Platform: check and return a reference from Screen::application().

Giving out a pointer implied excessive error checking in user code.
There's a new hasApplication() accessor that can be used to check for
application presence, moreover the ScreenedApplication is now
convertible to a pointer to provide backward compatiblity. This
conversion is marked as deprecated and will be removed in a future
release.

doc/changelog.dox

@@ -387,6 +387,11 @@ See also:
     in a RAII manner, without needing to explicitly call
     @ref Platform::BasicScreenedApplication::addScreen() and
     @ref Platform::BasicScreen::setPropagatedEvents() later
+-   @ref Platform::BasicScreen::application() now returns a reference instead
+    of a pointer, checking that the screen is actually added to the
+    application. Users are encouraged to call
+    @ref Platform::BasicScreen::hasApplication() to check if the application is
+    accessible.
 
 @subsubsection changelog-latest-changes-text Text library
 
@@ -575,6 +580,14 @@ See also:
 -   Passing @cpp nullptr @ce to @ref ImageView constructors is deprecated and
     will print a warning at runtime. Use a constructor without the @p data
     parameter instead.
+-   @ref Platform::BasicScreen::application() now returns a reference instead
+    of a pointer and together with @ref Platform::BasicScreen::redraw() checks
+    that the screen is actually added to the application instead of returning
+    @cpp nullptr @ce (and requiring excessive error checking in user code) or,
+    in case of @ref Platform::BasicScreen::redraw() "redraw()" silently doing
+    the wrong thing. For backwards compatibility
+    @ref Platform::BasicScreenedApplication is convertible to a pointer and
+    implements @cpp operator-> @ce, but this conversion is deprecated.
 
 @subsection changelog-latest-compatibility Potential compatibility breakages, removed APIs
 

src/Magnum/Platform/Screen.h

@@ -252,13 +252,33 @@ template<class Application> class BasicScreen:
          */
         void setPropagatedEvents(PropagatedEvents events) { _propagatedEvents = events; }
 
-        /** @brief Application holding this screen */
-        template<class T = BasicScreenedApplication<Application>> T* application() {
-            return static_cast<T*>(Containers::LinkedListItem<BasicScreen<Application>, BasicScreenedApplication<Application>>::list());
+        /**
+         * @brief Whether the screen is added to an application
+         *
+         * If not, the @ref application() accessor can't be used.
+         * @see @ref BasicScreenedApplication::addScreen(),
+         *      @ref BasicScreenedApplication::removeScreen()
+         */
+        bool hasApplication() {
+            return Containers::LinkedListItem<BasicScreen<Application>, BasicScreenedApplication<Application>>::list();
         }
+
+        /**
+         * @brief Application holding this screen
+         *
+         * Expects that the screen is added to an application.
+         * @see @ref hasApplication()
+         */
+        BasicScreenedApplication<Application>& application();
         /** @overload */
-        template<class T = BasicScreenedApplication<Application>> const T* application() const {
-            return static_cast<const T*>(Containers::LinkedListItem<BasicScreen<Application>, BasicScreenedApplication<Application>>::list());
+        const BasicScreenedApplication<Application>& application() const;
+        /** @overload */
+        template<class T = BasicScreenedApplication<Application>> T& application() {
+            return static_cast<T&>(application());
+        }
+        /** @overload */
+        template<class T = BasicScreenedApplication<Application>> const T& application() const {
+            return static_cast<const T&>(application());
         }
 
         /**
@@ -294,8 +314,13 @@ template<class Application> class BasicScreen:
         /** @{ @name Screen handling */
 
     protected:
-        /** @brief Request redraw */
-        virtual void redraw() { application()->redraw(); }
+        /**
+         * @brief Request redraw
+         *
+         * Expects that the screen is added to an application.
+         * @see @ref hasApplication()
+         */
+        virtual void redraw();
 
     private:
         /**

src/Magnum/Platform/ScreenedApplication.h

@@ -265,6 +265,18 @@ template<class Application> class BasicScreenedApplication:
             return static_cast<const Containers::LinkedList<BasicScreen<Application>>&>(*this);
         }
 
