Re: [boost] Re: Standard C Library and C++ / BOOST

30 Mar 2004

      Rob Stewart wrote:
...
From: "Reece Dunn" <msclrhd@hotmail.com>
...
The errorcheck class now has a policy-based implementation, so it would 
look
something like:
template< typename ErrorStorage, typename ErrorMsgStorage >
   class errorcheck: public Storage, public MsgStorage
   {
      private:
         inline errorcheck( const Storage & s, const MsgStorage & ms ):
            Storage( s ), MsgStorage( ms )
         {
         }
      public:
         inline errorcheck( typename Storage::error_type e ):
            Storage( e ), MsgStorage()
         {
            if( failed())
               throw( errorcheck< Storage, MsgStorage >( *this, *this 
));
         }
   };
which, IMHO, is more confusing than the straight throw( *this ), but 
does
not use the copy constructor, relying instead of the fully constructed
Storage and MsgStorage base classes.
I wouldn't call it confusing, though I'd expect that you'd need
to document why you couldn't do the more obvious thing.
I mean confusing in that you have errorcheck( *this, *this ) so it is harder 
to see what is going on. I have added a comment line explaining why this is 
done and would also put this in the docs.
...
I do have to ask why the private ctor takes a MsgStorage object
to copy since the MsgStorage in this case will be the default
constructed base subobject created in the initializer list of the
throwing ctor.  As written, the second argument to that ctor is
useless; the private ctor could default construct its MsgStorage
subobject.
...
Actually, though, I had this in mind:
...
template <typename ES, typename MS>
class error_check : public ES, public MS
{
   error_check(typename ES::error_type e, std::nothrow_t)
   : ES(e)
   {
   }
public:
   error_check(typename ES::error_type e)
   : ES(e)
   {
      if (failed())
      {
   throw error_check<ES,MS>(e, nothrow);
      }
   }
};
What about when you use boost::policy::errormsg_storage instead of 
boost::policy::no_errormsg_storage? Doing this allows the following:

   typedef boost::errorcheck< boost::policy::error_storage<>, 
boost::policy::errormsg_storage >
      errorcheckmsg;

   try
   {
      errorcheckmsg                    em;
      em( "oops!" ) = -3; // note the special syntax
   }
   catch( errorcheckmsg em )
   {
      std::cout << "error: " << em.get_msg() << '\n';
   }

resulting in "error: oops!" being outputted. Using your method, the message 
string will ont be passed and thus you would get "error: unspecified error"! 
That is why I implemented it as above.
...
(Note that you used "Storage" where you should have used
"ErrorStorage" in the excerpt above.  I just used abbreviations.)
That was my bad: ErrorStorage and Storage are the same.

Regards,
Reece

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