[escher/scroll_view] Put the decorators inside a union

escher/include/escher/scroll_view.h

@@ -8,6 +8,7 @@
 class ScrollView : public View {
 public:
   ScrollView(View * contentView, ScrollViewDataSource * dataSource);
+  ScrollView(ScrollView&& other);
   KDSize minimalSizeForOptimalDisplay() const override;
 
   void setTopMargin(KDCoordinate m) { m_topMargin = m; }
@@ -73,8 +74,11 @@ public:
     ScrollViewArrow m_leftArrow;
   };
 
-  Decorator * decorator();
-  void setDecoratorType(Decorator::Type t) { m_decoratorType = t; }
+  Decorator * decorator() { return m_decorators.activeDecorator(); }
+  void setDecoratorType(Decorator::Type t) {
+    m_decoratorType = t;
+    m_decorators.setActiveDecorator(t);
+  }
   virtual void setBackgroundColor(KDColor c) {
     m_backgroundColor = c;
     decorator()->setBackgroundColor(m_backgroundColor);
@@ -126,9 +130,19 @@ private:
 
   InnerView m_innerView;
   Decorator::Type m_decoratorType;
-  Decorator m_decorator;
-  BarDecorator m_barDecorator;
-  ArrowDecorator m_arrowDecorator;
+  union Decorators {
+  public:
+    /* Enforce trivial constructor and destructor that just leaves the memory unmodified. */
+    Decorators() {}
+    ~Decorators() {}
+    Decorator * activeDecorator() { return &m_none; }
+    void setActiveDecorator(Decorator::Type t);
+  private:
+    Decorator m_none;
+    BarDecorator m_bars;
+    ArrowDecorator m_arrows;
+  };
+  Decorators m_decorators;
   KDColor m_backgroundColor;
 };
 

escher/src/scroll_view.cpp

@@ -2,6 +2,8 @@
 #include <escher/palette.h>
 #include <escher/metric.h>
 
+#include <new>
+
 extern "C" {
 #include <assert.h>
 }
@@ -15,13 +17,24 @@ ScrollView::ScrollView(View * contentView, ScrollViewDataSource * dataSource) :
   m_bottomMargin(0),
   m_leftMargin(0),
   m_innerView(this),
-  m_decoratorType(Decorator::Type::Bars),
-  m_decorator(),
-  m_barDecorator(),
-  m_arrowDecorator(),
+  m_decorators(),
   m_backgroundColor(Palette::WallScreen)
 {
   assert(m_dataSource != nullptr);
+  setDecoratorType(Decorator::Type::Bars);
+}
+
+ScrollView::ScrollView(ScrollView&& other) :
+  m_contentView(other.m_contentView),
+  m_dataSource(other.m_dataSource),
+  m_topMargin(other.m_topMargin),
+  m_rightMargin(other.m_rightMargin),
+  m_bottomMargin(other.m_bottomMargin),
+  m_leftMargin(other.m_leftMargin),
+  m_innerView(this),
+  m_backgroundColor(other.m_backgroundColor)
+{
+  setDecoratorType(other.m_decoratorType);
 }
 
 KDSize ScrollView::minimalSizeForOptimalDisplay() const {
@@ -39,18 +52,6 @@ void ScrollView::setCommonMargins() {
   setLeftMargin(Metric::CommonLeftMargin);
 }
 
-ScrollView::Decorator * ScrollView::decorator() {
-  switch (m_decoratorType) {
-    case Decorator::Type::Bars:
-      return &m_barDecorator;
-    case Decorator::Type::Arrows:
-      return &m_arrowDecorator;
-    default:
-      assert(m_decoratorType == Decorator::Type::None);
-      return &m_decorator;
-  }
-}
-
 void ScrollView::scrollToContentPoint(KDPoint p, bool allowOverscroll) {
   if (!allowOverscroll && !m_contentView->bounds().contains(p)) {
     return;
@@ -197,6 +198,21 @@ void ScrollView::ArrowDecorator::setBackgroundColor(KDColor c) {
   }
 }
 
+void ScrollView::Decorators::setActiveDecorator(Decorator::Type t) {
+  /* We do NOT need to destroy the previous decorator because they don't have a destructor */
+  switch (t) {
+    case Decorator::Type::Bars:
+      new (&m_bars) BarDecorator();
+      break;
+    case Decorator::Type::Arrows:
+      new (&m_arrows) ArrowDecorator();
+      break;
+    default:
+      assert(t == Decorator::Type::None);
+      new (&m_none) Decorator();
+  }
+}
+
 #if ESCHER_VIEW_LOGGING
 const char * ScrollView::className() const {
   return "ScrollView";