On forcing all derived classes to implement a specific non-virtual method, part 1

<p>You may have a base class that implements only partial functionality and relies on the derived class to do the rest. How do you make sure that the derived class does the rest?</p> <p>For concreteness, let&#8217;s say that we are implementing <code>IValueConverter</code>, which has two methods:</p> <ul> <li><code>Convert()</code> to convert from the source to the destination.</li> <li><code>ConvertBack()</code> so that two-way conversions can convert from the destination to the source.</li> </ul> <p>Suppose you want to write a base class called <code>OneWayConverter</code>. Its implementation fails the <code>ConvertBack()</code> call, and you want to force the derived class to implement the forward conversion.</p> <pre>// C++/WRL struct WidgetColorConverter : Microsoft::WRL::RuntimeClass&lt; Microsoft::WRL::RuntimeClassFlags&lt;Microsoft::WRL::WinRt&gt;, ABI::Windows::UI::Xaml::Data::IValueConverter&gt;, OneWayConverter { // Require the derived class to implement Convert somehow HRESULT STDMETHODCALLTYPE Convert(IInspectable* value, ABI::Windows::UI::Xaml::Interop::TypeName targetType, IInspectable* parameter, HSTRING language, IInspectable** result) { ⟦ ... ⟧ } }; // C++/WinRT struct WidgetColorConverter : winrt::implements&lt;WidgetColorConverter&gt;, OneWayConverter { // Require the derived class to implement Convert somehow winrt::Windows::Foundation::IInspectable Convert( winrt::Windows::Foundation::IInspectable const&amp; value, winrt::Windows::UI::Xaml::Interop::TypeName const&amp; targetType, winrt::Windows::Foundation::IInspectable const&amp; parameter, winrt::hstring const&amp; language) { ⟦ ... ⟧ } } // Plain C++ analogous scenario struct WidgetColorConverter : OneWayConverter { // Require the derived class to implement Convert somehow Color Convert(Widget const&amp;amp value) { ⟦ ... ⟧ } }; </pre> <p>During a code review, I saw that somebody tried to do this just by writing a comment.</p> <pre>// C++/WRL struct OneWayConverter { // Derived classes must override this method. HRESULT STDMETHODCALLTYPE Convert(IInspectable* /*value*/, ABI::Windows::UI::Xaml::Interop::TypeName /*targetType*/, IInspectable* /*parameter*/, HSTRING /*language*/, IInspectable** result) { assert(false); *result = nullptr; return E_NOTIMPL; } // One-way converters cannot convert back HRESULT STDMETHODCALLTYPE ConvertBack(IInspectable* /*value*/, ABI::Windows::UI::Xaml::Interop::TypeName /*targetType*/, IInspectable* /*parameter*/, HSTRING /*language*/, IInspectable** result) { *result = nullptr; return E_NOTIMPL; } }; // C++/WinRT struct OneWayConverter { // Derived classes must override this method. winrt::Windows::Foundation::IInspectable Convert( winrt::Windows::Foundation::IInspectable const&amp; /*value*/, winrt::Windows::UI::Xaml::Interop::TypeName const&amp; /*targetType*/, winrt::Windows::Foundation::IInspectable const&amp; /*parameter*/, winrt::hstring const&amp; /*language*/) { assert(false); throw winrt::hresult_not_implemented(); } // One-way converters cannot convert back winrt::Windows::Foundation::IInspectable ConvertBack( winrt::Windows::Foundation::IInspectable const&amp; /*value*/, winrt::Windows::UI::Xaml::Interop::TypeName const&amp; /*targetType*/, winrt::Windows::Foundation::IInspectable const&amp; /*parameter*/, winrt::hstring const&amp; /*language*/) { throw winrt::hresult_not_implemented(); } }; // Plain C++ analogous scenario struct OneWayConverter { // Derived classes must override this method. Color Convert(Widget const&amp;amp /*value*/) { assert(false); throw std::exception("not implemented"); } // One-way converters cannot convert back Widget ConvertBack(Color const&amp; /*color*/) { throw std::exception("not implemented"); } }; </pre> <p>I pointed out that they were doing too much work.