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POC++: Can virtual inheritance be detected at compile time?
 

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  1. POC++: Can virtual inheritance be detected at compile time?
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    <p>I would like to determine at compile time if a pointer to Derived can be cast from a pointer to Base without dynamic_cast&lt;>. Is this possible using templates and metaprogramming? This isn't exactly the same problem as determining if Base is a virtual base class of Derived, because Base could be the super class of a virtual base class of Derived.</p> <p>Thanks, Tim Update: I felt good about this method:</p> <pre><code>#include &lt;iostream&gt; using namespace std; class Foo { }; class Bar : public Foo { }; class Baz : public virtual Foo { }; class Autre : public virtual Bar { }; typedef char Small; class Big { char dummy[2]; }; template&lt;typename B, typename D&gt; struct is_static_castable { const B* foo; char bar[1]; static Small test(char(*)[sizeof(static_cast&lt;const D*&gt;(foo)) == sizeof(const D*)]); static Big test(...); enum { value = (sizeof(test(&amp;bar)) == sizeof(Small)) }; }; int main() { cout &lt;&lt; "Foo -&gt; Bar: " &lt;&lt; is_static_castable&lt;Foo, Bar&gt;::value &lt;&lt; "\n"; cout &lt;&lt; "Foo -&gt; Baz: " &lt;&lt; is_static_castable&lt;Foo, Baz&gt;::value &lt;&lt; "\n"; cout &lt;&lt; "Foo -&gt; Autre: " &lt;&lt; is_static_castable&lt;Foo, Autre&gt;::value &lt;&lt; "\n"; } </code></pre> <p>But it doesn't work with gcc:</p> <pre><code>multi-fun.cpp: In instantiation of ‘is_static_castable&lt;Foo, Baz&gt;’: multi-fun.cpp:38: instantiated from here multi-fun.cpp:29: error: cannot convert from base ‘Foo’ to derived type ‘Baz’ via virtual base ‘Foo’ multi-fun.cpp:29: error: array bound is not an integer constant multi-fun.cpp: In instantiation of ‘is_static_castable&lt;Foo, Autre&gt;’: multi-fun.cpp:39: instantiated from here multi-fun.cpp:29: error: cannot convert from base ‘Foo’ to derived type ‘Autre’ via virtual base ‘Bar’ multi-fun.cpp:29: error: array bound is not an integer constant </code></pre> <p>Am I confused about what can be done with the sizeof() trick?</p>
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      1. USKirill V. Lyadvinsky
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    1. COI think if `Base` is merely a superclass of a virtual base class of `Derived`, this should work with `static_cast`. At least i can't find the Standard say otherwise. You can use `boost::is_virtual_base_of` for checking the other case.
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      1. USJohannes Schaub - litb
    2. COI can almost think of some ways, but they're ugly. If you can give me moar info about what you want to do with this, I might be able to come up with something.
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      1. USNarfanator
    3. COIn the case where Base is a superclass of a virtual base of Derived, gcc produces the error (for example): error: cannot convert from base ‘osg::Referenced’ to derived type ‘osgViewer::ViewerBase’ via virtual base ‘osg::Object’ Using the mental model that virtual base classes are implemented using pointers to the bases, the error makes sense to me: static_cast can convert the "super base" to the virtual base without a problem, but then it's in the same situation with respect to converting the pointer to the derived type.
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