+        #if defined(MAGNUM_BUILD_DEPRECATED) && !defined(DOXYGEN_GENERATING_OUTPUT)
+        CORRADE_DEPRECATED("Platform::Screen::application() returns a reference now") BasicScreenedApplication<Application>* operator->() { return this; }
+        CORRADE_DEPRECATED("Platform::Screen::application() returns a reference now") const BasicScreenedApplication<Application>* operator->() const { return this; }
+        CORRADE_DEPRECATED("Platform::Screen::application() returns a reference now") BasicScreenedApplication<Application>& operator*() { return *this; }
+        CORRADE_DEPRECATED("Platform::Screen::application() returns a reference now") const BasicScreenedApplication<Application>& operator*() const { return *this; }
+        CORRADE_DEPRECATED("Platform::Screen::application() returns a reference now") operator BasicScreenedApplication<Application>*() { return this; }
+        CORRADE_DEPRECATED("Platform::Screen::application() returns a reference now") operator const BasicScreenedApplication<Application>*() const { return this; }
+        template<class T, class = typename std::enable_if<std::is_base_of<BasicScreenedApplication<Application>, T>::value>::type> CORRADE_DEPRECATED("Platform::Screen::application() returns a reference now") operator T*() { return static_cast<T*>(this); }
+        template<class T, class = typename std::enable_if<std::is_base_of<BasicScreenedApplication<Application>, T>::value>::type> CORRADE_DEPRECATED("Platform::Screen::application() returns a reference now") operator const T*() const { return static_cast<const T*>(this); }
+        CORRADE_DEPRECATED("Platform::Screen::application() returns a reference now, use hasApplication() instead") bool operator!() const { return false; }
+        #endif
+
     protected:
         /* Nobody will need to have (and delete) ScreenedApplication*, thus
            this is faster than public pure virtual destructor */

src/Magnum/Platform/ScreenedApplication.hpp

@@ -117,6 +117,24 @@ template<class Application> BasicScreen<Application>::BasicScreen(BasicScreenedA
 
 template<class Application> BasicScreen<Application>::~BasicScreen() = default;
 
+template<class Application> BasicScreenedApplication<Application>& BasicScreen<Application>::application() {
+    BasicScreenedApplication<Application>* application = Containers::LinkedListItem<BasicScreen<Application>, BasicScreenedApplication<Application>>::list();
+    CORRADE_ASSERT(application, "Platform::Screen::application(): the screen is not added to any application", *application);
+    return *application;
+}
+
+template<class Application> const BasicScreenedApplication<Application>& BasicScreen<Application>::application() const {
+    const BasicScreenedApplication<Application>* application = Containers::LinkedListItem<BasicScreen<Application>, BasicScreenedApplication<Application>>::list();
+    CORRADE_ASSERT(application, "Platform::Screen::application(): the screen is not added to any application", *application);
+    return *application;
+}
+
+template<class Application> void BasicScreen<Application>::redraw() {
+    BasicScreenedApplication<Application>* application = Containers::LinkedListItem<BasicScreen<Application>, BasicScreenedApplication<Application>>::list();
+    CORRADE_ASSERT(application, "Platform::Screen::redraw(): the screen is not added to any application", );
+    application->redraw();
+}
+
 template<class Application> void BasicScreen<Application>::focusEvent() {}
 template<class Application> void BasicScreen<Application>::blurEvent() {}
 
@@ -149,7 +167,7 @@ template<class Application> BasicScreenedApplication<Application>::BasicScreened
 template<class Application> BasicScreenedApplication<Application>::~BasicScreenedApplication() = default;
 
 template<class Application> BasicScreenedApplication<Application>& BasicScreenedApplication<Application>::addScreen(BasicScreen<Application>& screen) {
-    CORRADE_ASSERT(!screen.application(),
+    CORRADE_ASSERT(!screen.hasApplication(),
         "Platform::ScreenedApplication::addScreen(): screen already added to an application", *this);
 
     /* A subset of this (except focusEvent()) is done in
@@ -162,7 +180,7 @@ template<class Application> BasicScreenedApplication<Application>& BasicScreened
 }
 
 template<class Application> BasicScreenedApplication<Application>& BasicScreenedApplication<Application>::removeScreen(BasicScreen<Application>& screen) {
-    CORRADE_ASSERT(screen.application() == this,
+    CORRADE_ASSERT(screen.hasApplication() && &screen.application() == this,
         "Platform::ScreenedApplication::removeScreen(): screen not owned by this application", *this);
 
     screen.blurEvent();
@@ -172,7 +190,7 @@ template<class Application> BasicScreenedApplication<Application>& BasicScreened
 }
 
 template<class Application> BasicScreenedApplication<Application>& BasicScreenedApplication<Application>::focusScreen(BasicScreen<Application>& screen) {
-    CORRADE_ASSERT(screen.application() == this,
+    CORRADE_ASSERT(screen.hasApplication() && &screen.application() == this,
         "Platform::ScreenedApplication::focusScreen(): screen not owned by this application", *this);
 
     /* Already focused, nothing to do */