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POMis-indexed char[] arrays in Visual Studio 2005
 

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  1. POMis-indexed char[] arrays in Visual Studio 2005
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    <p>I'm at a loss to explain what seems to be mis-addressing within the <code>char[]</code> arrays of a C++ class I've inherited. I'm using Visual Studio 2005 and have narrowed the problem down to this:</p> <p><strong>MyClass.h:</strong></p> <pre><code>class MyClass { public: MyClass(); virtual ~MyClass(); void Reset(); // More member functions. . . char m_szString[128]; // More member variables. . . } </code></pre> <p><strong>MyClass.cpp:</strong></p> <pre><code>MyClass::MyClass() { Reset(); } MyClass::Reset() { m_szString[0] = 'X'; } // . . . </code></pre> <p>As I single-step through the program, I find that the <code>Reset()</code> function actually sets <code>m_szString[</code><strong><code>4</code></strong><code>]</code> to <code>'X'</code> &mdash; not <code>m_szString[</code><strong><code>0</code></strong><code>]</code> as I expected. According to the watch window, the only element in the class before <code>m_szString[]</code> is a pointer to the vftable, <code>__vfptr</code>, which happens to be 4 bytes. </p> <p>If I add more member variables to <code>MyClass</code>, subsequent strings are mis-addressed by various, always-increasing multiples of 4 bytes. Not simply <em>aligned</em> to 4-byte boundaries, but actually <em>offset</em> by multiples of 4. It's as if the compiler is skipping twice the necessary space for each vftable ... but that's purely a guess.</p> <p>Somewhat similar problems have been reported (Google, MSDN), but I haven't found any answers.</p> <hr> <p><strong>Additional Information / Partial Solution:</strong> The class is the only member variable of a wrapper class that becomes a DLL. The parent was originally declared as</p> <pre><code>class ATL_NO_VTABLE CWrapperClass </code></pre> <p>Removing <code>ATL_NO_VTABLE</code> fixed the alignment problem.</p> <p>I'm still seeing buffer overflows, but those should be fairly easy to track down.</p> <p>Can you explain <code>ATL_NO_VTABLE</code> in terms understandable to an embedded C developer who's had very limited experience with COM beyond BSTRs, or better yet, provide a pointer (sorry) to a good reference?</p> <hr> <p><em>Still More:</em> <a href="https://stackoverflow.com/questions/7525/">This question</a> provides some helpful debugging information.</p> <p>Thanks for your help.</p>
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    1. USAdam Liss
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    1. COCan you use print statements and try this in release mode?
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    2. COCan you give a *complete* working example demonstrating the problem?
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      1. USAdam Rosenfield
    3. COThere has to be more than you're mentioning here - the comment about "adjustor thunks" makes me suspicious for example.
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      1. USAndrew Grant
 

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