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POHow to obtain a pointer out of a C++ vtable?
 

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  1. POHow to obtain a pointer out of a C++ vtable?
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    <p>Say you have a C++ class like:</p> <pre><code>class Foo { public: virtual ~Foo() {} virtual DoSomething() = 0; }; </code></pre> <p>The C++ compiler translates a call into a vtable lookup:</p> <pre><code>Foo* foo; // Translated by C++ to: // foo-&gt;vtable-&gt;DoSomething(foo); foo-&gt;DoSomething(); </code></pre> <p>Suppose I was writing a JIT compiler and I wanted to obtain the address of the DoSomething() function for a particular instance of class Foo, so I can generate code that jumps to it directly instead of doing a table lookup and an indirect branch.</p> <p>My questions are:</p> <ol> <li><p>Is there any standard C++ way to do this (I'm almost sure the answer is no, but wanted to ask for the sake of completeness).</p></li> <li><p>Is there any remotely compiler-independent way of doing this, like a library someone has implemented that provides an API for accessing a vtable?</p></li> </ol> <p>I'm open completely to hacks, if they will work. For example, if I created my own derived class and could determine the address of its DoSomething method, I could assume that the vtable is the first (hidden) member of Foo and search through its vtable until I find my pointer value. However, I don't know a way of getting this address: if I write <code>&amp;DerivedFoo::DoSomething</code> I get a pointer-to-member, which is something totally different.</p> <p>Maybe I could turn the pointer-to-member into the vtable offset. When I compile the following:</p> <pre><code>class Foo { public: virtual ~Foo() {} virtual void DoSomething() = 0; }; void foo(Foo *f, void (Foo::*member)()) { (f-&gt;*member)(); } </code></pre> <p>On GCC/x86-64, I get this assembly output:</p> <pre><code>Disassembly of section .text: 0000000000000000 &lt;_Z3fooP3FooMS_FvvE&gt;: 0: 40 f6 c6 01 test sil,0x1 4: 48 89 74 24 e8 mov QWORD PTR [rsp-0x18],rsi 9: 48 89 54 24 f0 mov QWORD PTR [rsp-0x10],rdx e: 74 10 je 20 &lt;_Z3fooP3FooMS_FvvE+0x20&gt; 10: 48 01 d7 add rdi,rdx 13: 48 8b 07 mov rax,QWORD PTR [rdi] 16: 48 8b 74 30 ff mov rsi,QWORD PTR [rax+rsi*1-0x1] 1b: ff e6 jmp rsi 1d: 0f 1f 00 nop DWORD PTR [rax] 20: 48 01 d7 add rdi,rdx 23: ff e6 jmp rsi </code></pre> <p>I don't fully understand what's going on here, but if I could reverse-engineer this or use an ABI spec I could generate a fragment like the above for each separate platform, as a way of obtaining a pointer out of a vtable.</p>
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    1. COThis is not really an answer, but you should read http://codesourcery.com/cxx-abi/. Ignore mentions of Itanium; GCC uses that ABI (with appropriate processor-specific tweaks) universally, and so do pretty much all other compilers for \*nix nowadays. MSVC is of course different, but probably not *that* different -- there are only so many ways to do it.
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    2. COYou should look into how to use debug symbols. This sort of thing is quite easy if symbols are available. I answered this question http://stackoverflow.com/questions/5740155/access-v-table-at-run-time/5740839#5740839 with references for how to access vtable pointer's, their layout, where they are derived from etc... Check out my reference to wikipedia's page on C++ RTT information, this may help you out http://en.wikipedia.org/wiki/Run-time_type_information
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