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| <h3 class="section">8.13 Virtual Base Classes</h3> |
| |
| <p>A derived class object consists of a concatenation in memory of the data |
| areas defined by each base class, starting with the leftmost and ending |
| with the rightmost in the list of base classes. The exception to this |
| rule is for virtual inheritance. In the example above, class <code>D</code> |
| inherits virtually from base class <code>B</code>. This means that an |
| instance of a <code>D</code> object will not contain its own <code>B</code> part but |
| merely a pointer to a <code>B</code> part, known as a virtual base pointer. |
| |
| <p>In a derived class stab, the base offset part of the derivation |
| information, described above, shows how the base class parts are |
| ordered. The base offset for a virtual base class is always given as 0. |
| Notice that the base offset for <code>B</code> is given as 0 even though |
| <code>B</code> is not the first base class. The first base class <code>A</code> |
| starts at offset 0. |
| |
| <p>The field information part of the stab for class <code>D</code> describes the field |
| which is the pointer to the virtual base class <code>B</code>. The vbase pointer |
| name is ‘<samp><span class="samp">$vb</span></samp>’ followed by a type reference to the virtual base class. |
| Since the type id for <code>B</code> in this example is 25, the vbase pointer name |
| is ‘<samp><span class="samp">$vb25</span></samp>’. |
| |
| <!-- FIXME!! fake linebreaks below --> |
| <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> |
| <p>Following the name and a semicolon is a type reference describing the |
| type of the virtual base class pointer, in this case 24. Type 24 was |
| defined earlier as the type of the <code>B</code> class <code>this</code> pointer. The |
| <code>this</code> pointer for a class is a pointer to the class type. |
| |
| <pre class="example"> .stabs "this:P24=*25=xsB:",64,0,0,8 |
| </pre> |
| <p>Finally the field offset part of the vbase pointer field description |
| shows that the vbase pointer is the first field in the <code>D</code> object, |
| before any data fields defined by the class. The layout of a <code>D</code> |
| class object is a follows, <code>Adat</code> at 0, the vtable pointer for |
| <code>A</code> at 32, <code>Cdat</code> at 64, the vtable pointer for C at 96, the |
| virtual base pointer for <code>B</code> at 128, and <code>Ddat</code> at 160. |
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