Combine snapshot classes into a template class

installer/src/installer-lib/handle.h

 /**
  * \file handle.h The "install session" is the context for all custom installation behavior.
  */
 
 #ifndef HANDLE_H
 #define HANDLE_H
 
 #include "windows.h"
 #include "msi.h"
 
@@ skipping 123 matching lines @@
   
 public:
   /**
    * Conversion operator to underlying handle type.
    */
   operator T()
   {
     return _handle ;
   }
 
+  /**
+   * Error thrown when initialize or assigning a null handle against policy.
+   *
+   * Note that this error is a logic_error, not a runtime error.
+   * If it's against policy for a handle to be null, it's an error for the caller to try to make it null.
+   * Policy enforcment here is not a substitute for good error handling by the caller.
+   * In many cases, the caller ought to be throwing windows_api_error.
+   */
   struct null_handle_error
-    : public std::runtime_error
+    : public std::logic_error
   {
     null_handle_error()
-      : std::runtime_error( "May not initialize with null handle" ) 
+      : std::logic_error( "May not initialize with null handle" ) 
     {}
   } ;
 } ;
 
 /*
  * Handle class
  */
 template<
   class T,
   template <class> class Null_Policy,
@@ skipping 88 matching lines @@
    * The check for a non-zero handle compiles out conditionally based on Null_Policy.
    * The specific function used to close the handle is given by the Destruction_Policy.
    */
   ~handle()
   {
     if ( Null_Policy< T >::prohibited_always() || ( _handle != 0 ) ) {
       Destruction_Policy< T >::close( _handle ) ;
     }
   }
 
+  /**
+   * Expose the underlying handle type.
+   */
+  typedef T handle_type ;
 } ;
 
 //-------------------------------------------------------
 // Common instantiations of handle
 //-------------------------------------------------------
 typedef handle< HANDLE, Disallow_Null, Windows_Generic_Destruction > Windows_Handle ;
 
 #endif