</p> <p>The way to force somebody to implement a method in the derived class is simply not to implement the method in the base class in the first place.</p> <pre>// C++/WRL struct OneWayConverter { // Derived classes must implement Convert() // One-way converters cannot convert back HRESULT STDMETHODCALLTYPE ConvertBack(IInspectable* /*value*/, ABI::Windows::UI::Xaml::Interop::TypeName /*targetType*/, IInspectable* /*parameter*/, HSTRING /*language*/, IInspectable** result) { *result = nullptr; return E_NOTIMPL; } }; // C++/WinRT struct OneWayConverter { // Derived classes must implement Convert() // One-way converters cannot convert back winrt::Windows::Foundation::IInspectable ConvertBack( winrt::Windows::Foundation::IInspectable const&amp; /*value*/, winrt::Windows::UI::Xaml::Interop::TypeName const&amp; /*targetType*/, winrt::Windows::Foundation::IInspectable const&amp; /*parameter*/, winrt::hstring const&amp; /*language*/) { throw winrt::hresult_not_implemented(); } }; // Plain C++ analogous scenario struct OneWayConverter { // Derived classes must implement Convert() // One-way converters cannot convert back Widget ConvertBack(Color const&amp; /*color*/) { throw std::exception("not implemented"); } }; </pre> <p>The error message if they forget to implement it depends on the library.</p> <pre>// C++/WRL struct WidgetColorConverter : Microsoft::WRL::RuntimeClass&lt; Microsoft::WRL::RuntimeClassFlags&lt;Microsoft::WRL::WinRt&gt;, OneWayConverter&gt; { // Oops, forgot to implement Convert() }; </pre> <p>The error occurs when you call <code>WRL::<wbr />Make</code> to try to create the defective <code>Widget­Color­Converter</code>.</p> <pre style="white-space: pre-wrap;">wrl\implements.h(2512,32): error C2259: 'WidgetColorConverter': cannot instantiate abstract class see declaration of 'WidgetColorConverter' due to following members: wrl\implements.h(2512,32): 'HRESULT ABI::<wbr />Windows::<wbr />UI::<wbr />Xaml::<wbr />Data::<wbr />IValueConverter::<wbr />Convert(IInspectable *,ABI::<wbr />Windows::<wbr />UI::<wbr />Xaml::<wbr />Interop::<wbr />TypeName,<wbr />IInspectable *,<wbr />HSTRING,<wbr />IInspectable **)': is abstract windows.ui.xaml.data.h(2778,59): see declaration of 'ABI::<wbr />Windows::<wbr />UI::<wbr />Xaml::<wbr />Data::<wbr />IValueConverter::<wbr />Convert' wrl\implements.h(2512,32): the template instantiation context (the oldest one first) is test(41,30): see reference to function template instantiation 'Microsoft::<wbr />WRL::<wbr />ComPtr&lt;<wbr />WidgetColorConverter&gt; Microsoft::<wbr />WRL::<wbr />Details::<wbr />Make&lt;WidgetColorConverter,&gt;(void)' being compiled </pre> <p>&#8220;Cannot instantiate abstract class due to the following members&#8221; is the standard error for failing to implement all the necessary pure virtual methods inherited from a base class, so one could expect that people who encounter this error will understand what it means.</p> <pre>// C++/WinRT struct WidgetColorConverter : winrt::implements&lt;WidgetColorConverter, winrt::Windows::UI::Xaml::Data::IValueConverter&gt;, OneWayConverter { // Oops, forgot to implement Convert() }; </pre> <p>The error occurs when you call <code>winrt::<wbr />make</code> to try to create the defective <code>Widget­Color­Converter</code>.