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