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POC++: is a class with virtual base but without virtual functions polymorphic and has VTable?
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  1. POC++: is a class with virtual base but without virtual functions polymorphic and has VTable?
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    copied!<p>Consider the following code:</p> <pre><code>#include &lt;iostream&gt; #include &lt;typeinfo&gt; #include &lt;type_traits&gt; using namespace std; struct A { int data; }; struct B1 : A {}; struct B2 : virtual A {}; struct Base1 : virtual A {}; struct Base2 : virtual A {}; struct Derived : Base1, Base2 {}; int main() { cout &lt;&lt; sizeof(B1) &lt;&lt; endl; cout &lt;&lt; sizeof(B2) &lt;&lt; endl; cout &lt;&lt; sizeof(Derived) &lt;&lt; endl; cout &lt;&lt; std::is_polymorphic&lt;B1&gt;::value &lt;&lt; endl; cout &lt;&lt; std::is_polymorphic&lt;B2&gt;::value &lt;&lt; endl; cout &lt;&lt; std::is_polymorphic&lt;Derived&gt;::value &lt;&lt; endl; return 0; } </code></pre> <p>On my system it prints</p> <pre><code>4 8 12 0 0 0 </code></pre> <p>Which means that none of these classes is polymorphic. However, sizes of B1 and B2 differ by exactly size of a pointer, which is probably a pointer to vtable. I've ran gcc with <code>-fdump-class-hierarchy</code> and got:</p> <pre><code>Vtable for B2 B2::_ZTV2B2: 3u entries 0 4u 4 (int (*)(...))0 8 (int (*)(...))(&amp; _ZTI2B2) VTT for B2 B2::_ZTT2B2: 1u entries 0 ((&amp; B2::_ZTV2B2) + 12u) Class B2 size=8 align=4 base size=4 base align=4 B2 (0x0x3ca5400) 0 nearly-empty vptridx=0u vptr=((&amp; B2::_ZTV2B2) + 12u) A (0x0x3c972d8) 4 virtual vbaseoffset=-12 </code></pre> <p>Here is a question: what is a polymorphic class? Does 'having vtable' means 'be polymorphic and have RTTI' and vice-versa? If not, why do it have to have at least one virtual function to be polymorphic, if it's going to have vtable anyway?</p> <p>Looks like definition of polymorphic class differs from 'the class that has a vtable', because, as we can see above, B2 is not a polymorphic, but has a vtable and, as I think, should have a RTTI. In <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2003/n1529.html#1.2.4.1" rel="noreferrer">this</a> document it's clearly stated that "polymorphic - i.e. have at least one virtual function. (This is necessary to allow the generated dispatch code to use RTTI on the classes.)".</p>
 

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