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POData member offset in C++
 

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    <p>"<em>Inside the C++ Object Model</em>" says that the offset of a data member in a class is always 1 more than the actual offset in order to distinguish between the pointer to 0 and the pointer to the first data member, here is the example:</p> <pre><code>class Point3d { public: virtual ~Point3d(); public: static Point3d origin; float x, y, z; }; //to be used after, ignore it for the first question int main(void) { /*cout &lt;&lt; "&amp;Point3d::x = " &lt;&lt; &amp;Point3d::x &lt;&lt; endl; cout &lt;&lt; "&amp;Point3d::y = " &lt;&lt; &amp;Point3d::y &lt;&lt; endl; cout &lt;&lt; "&amp;Point3d::z = " &lt;&lt; &amp;Point3d::z &lt;&lt; endl;*/ printf("&amp;Point3d::x = %p\n", &amp;Point3d::x); printf("&amp;Point3d::y = %p\n", &amp;Point3d::y); printf("&amp;Point3d::z = %p\n", &amp;Point3d::z); getchar(); } </code></pre> <p>So in order to distinguish the two pointers below, the offset of a data member is always 1 more.</p> <pre><code>float Point3d::*p1 = 0; float Point3d::*p2 = &amp;Point3d::x; </code></pre> <p>The main function above is attempt to get the offset of the members to verify this argument, which is supposed to output: 5, 9, 13(Consider the vptr of 4bytes at the beginning). In <strong>MS Visual Studio 2012</strong> however, the output is:</p> <pre><code>&amp;Point3d::x = 00000004 &amp;Point3d::y = 00000008 &amp;Point3d::z = 0000000C </code></pre> <p><strong>Question: So is MS C++ compiler did some optimization or something to prevent this mechanism?</strong></p>
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    1. COseems some gotcha..is it because that there is already a vptr there in the beginning of the class, so we don't need this mechanism any more?
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    2. COif "Inside the C++ Object Model" says that the address of the object is different from the address of it's first member, it's wrong. I think more likely, you misunderstood what it was saying.
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    3. CO@MooingDuck No, this is a syntax that gives the offset of data members, don't need a object at all.
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