128 lines
6.1 KiB
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128 lines
6.1 KiB
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<html lang="en">
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<title>Virtual Methods - STABS</title>
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<meta name="description" content="STABS">
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<link rel="up" href="Cplusplus.html#Cplusplus" title="Cplusplus">
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<link rel="prev" href="Method-Modifiers.html#Method-Modifiers" title="Method Modifiers">
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<link rel="next" href="Inheritance.html#Inheritance" title="Inheritance">
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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-Methods"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Inheritance.html#Inheritance">Inheritance</a>,
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Previous: <a rel="previous" accesskey="p" href="Method-Modifiers.html#Method-Modifiers">Method Modifiers</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">8.11 Virtual Methods</h3>
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<p><< The following examples are based on a4.C >>
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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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<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>
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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><span class="samp">Adat</span></samp>’, an integer, starting at structure offset 0 and
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occupying 32 bits.
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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><span class="samp">$vf</span></samp>’ and continues with a
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type reference to the class it is part of. In this example the type
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reference for class A is 20 so the name of its vtable pointer field is
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‘<samp><span class="samp">$vf20</span></samp>’, followed by the usual colon.
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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>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>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>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><span class="samp">A</span></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>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>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>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>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><span class="samp">~%</span></samp>’ and followed by a final semi-colon.
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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>
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<!-- FIXME: bogus line break. -->
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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>
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</body></html>
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