added notes about protected destructors in base classes.

git-svn-id: svn://bind10.isc.org/svn/bind10/branches/trac221b@2554 e5f2f494-b856-4b98-b285-d166d9295462

src/lib/cc/session.h

@@ -60,8 +60,14 @@ namespace isc {
             AbstractSession(const AbstractSession& source);
             AbstractSession& operator=(const AbstractSession& source);
         protected:
+            /// \brief The default constructor.
+            ///
+            /// This is intentionally defined as \c protected as this base
+            /// class should never be instantiated (except as part of a
+            /// derived class).
             AbstractSession() {}
         public:
+            /// \brief The destructor.
             virtual ~AbstractSession() {}
             //@}
             virtual void establish(const char* socket_file) = 0;

src/lib/xfr/xfrout_client.h

@@ -51,8 +51,13 @@ private:
     AbstractXfroutClient(const AbstractXfroutClient& source);
     AbstractXfroutClient& operator=(const AbstractXfroutClient& source);
 protected:
+    /// \brief The default constructor.
+    ///
+    /// This is intentionally defined as \c protected as this base class should
+    /// never be instantiated (except as part of a derived class).
     AbstractXfroutClient() {}
 public:
+    /// \brief The destructor.
     virtual ~AbstractXfroutClient() {}
     //@}
     virtual void connect() = 0;