Extract "external field" approach in a reusable class

Add wxExternalField<> template that can be used for other fields and/or
in other ports too.

include/wx/private/extfield.h

@@ -0,0 +1,84 @@
+///////////////////////////////////////////////////////////////////////////////
+// Name:        wx/private/extfield.h
+// Purpose:     Declare wxExternalField helper
+// Author:      Vadim Zeitlin
+// Created:     2017-11-21
+// Copyright:   (c) 2017 Vadim Zeitlin <vadim@wxwidgets.org>
+// Licence:     wxWindows licence
+///////////////////////////////////////////////////////////////////////////////
+
+#ifndef _WX_PRIVATE_EXTFIELD_H_
+#define _WX_PRIVATE_EXTFIELD_H_
+
+// ----------------------------------------------------------------------------
+// wxExternalField: store object data outside of it
+// ----------------------------------------------------------------------------
+
+// This class allows to store some data without consuming space for the objects
+// that don't need it and can be useful for avoiding to add rarely used fields
+// to the classes that are used by many objects, e.g. wxWindow.
+//
+// Note that using this class costs in speed and convenience of access to the
+// field, which requires a hash lookup instead of accessing it directly. It
+// also only currently works for heap-allocated fields as it's probably never
+// worth using it for fields of simple types.
+//
+// Template parameter Object is the class that "contains" the field, Field is
+// the type of the field itself and FieldMap is the hash map, defined
+// separately using WX_DECLARE_HASH_MAP(), with Object* as the key and Field*
+// as the value type.
+template <typename Object, typename Field, typename FieldMap>
+class wxExternalField
+{
+public:
+    typedef Object ObjectType;
+    typedef Field FieldType;
+    typedef FieldMap MapType;
+
+    // Store the field object to be used for the given object, replacing the
+    // existing one, if any.
+    //
+    // This method takes ownership of the field pointer which will be destroyed
+    // by EraseForWindow().
+    static void StoreForWindow(ObjectType* obj, FieldType* field)
+    {
+        const typename MapType::iterator it = ms_map.find(obj);
+        if ( it != ms_map.end() )
+        {
+            delete it->second;
+            it->second = field;
+        }
+        else
+        {
+            ms_map.insert(typename MapType::value_type(obj, field));
+        }
+    }
+
+    // Find the object for the corresponding window.
+    static FieldType* FromWindow(ObjectType* obj)
+    {
+        const typename MapType::const_iterator it = ms_map.find(obj);
+        return it == ms_map.end() ? NULL : it->second;
+    }
+
+    // Erase the object used for the corresponding window, return true if there
+    // was one or false otherwise.
+    static bool EraseForWindow(ObjectType* obj)
+    {
+        const typename MapType::iterator it = ms_map.find(obj);
+        if ( it == ms_map.end() )
+            return false;
+
+        delete it->second;
+        ms_map.erase(it);
+        return true;
+    }
+
+private:
+    static FieldMap ms_map;
+};
+
+template <typename O, typename F, typename M>
+M wxExternalField<O, F, M>::ms_map;
+
+#endif // _WX_PRIVATE_EXTFIELD_H_