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163 lines
7.5 KiB
HTML
163 lines
7.5 KiB
HTML
4 years ago
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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) 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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Texts. A copy of the license is included in the section entitled "GNU
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Free Documentation License". -->
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Virtual Methods (STABS)</title>
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<meta name="description" content="Virtual Methods (STABS)">
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<meta name="keywords" content="Virtual Methods (STABS)">
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<meta name="resource-type" content="document">
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<meta name="distribution" content="global">
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<meta name="Generator" content="makeinfo">
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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="Symbol-Types-Index.html#Symbol-Types-Index" rel="index" title="Symbol Types Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Cplusplus.html#Cplusplus" rel="up" title="Cplusplus">
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<link href="Inheritance.html#Inheritance" rel="next" title="Inheritance">
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<link href="Method-Modifiers.html#Method-Modifiers" rel="prev" title="Method Modifiers">
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<style type="text/css">
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a.summary-letter {text-decoration: none}
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-->
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</style>
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</head>
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<body lang="en">
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<a name="Virtual-Methods"></a>
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<div class="header">
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<p>
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Next: <a href="Inheritance.html#Inheritance" accesskey="n" rel="next">Inheritance</a>, Previous: <a href="Method-Modifiers.html#Method-Modifiers" accesskey="p" rel="prev">Method Modifiers</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>
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</div>
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<hr>
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<a name="Virtual-Methods-1"></a>
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<h3 class="section">8.11 Virtual Methods</h3>
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<p><< The following examples are based on a4.C >>
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</p>
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<p>The presence of virtual methods in a class definition adds additional
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data to the class description. The extra data is appended to the
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description of the virtual method and to the end of the class
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description. Consider the class definition below:
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</p>
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<div class="example">
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<pre class="example">class A {
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public:
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int Adat;
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virtual int A_virt (int arg) { return arg; };
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};
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</pre></div>
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<p>This results in the stab below describing class A. It defines a new
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type (20) which is an 8 byte structure. The first field of the class
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struct is ‘<samp>Adat</samp>’, an integer, starting at structure offset 0 and
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occupying 32 bits.
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</p>
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<p>The second field in the class struct is not explicitly defined by the
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C<tt>++</tt> class definition but is implied by the fact that the class
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contains a virtual method. This field is the vtable pointer. The
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name of the vtable pointer field starts with ‘<samp>$vf</samp>’ and continues with a
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type reference to the class it is part of. In this example the type
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reference for class A is 20 so the name of its vtable pointer field is
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‘<samp>$vf20</samp>’, followed by the usual colon.
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</p>
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<p>Next there is a type definition for the vtable pointer type (21).
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This is in turn defined as a pointer to another new type (22).
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</p>
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<p>Type 22 is the vtable itself, which is defined as an array, indexed by
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a range of integers between 0 and 1, and whose elements are of type
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17. Type 17 was the vtable record type defined by the boilerplate C<tt>++</tt>
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type definitions, as shown earlier.
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</p>
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<p>The bit offset of the vtable pointer field is 32. The number of bits
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in the field are not specified when the field is a vtable pointer.
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</p>
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<p>Next is the method definition for the virtual member function <code>A_virt</code>.
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Its description starts out using the same format as the non-virtual
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member functions described above, except instead of a dot after the
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‘<samp>A</samp>’ there is an asterisk, indicating that the function is virtual.
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Since is is virtual some addition information is appended to the end
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of the method description.
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</p>
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<p>The first number represents the vtable index of the method. This is a
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32 bit unsigned number with the high bit set, followed by a
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semi-colon.
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</p>
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<p>The second number is a type reference to the first base class in the
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inheritance hierarchy defining the virtual member function. In this
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case the class stab describes a base class so the virtual function is
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not overriding any other definition of the method. Therefore the
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reference is to the type number of the class that the stab is
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describing (20).
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</p>
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<p>This is followed by three semi-colons. One marks the end of the
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current sub-section, one marks the end of the method field, and the
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third marks the end of the struct definition.
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</p>
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<p>For classes containing virtual functions the very last section of the
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string part of the stab holds a type reference to the first base
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class. This is preceded by ‘<samp>~%</samp>’ and followed by a final semi-colon.
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</p>
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<div class="display">
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<pre class="display">.stabs "class_name(A):type_def(20)=sym_desc(struct)struct_bytes(8)
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field_name(Adat):type_ref(int),bit_offset(0),field_bits(32);
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field_name(A virt func ptr):type_def(21)=type_desc(ptr to)type_def(22)=
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sym_desc(array)index_type_ref(range of int from 0 to 1);
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elem_type_ref(vtbl elem type),
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bit_offset(32);
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meth_name(A_virt)::typedef(23)=sym_desc(method)returning(int);
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:arg_type(int),protection(public)normal(yes)virtual(yes)
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vtable_index(1);class_first_defining(A);;;~%first_base(A);",
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N_LSYM,NIL,NIL,NIL
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</pre></div>
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<div class="example">
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<pre class="example">.stabs "A:t20=s8Adat:1,0,32;$vf20:21=*22=ar1;0;1;17,32;
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A_virt::23=##1;:i;2A*-2147483647;20;;;~%20;",128,0,0,0
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</pre></div>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Inheritance.html#Inheritance" accesskey="n" rel="next">Inheritance</a>, Previous: <a href="Method-Modifiers.html#Method-Modifiers" accesskey="p" rel="prev">Method Modifiers</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>
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</div>
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</body>
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</html>
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