rectangles: Move enable_shared_from_this to base

In preparation of making a recursive find_rect() function which will need
shared_from_this in the base class, we need to move enable_shared_from_this into
the base Rectangle class.

CompositeRectangle will still get a type-safe shared_from_this()
function using static_pointer_cast, which is safe because
CompositeRectangle::shared_from_this is guaranteed to be called with a
CompositeRectangle instance.

src/imgpack/algorithm/rectangles.cc

@@ -101,6 +101,19 @@ void CompositeRectangle::recalculate_size ()
         recalculate_size_impl ();
 }
 
+std::shared_ptr<const CompositeRectangle>
+CompositeRectangle::shared_from_this () const
+{
+    return std::static_pointer_cast<const CompositeRectangle>
+        (Rectangle::shared_from_this ());
+}
+
+CompositeRectangle::Ptr CompositeRectangle::shared_from_this ()
+{
+    return std::static_pointer_cast<CompositeRectangle>
+        (Rectangle::shared_from_this ());
+}
+
 
 
 // HCompositeRectangle definitions

src/imgpack/algorithm/rectangles.hh

@@ -12,7 +12,8 @@ namespace ImgPack
     {
         class CompositeRectangle;
 
-        class Rectangle
+        class Rectangle :
+            public std::enable_shared_from_this<Rectangle>
         {
         public:
             typedef std::shared_ptr<Rectangle> Ptr;
@@ -51,8 +52,7 @@ namespace ImgPack
 
 
         class CompositeRectangle :
-            public Rectangle,
-            public std::enable_shared_from_this<CompositeRectangle>
+            public Rectangle
         {
         public:
             typedef std::shared_ptr<CompositeRectangle> Ptr;
@@ -69,6 +69,10 @@ namespace ImgPack
             void orphan_child (Rectangle &child);
 
             void recalculate_size ();
+
+            std::shared_ptr<const CompositeRectangle> shared_from_this () const;
+            Ptr shared_from_this ();
+
         protected:
             virtual void recalculate_size_impl () = 0;
             std::pair<Rectangle::Ptr, Rectangle::Ptr> _children;