</p> <pre style="white-space: pre-wrap;">windows.ui.xaml.data.h(1469,90): error C2039: 'Convert': is not a member of 'WidgetColorConverter' test.cpp(66,8): see declaration of 'WidgetColorConverter' windows.ui.xaml.data.h(1469,90): the template instantiation context (the oldest one first) is test.cpp(66,31): see reference to class template instantiation 'winrt::<wbr />implements&lt;WidgetColorConverter,<wbr />winrt::<wbr />Windows::<wbr />UI::<wbr />Xaml::<wbr />Data::<wbr />IValueConverter&gt;' being compiled winrt\base.h(8088,31): see reference to class template instantiation 'winrt::<wbr />impl::<wbr />producers_base&lt;D,<wbr />std::tuple&lt;<wbr />winrt::<wbr />Windows::<wbr />UI::<wbr />Xaml::<wbr />Data::<wbr />IValueConverter&gt;&gt;' being compiled with [ D=WidgetColorConverter ] winrt\base.h(6763,50): see reference to class template instantiation 'winrt::<wbr />impl::<wbr />producer_convert&lt;D,<wbr />winrt::<wbr />Windows::<wbr />UI::<wbr />Xaml::<wbr />Data::<wbr />IValueConverter,<wbr />void&gt;' being compiled with [ D=WidgetColorConverter ] winrt\base.h(6734,31): see reference to class template instantiation 'winrt::<wbr />impl::<wbr />producer&lt;D,<wbr />winrt::<wbr />Windows::<wbr />UI::<wbr />Xaml::<wbr />Data::<wbr />IValueConverter,<wbr />void&gt;' being compiled with [ D=WidgetColorConverter ] winrt\base.h(7137,23): see reference to class template instantiation 'winrt::<wbr />impl::<wbr />produce&lt;D,I&gt;' being compiled with [ D=WidgetColorConverter, I=winrt::<wbr />Windows::<wbr />UI::<wbr />Xaml::<wbr />Data::<wbr />IValueConverter ] winrt\windows.ui.xaml.data.h(1465,32): while compiling class template member function 'int32_t winrt::<wbr />impl::<wbr />produce&lt;D,I&gt;::<wbr />Convert(<wbr />void *,<wbr />winrt::<wbr />impl::<wbr />struct_<wbr />Windows_<wbr />UI_<wbr />Xaml_<wbr />Interop_<wbr />TypeName,<wbr />void *,<wbr />void *,<wbr />void **) noexcept' with [ D=WidgetColorConverter, I=winrt::<wbr />Windows::<wbr />UI::<wbr />Xaml::<wbr />Data::<wbr />IValueConverter ] </pre> <p>&#8220;⟦Name⟧ is not a member of&#8221; is typical of a CRTP error, since the template is trying to call a method on the derived class, but it&#8217;s not there. Again, one could expect that people who encounter this error will understand what it means.</p> <p>For the plain C++ case, you might have this:</p> <pre>// Plain C++ analogous scenario struct WidgetColorConverter : OneWayConverter { // Oops, forgot to implement Convert() }; </pre> <p>And everything works great until somebody tries to call the <code>Convert</code> method on a <code>Widget­Color­Converter</code> and it&#8217;s not there.</p> <pre style="white-space: pre-wrap;">test.cpp(79,20): error C2039: 'Convert': is not a member of 'WidgetColorConverter' </pre> <p>Again, this is a common error in C++ so you would hope that people understand what it means.</p> <p>Great, so we were able to convert all of these authoring errors into compile-time errors, thereby avoiding the danger that somebody will use the base class and fail to implement all of the expected methods.</p> <p>But wait, we can do better. We&#8217;ll look at this some more next time.</p> <p>The post <a href="https://devblogs.microsoft.com/oldnewthing/20260827-00/?p=112651">On forcing all derived classes to implement a specific non-virtual method, part 1</a> appeared first on <a href="https://devblogs.microsoft.com/oldnewthing">The Old New Thing</a>.</p>