Fix PropertyObject bug and interface improvements.

- Fix overwriting of parent node if PropertyObject::node() is
   called for a non-existing node.
 - Prevent implicit conversion from const char* and
   SGPropertyNode*

simgear/props/propertyObject.cxx

@@ -66,15 +66,17 @@ SGPropertyNode* PropertyObjectBase::node(bool aCreate) const
     return _prop;
   }
   
-  SGPropertyNode* r = _prop ? _prop : static_defaultRoot;
+  SGPropertyNode *r = _prop ? _prop : static_defaultRoot,
-  _prop = r->getNode(_path, aCreate);
+                 *prop = r->getNode(_path, aCreate);
   
-  if (_prop) {
+  if( prop )
-    // resolve worked, we will cache from now on, so clear _path
+  {
+    // resolve worked, we will cache from now on, so clear _path and cache prop
     _path = NULL;
+    _prop = prop;
   }
 
-  return _prop;
+  return prop;
 }
 
 SGPropertyNode* PropertyObjectBase::getOrThrow() const

simgear/props/propertyObject.hxx

@@ -59,19 +59,20 @@ template <typename T>
 class PropertyObject : PropertyObjectBase
 {
 public:
-  PropertyObject();
+  PropertyObject()
+  {}
   
   /**
    * Create from path relative to the default root, and option default value
    */
-  PropertyObject(const char* aChild) :
+  explicit PropertyObject(const char* aChild) :
     PropertyObjectBase(aChild)
   { }
   
   /**
    * Create from a node, with optional relative path
    */
-  PropertyObject(SGPropertyNode* aNode, const char* aChild = NULL) :
+  explicit PropertyObject(SGPropertyNode* aNode, const char* aChild = NULL) :
     PropertyObjectBase(aNode, aChild)
   {
   
@@ -115,6 +116,7 @@ public:
   {
     SGPropertyNode* n = PropertyObjectBase::node(true);
     if (!n) {
+      std::cout << "no node" << std::endl;
       return aValue;
     }
     
@@ -157,13 +159,13 @@ template <>
 class PropertyObject<std::string> : PropertyObjectBase
 {
 public:
-  PropertyObject(const char* aChild) :
+  explicit PropertyObject(const char* aChild) :
     PropertyObjectBase(aChild)
   { }
   
 
   
-  PropertyObject(SGPropertyNode* aNode, const char* aChild = NULL) :
+  explicit PropertyObject(SGPropertyNode* aNode, const char* aChild = NULL) :
     PropertyObjectBase(aNode, aChild)
   {