritance</a>, Up: <a href="Cplusplus.html#Cplusplus" accesskey="u" rel="up">Cplusplus</a> [<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> 66</div> 67<hr> 68<a name="Virtual-Base-Classes-1"></a> 69<h3 class="section">8.13 Virtual Base Classes</h3> 70 71<p>A derived class object consists of a concatenation in memory of the data 72areas defined by each base class, starting with the leftmost and ending 73with the rightmost in the list of base classes. The exception to this 74rule is for virtual inheritance. In the example above, class <code>D</code> 75inherits virtually from base class <code>B</code>. This means that an 76instance of a <code>D</code> object will not contain its own <code>B</code> part but 77merely a pointer to a <code>B</code> part, known as a virtual base pointer. 78</p> 79<p>In a derived class stab, the base offset part of the derivation 80information, described above, shows how the base class parts are 81ordered. The base offset for a virtual base class is always given as 0. 82Notice that the base offset for <code>B</code> is given as 0 even though 83<code>B</code> is not the first base class. The first base class <code>A</code> 84starts at offset 0. 85</p> 86<p>The field information part of the stab for class <code>D</code> describes the field 87which is the pointer to the virtual base class <code>B</code>. The vbase pointer 88name is ‘<samp>$vb</samp>’ followed by a type reference to the virtual base class. 89Since the type id for <code>B</code> in this example is 25, the vbase pointer name 90is ‘<samp>$vb25</samp>’. 91</p> 92<div class="smallexample"> 93<pre class="smallexample">.stabs "D:Tt31=s32!3,000,20;100,25;0264,28;$vb25:24,128;Ddat:1, 94 160,32;A_virt::32=##1;:i;2A*-2147483647;20;;B_virt::32:i; 95 2A*-2147483647;25;;C_virt::32:i;2A*-2147483647;28;;D_virt: 96 :32:i;2A*-2147483646;31;;;~%20;",128,0,0,0 97</pre></div> 98 99<p>Following the name and a semicolon is a type reference describing the 100type of the virtual base class pointer, in this case 24. Type 24 was 101defined earlier as the type of the <code>B</code> class <code>this</code> pointer. The 102<code>this</code> pointer for a class is a pointer to the class type. 103</p> 104<div class="example"> 105<pre class="example">.stabs "this:P24=*25=xsB:",64,0,0,8 106</pre></div> 107 108<p>Finally the field offset part of the vbase pointer field description 109shows that the vbase pointer is the first field in the <code>D</code> object, 110before any data fields defined by the class. The layout of a <code>D</code> 111class object is a follows, <code>Adat</code> at 0, the vtable pointer for 112<code>A</code> at 32, <code>Cdat</code> at 64, the vtable pointer for C at 96, the 113virtual base pointer for <code>B</code> at 128, and <code>Ddat</code> at 160. 114</p> 115 116<hr> 117<div class="header"> 118<p> 119Next: <a href="Static-Members.html#Static-Members" accesskey="n" rel="next">Static Members</a>, Previous: <a href="Inheritance.html#Inheritance" accesskey="p" rel="previous">Inheritance</a>, Up: <a href="Cplusplus.html#Cplusplus" accesskey="u" rel="up">Cplusplus</a> [<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> 120</div> 121 122 123 124</body> 125</html> 126