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POhelp needed on deciphering the g++ vtable dumps
 

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  1. POhelp needed on deciphering the g++ vtable dumps
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    <h3>Source Code</h3> <pre><code>class W { public: virtual void f() { cout &lt;&lt; "W::f()" &lt;&lt; endl; } virtual void g() { cout &lt;&lt; "W::g()" &lt;&lt; endl; } }; class AW : public virtual W { public: void g() { cout &lt;&lt; "AW::g()" &lt;&lt; endl; } }; class BW : public virtual W { public: void f() { cout &lt;&lt; "BW::f()" &lt;&lt; endl; } }; class CW : public AW, public BW { }; </code></pre> <h3>g++ -fdump-class-hierarchy</h3> <pre><code>Vtable for W W::_ZTV1W: 4u entries 0 (int (*)(...))0 4 (int (*)(...))(&amp; _ZTI1W) 8 W::f 12 W::g Vtable for AW AW::_ZTV2AW: 7u entries 0 0u 4 0u 8 0u 12 (int (*)(...))0 16 (int (*)(...))(&amp; _ZTI2AW) 20 W::f 24 AW::g VTT for AW AW::_ZTT2AW: 2u entries 0 ((&amp; AW::_ZTV2AW) + 20u) 4 ((&amp; AW::_ZTV2AW) + 20u) Vtable for BW BW::_ZTV2BW: 7u entries 0 0u 4 0u 8 0u 12 (int (*)(...))0 16 (int (*)(...))(&amp; _ZTI2BW) 20 BW::f 24 W::g VTT for BW BW::_ZTT2BW: 2u entries 0 ((&amp; BW::_ZTV2BW) + 20u) 4 ((&amp; BW::_ZTV2BW) + 20u) Vtable for CW CW::_ZTV2CW: 14u entries 0 0u 4 0u 8 4u 12 (int (*)(...))0 16 (int (*)(...))(&amp; _ZTI2CW) 20 BW::_ZTv0_n12_N2BW1fEv 24 AW::g 28 4294967292u 32 4294967292u 36 0u 40 (int (*)(...))-0x00000000000000004 44 (int (*)(...))(&amp; _ZTI2CW) 48 BW::f 52 0u Construction vtable for AW (0xb6dbf8c0 instance) in CW CW::_ZTC2CW0_2AW: 7u entries 0 0u 4 0u 8 0u 12 (int (*)(...))0 16 (int (*)(...))(&amp; _ZTI2AW) 20 W::f 24 AW::g Construction vtable for BW (0xb6dbf900 instance) in CW CW::_ZTC2CW4_2BW: 13u entries 0 4294967292u 4 4294967292u 8 0u 12 (int (*)(...))0 16 (int (*)(...))(&amp; _ZTI2BW) 20 BW::f 24 0u 28 0u 32 4u 36 (int (*)(...))4 40 (int (*)(...))(&amp; _ZTI2BW) 44 BW::_ZTv0_n12_N2BW1fEv 48 W::g VTT for CW CW::_ZTT2CW: 7u entries 0 ((&amp; CW::_ZTV2CW) + 20u) 4 ((&amp; CW::_ZTC2CW0_2AW) + 20u) 8 ((&amp; CW::_ZTC2CW0_2AW) + 20u) 12 ((&amp; CW::_ZTC2CW4_2BW) + 20u) 16 ((&amp; CW::_ZTC2CW4_2BW) + 44u) 20 ((&amp; CW::_ZTV2CW) + 20u) 24 ((&amp; CW::_ZTV2CW) + 48u) </code></pre> <h2>Questions</h2> <p>What are each of the entries in:</p> <pre><code>Vtable for AW AW::_ZTV2AW: 7u entries 0 0u // whats this for??? 4 0u // ? 8 0u // ? Vtable for CW CW::_ZTV2CW: 14u entries 0 0u // ? 4 0u // ? 8 4u // what is the significance of this ??? 12 (int (*)(...))0 16 (int (*)(...))(&amp; _ZTI2CW) 20 BW::_ZTv0_n12_N2BW1fEv // ? 24 AW::g 28 4294967292u // ? 32 4294967292u // ? 36 0u // ? 40 (int (*)(...))-0x00000000000000004 // some delta 44 (int (*)(...))(&amp; _ZTI2CW) 48 BW::f 52 0u // ? CW *pcw = new CW; AW *paw = pcw; paw-&gt;f(); // Invokes B::f why not w::f </code></pre> <p>paw is pointing to AW's vtable and it has a entry of W::f. How does B::f() get called?</p> <p>Similarly:</p> <pre><code> BW* pbw = pcw; pbw-&gt;g(); // invokes AW::g() why not w::g ??? </code></pre> <p>What is the use of VTT and how its used. VTT of AW contains pointer to vtable of AW<br> AW::_ZTT2AW: 2u entries<br> 0 ((&amp; AW::_ZTV2AW) + 20u)<br> 4 ((&amp; AW::_ZTV2AW) + 20u)<br> and both entries at offset 0 and 4 points to vtable base address+20 u = W::f. Why both the entries point to same function of the base class?</p>
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    1. USGanesh Kundapur
    1. USGanesh Kundapur
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    1. CORelated: http://stackoverflow.com/questions/4609631/deciphering-vtable-dumps/4609777#4609777
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      1. UStemplatetypedef
    2. CO@trojanfoe I think because he wrote "Thanks" in the end of the question.
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      1. USNarek
    3. COforgot to mark it earlier. Now i did for all
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      1. USGanesh Kundapur
 

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