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

From: Reece Dunn (msclrhd_at_[hidden])
Date: 2004-03-30 16:29:08

Rob Stewart wrote:
>From: "Reece Dunn" <msclrhd_at_[hidden]>
> > 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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