Cache wxDisplayImpl object for faster access

Avoid a heap allocation on every wxDisplay creation by caching the
wxDisplayImpl objects once they're created.

Currently we never invalidate the cache, but we should add a way to do
it in the future.

This speeds up wxDisplay::GetGeometry() benchmark by a factor of 4.

include/wx/display.h

@@ -50,7 +50,6 @@ public:
 
     // dtor is not virtual as this is a concrete class not meant to be derived
     // from
-    ~wxDisplay();
 
 
     // return the number of available displays, valid parameters to

include/wx/private/display.h

@@ -11,6 +11,7 @@
 #define _WX_PRIVATE_DISPLAY_H_
 
 #include "wx/gdicmn.h"      // for wxRect
+#include "wx/vector.h"
 
 // ----------------------------------------------------------------------------
 // wxDisplayFactory: allows to create wxDisplay objects
@@ -20,12 +21,20 @@ class WXDLLIMPEXP_CORE wxDisplayFactory
 {
 public:
     wxDisplayFactory() { }
-    virtual ~wxDisplayFactory() { }
+    virtual ~wxDisplayFactory();
 
-    // create a new display object
-    //
-    // it can return a NULL pointer if the display creation failed
-    virtual wxDisplayImpl *CreateDisplay(unsigned n) = 0;
+    // Create the display if necessary using CreateDisplay(), otherwise just
+    // get it from cache.
+    wxDisplayImpl* GetDisplay(unsigned n)
+    {
+        if ( m_impls.empty() )
+            m_impls.resize(GetCount());
+        else if ( m_impls[n] )
+            return m_impls[n];
+
+        m_impls[n] = CreateDisplay(n);
+        return m_impls[n];
+    }
 
     // get the total number of displays
     virtual unsigned GetCount() = 0;
@@ -37,6 +46,18 @@ public:
     //
     // the window pointer must not be NULL (i.e. caller should check it)
     virtual int GetFromWindow(const wxWindow *window);
+
+protected:
+    // create a new display object
+    //
+    // it can return a NULL pointer if the display creation failed
+    virtual wxDisplayImpl *CreateDisplay(unsigned n) = 0;
+
+private:
+    // On-demand populated vector of wxDisplayImpl objects.
+    wxVector<wxDisplayImpl*> m_impls;
+
+    wxDECLARE_NO_COPY_CLASS(wxDisplayFactory);
 };
 
 // ----------------------------------------------------------------------------

src/common/dpycmn.cpp

@@ -118,12 +118,7 @@ wxDisplay::wxDisplay(unsigned n)
     wxASSERT_MSG( n == 0 || n < GetCount(),
                     wxT("An invalid index was passed to wxDisplay") );
 
-    m_impl = Factory().CreateDisplay(n);
-}
-
-wxDisplay::~wxDisplay()
-{
-    delete m_impl;
+    m_impl = Factory().GetDisplay(n);
 }
 
 // ----------------------------------------------------------------------------
@@ -230,6 +225,15 @@ bool wxDisplay::ChangeMode(const wxVideoMode& mode)
 // wxDisplayFactory implementation
 // ============================================================================
 
+wxDisplayFactory::~wxDisplayFactory()
+{
+    for ( size_t n = 0; n < m_impls.size(); ++n )
+    {
+        // It can be null, that's ok.
+        delete m_impls[n];
+    }
+}
+
 int wxDisplayFactory::GetFromWindow(const wxWindow *window)
 {
     // consider that the window belongs to the display containing its centre