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copied!<p>Your virtual function <code>A::foo()</code> returns an <code>A*</code>, while function <code>B<C>::foo()</code>, which is meant to override it, returns a <code>C*</code>.</p> <p>This in theory does respect the principle of covariance, since <code>C</code> is indeed a specialization of (derives from) <code>A</code>, but at the point of instantiation, this is not known, because <code>C</code> is an incomplete type.</p> <p>One possible way to re-think your design is to make <code>A</code> a class template as well, and let <code>B</code> propagate the template argument for <code>T</code> up to <code>A</code>:</p> <pre><code>template<typename T> class A { virtual T* foo() = 0; }; template<class T> class B : public A<T> { virtual T* foo() { return nullptr; } }; </code></pre> <p>Concerning your workaround:</p> <blockquote> <p>What does the standard states about this situation? Should this code compile? If not, why?</p> </blockquote> <p>It shouldn't compile, because the mere fact of making <code>C</code> also derive from <code>A</code> explicitly (notice, that you would end up with <em>two</em> distinct base sub-objects of type <code>A</code> inside <code>C</code>) does not make <code>C</code> a complete type when instantiating <code>B<C></code>. Per Paragraph 9.2/2 of the C++11 Standard:</p> <blockquote> <p><strong>A class is considered a completely-defined object type (3.9) (or complete type) at the closing <code>}</code> of the <em>class-specifier</em>.</strong> Within the class <em>member-specification</em>, the class is regarded as complete within function bodies, default arguments, and <em>brace-or-equal-initializers</em> for non-static data members (including such things in nested classes). Otherwise it is regarded as incomplete within its own class <em>member-specification</em>. </p> </blockquote>

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