147 lines
6.4 KiB
HTML
147 lines
6.4 KiB
HTML
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<html lang="en">
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<head>
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<title>Inheritance - STABS</title>
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<meta http-equiv="Content-Type" content="text/html">
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<meta name="description" content="STABS">
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<meta name="generator" content="makeinfo 4.13">
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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="Virtual-Methods.html#Virtual-Methods" title="Virtual Methods">
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<link rel="next" href="Virtual-Base-Classes.html#Virtual-Base-Classes" title="Virtual Base Classes">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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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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Texts. A copy of the license is included in the section entitled ``GNU
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Free Documentation License''.-->
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</head>
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<body>
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<div class="node">
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<a name="Inheritance"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Virtual-Base-Classes.html#Virtual-Base-Classes">Virtual Base Classes</a>,
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Previous: <a rel="previous" accesskey="p" href="Virtual-Methods.html#Virtual-Methods">Virtual Methods</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">8.12 Inheritance</h3>
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<p>Stabs describing C<tt>++</tt> derived classes include additional sections that
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describe the inheritance hierarchy of the class. A derived class stab
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also encodes the number of base classes. For each base class it tells
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if the base class is virtual or not, and if the inheritance is private
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or public. It also gives the offset into the object of the portion of
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the object corresponding to each base class.
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<p>This additional information is embedded in the class stab following the
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number of bytes in the struct. First the number of base classes
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appears bracketed by an exclamation point and a comma.
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<p>Then for each base type there repeats a series: a virtual character, a
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visibility character, a number, a comma, another number, and a
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semi-colon.
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<p>The virtual character is ‘<samp><span class="samp">1</span></samp>’ if the base class is virtual and
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‘<samp><span class="samp">0</span></samp>’ if not. The visibility character is ‘<samp><span class="samp">2</span></samp>’ if the derivation
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is public, ‘<samp><span class="samp">1</span></samp>’ if it is protected, and ‘<samp><span class="samp">0</span></samp>’ if it is private.
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Debuggers should ignore virtual or visibility characters they do not
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recognize, and assume a reasonable default (such as public and
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non-virtual) (GDB 4.11 does not, but this should be fixed in the next
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GDB release).
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<p>The number following the virtual and visibility characters is the offset
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from the start of the object to the part of the object pertaining to the
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base class.
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<p>After the comma, the second number is a type_descriptor for the base
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type. Finally a semi-colon ends the series, which repeats for each
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base class.
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<p>The source below defines three base classes <code>A</code>, <code>B</code>, and
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<code>C</code> and the derived class <code>D</code>.
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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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class B {
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public:
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int B_dat;
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virtual int B_virt (int arg) {return arg; };
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};
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class C {
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public:
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int Cdat;
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virtual int C_virt (int arg) {return arg; };
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};
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class D : A, virtual B, public C {
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public:
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int Ddat;
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virtual int A_virt (int arg ) { return arg+1; };
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virtual int B_virt (int arg) { return arg+2; };
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virtual int C_virt (int arg) { return arg+3; };
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virtual int D_virt (int arg) { return arg; };
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};
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</pre>
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<p>Class stabs similar to the ones described earlier are generated for
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each base class.
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<!-- FIXME!!! the linebreaks in the following example probably make the -->
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<!-- examples literally unusable, but I don't know any other way to get -->
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<!-- them on the page. -->
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<!-- One solution would be to put some of the type definitions into -->
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<!-- separate stabs, even if that's not exactly what the compiler actually -->
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<!-- emits. -->
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<pre class="smallexample"> .stabs "A:T20=s8Adat:1,0,32;$vf20:21=*22=ar1;0;1;17,32;
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A_virt::23=##1;:i;2A*-2147483647;20;;;~%20;",128,0,0,0
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.stabs "B:Tt25=s8Bdat:1,0,32;$vf25:21,32;B_virt::26=##1;
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:i;2A*-2147483647;25;;;~%25;",128,0,0,0
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.stabs "C:Tt28=s8Cdat:1,0,32;$vf28:21,32;C_virt::29=##1;
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:i;2A*-2147483647;28;;;~%28;",128,0,0,0
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</pre>
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<p>In the stab describing derived class <code>D</code> below, the information about
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the derivation of this class is encoded as follows.
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<pre class="display"> .stabs "derived_class_name:symbol_descriptors(struct tag&type)=
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type_descriptor(struct)struct_bytes(32)!num_bases(3),
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base_virtual(no)inheritance_public(no)base_offset(0),
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base_class_type_ref(A);
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base_virtual(yes)inheritance_public(no)base_offset(NIL),
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base_class_type_ref(B);
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base_virtual(no)inheritance_public(yes)base_offset(64),
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base_class_type_ref(C); <small class="dots">...</small>
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</pre>
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<!-- FIXME! fake linebreaks. -->
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<pre class="smallexample"> .stabs "D:Tt31=s32!3,000,20;100,25;0264,28;$vb25:24,128;Ddat:
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1,160,32;A_virt::32=##1;:i;2A*-2147483647;20;;B_virt:
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:32:i;2A*-2147483647;25;;C_virt::32:i;2A*-2147483647;
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28;;D_virt::32:i;2A*-2147483646;31;;;~%20;",128,0,0,0
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</pre>
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</body></html>
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