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<title>STABS: Inheritance</title>
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<a name="Inheritance"></a>
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<div class="header">
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<p>
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Next: <a href="Virtual-Base-Classes.html#Virtual-Base-Classes" accesskey="n" rel="next">Virtual Base Classes</a>, Previous: <a href="Virtual-Methods.html#Virtual-Methods" accesskey="p" rel="prev">Virtual Methods</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="Inheritance-1"></a>
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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>
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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>
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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>
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<p>The virtual character is ‘<samp>1</samp>’ if the base class is virtual and
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‘<samp>0</samp>’ if not. The visibility character is ‘<samp>2</samp>’ if the derivation
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is public, ‘<samp>1</samp>’ if it is protected, and ‘<samp>0</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>
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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>
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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>
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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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</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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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></div>
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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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</p>
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<div class="smallexample">
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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;$vf28:21,32;C_virt::29=##1;
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:i;2A*-2147483647;28;;;~%28;",128,0,0,0
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</pre></div>
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<p>In the stab describing derived class <code>D</code> below, the information about
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the derivation of this class is encoded as follows.
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</p>
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<div class="display">
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<pre class="display">.stabs "derived_class_name:symbol_descriptors(struct tag&type)=
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type_descriptor(struct)struct_bytes(32)!num_bases(3),
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base_virtual(no)inheritance_public(no)base_offset(0),
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base_class_type_ref(A);
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base_virtual(yes)inheritance_public(no)base_offset(NIL),
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base_class_type_ref(B);
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base_virtual(no)inheritance_public(yes)base_offset(64),
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base_class_type_ref(C); …
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</pre></div>
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<div class="smallexample">
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<pre class="smallexample">.stabs "D:Tt31=s32!3,000,20;100,25;0264,28;$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></div>
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<div class="header">
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<p>
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Next: <a href="Virtual-Base-Classes.html#Virtual-Base-Classes" accesskey="n" rel="next">Virtual Base Classes</a>, Previous: <a href="Virtual-Methods.html#Virtual-Methods" accesskey="p" rel="prev">Virtual Methods</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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