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POHow does the Visual C++ compiler pass the this ptr to the called function?
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  1. POHow does the Visual C++ compiler pass the this ptr to the called function?
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    copied!<p>I'm learning C++ using Eckel's "Thinking in C++". It states the following:</p> <ul> <li>If a class contains virtual methods, a virtual function table is created for that class etc. The workings of the function table are explained roughly. (I know a vtable is not mandatory, but Visual C++ creates one.)</li> <li>The calling object is passed to the called function as an argument. (This might not be true for Visual C++ (or any compiler).) I'm trying to find out how VC++ passes the calling object to the function. </li> </ul> <p>To test both points in Visual C++, I've created the following class (using Visual Studio 2010, WinXP Home 32bit):</p> <p>ByteExaminer.h:</p> <pre><code>#pragma once class ByteExaminer { public: short b[2]; ByteExaminer(void); virtual void f() const; virtual void g() const; void bruteFG(); }; </code></pre> <p>ByteExaminer.cpp:</p> <pre><code>#include "StdAfx.h" #include "ByteExaminer.h" using namespace std; ByteExaminer::ByteExaminer(void) { b[0] = 25; b[1] = 26; } void ByteExaminer::f(void) const { cout &lt;&lt; "virtual f(); b[0]: " &lt;&lt; hex &lt;&lt; b[0] &lt;&lt; endl; } void ByteExaminer::g(void) const { cout &lt;&lt; "virtual g(); b[1]: " &lt;&lt; hex &lt;&lt; b[1] &lt;&lt; endl; } void ByteExaminer::bruteFG(void) { int *mem = reinterpret_cast&lt;int*&gt;(this); void (*fg[])(ByteExaminer*) = { (void (*)(ByteExaminer*))(*((int *)*mem)), (void (*)(ByteExaminer*))(*((int *)(*mem + 4))) }; fg[0](this); fg[1](this); } </code></pre> <p>The navigation through the vtable in <code>bruteFG()</code> works - when I call <code>fg[0](this)</code>, <code>f()</code> is called. What does NOT work, however, is the passing of <code>this</code> to the function - meaning that <code>this-&gt;b[0]</code> is not printed correctly (garbage comes out instead. I'm actually lucky this doesn't produce a segfault). </p> <p>So the actual output for </p> <pre><code>ByteExaminer be; be.bruteFG(); </code></pre> <p>is:</p> <pre><code>virtual f(); b[0]: 1307 virtual g(); b[1]: 0 </code></pre> <p>So how should I proceed to get the correct result? How are the <code>this</code> pointers passed to functions in VC++?</p> <p>(Nota bene: I'm NOT going to program this way seriously, ever. This is "for the lulz"; or for the learning experience. So don't try to convert me to proper C++ianity :))</p>
 

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