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POunderstanding c++ vtables and RTTI
 

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  1. POunderstanding c++ vtables and RTTI
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    <p><br> <br> I was messing around with vtables lately in order to better understand what a compiler/the process needs to do to implement classes and inheritance.<br> <br> <strong>This is what I was trying to accomplish:</strong><br> I wanted to write my own little vtable in order to force a static behaviour on an object:<br></p> <pre><code>class A { public: virtual void foo() { cout &lt;&lt; "A.foo()" &lt;&lt; endl; } virtual void bar() { cout &lt;&lt; "A.bar()" &lt;&lt; endl; } }; class B : public A { public: void foo() { cout &lt;&lt; "B.foo()" &lt;&lt; endl; } void bar() { cout &lt;&lt; "B.bar()" &lt;&lt; endl; } }; typedef void (A::*func)(); int main() { A&amp; b_as_a = *(new B()); long* p = (long*)(&amp;b_as_a); func* vtab = (func*)(p[0]); b_as_a.foo(); b_as_a.bar(); func* my_vtab = new func[4]; my_vtab[0] = vtab[0]; // \ I added these lines in step two after i got an my_vtab[1] = vtab[1]; // / access violation my_vtab[2] = &amp;A::bar; my_vtab[3] = &amp;A::foo; p[0] = (long)(my_vtab); b_as_a.foo(); b_as_a.bar(); delete[] my_vtab; delete &amp;b_as_a; return EXIT_SUCCESS; } </code></pre> <p>and here a dump of <code>g++ -std=c++11 -fdump-class-hierarchy</code></p> <pre><code>Vtable for A A::_ZTV1A: 4u entries 0 (int (*)(...))0 8 (int (*)(...))(&amp; _ZTI1A) 16 (int (*)(...))A::foo 24 (int (*)(...))A::bar Class A size=8 align=8 base size=8 base align=8 A (0x0x7f40b60fe000) 0 nearly-empty vptr=((&amp; A::_ZTV1A) + 16u) Vtable for B B::_ZTV1B: 4u entries 0 (int (*)(...))0 8 (int (*)(...))(&amp; _ZTI1B) 16 (int (*)(...))B::foo 24 (int (*)(...))B::bar Class B size=8 align=8 base size=8 base align=8 B (0x0x7f40b60dfbc8) 0 nearly-empty vptr=((&amp; B::_ZTV1B) + 16u) A (0x0x7f40b60fe060) 0 nearly-empty primary-for B (0x0x7f40b60dfbc8) </code></pre> <p><br> This did not work... so I researched a little.<br> I found this post: <a href="https://stackoverflow.com/questions/5712808/understanding-the-vtable-entries">What is the first (int (*)(...))0 vtable entry in the output of g++ -fdump-class-hierarchy?</a> <br> It explains the first two entries in a vtable. I understood what the first one does but all I know about the second entry is, that it's some sort of pointer to class information.<br> I guess that is the reason why it doesn't work.<br> <br> <strong>The question that remains is:</strong><br> What does the second entry in the vtable do that the funktionpointers underneath aren't read anymore???<br> <br> <br> Extra info: I am using g++ on openSuse 12.3</p>
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      1. POunderstanding c++ vtables and RTTI
      1. USPierre Fourgeaud
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    1. COI am having problems understanding what the question is. Can you further explain what you mean to ask?
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      1. POunderstanding c++ vtables and RTTI
      1. USDavid Rodríguez - dribeas
 

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