Re: Two questions

["chris.danx" <chris.danx@ntlworld.com>]

I misread it.

Thanks for your help,
Chris Campbell

"John English" <je@bton.ac.uk> wrote in message
news:3AACECE3.6D6E274E@bton.ac.uk...
> "chris.danx" wrote:
> >
> > Hi,
> >     two questions, one on exceptions and one on objects or tagged types.
> >
> > Where exactly can i put an exception handler?  I read in JE's book that
you
> > can put them in loop ... end loop statements after an exit statement.
>
> You've misread the book then, or written "can" instead of "can't" by
mistake...
>
> >  I
> > normally put handlers at the end of a routine, and i was supprised to
find
> > you could put them elsewhere.  What i want to know is where else can i
put
> > them?  In a while loop?  In a for loop?  In an if?  ... etc.
>
> Inside a begin...end block (which can go inside a loop or whatever), e.g.
> like this:
>
>   loop
>     begin
>       Put("Enter an integer: ");
>       Get(X);
>       exit;     -- get out of loop if this line is reached
>     exception
>       when Constraint_Error | Data_Error =>
>         Put_Line ("Error in input -- please try again");
>         Skip_Line;       -- end up here if "Get" fails,
>     end;                 -- so don't exit from the loop
>   end loop;
>
> (This is the example from p.40 of my book.)
>
> > The second is to do with tagged types.  Sometimes it is suggested that
you
> > put your 'create routine' -- the initialisation routine -- in a nested
> > package.  Others suggest putting it in the same package as the tagged
type.
> >
> > Example
> >
> > package something_cool is
> >
> >     type cool is tagged private;
> >
> >     package constructor is
> >
> >         procedure create (c : out cool; ... ... );
> >
> >     end constructor;
> >
> >     private
> >         ...
> >         ...
> >
> > end something_cool;
> >
> > or
> >
> > package something_cool is
> >
> >     type cool is tagged private;
> >
> >     procedure create (c : out cool; ... ... );
> >
> >     private
> >         ...
> >         ...
> >
> > end something_cool;
> >
> > What's the difference?  Where should i use method 1 and where should i
use
> > method 2?
>
> Generally, use the first. If you use the second approach, "create"
> is inherited by types derived from Cool, but it may well need extra
> parameters for a derived type... so you end up with two versions of
> Create, one with the old set of parameters (which will presumably do
> incomplete initialisation) which you might call accidentally. The
> second technique just avoids making Create a primitive of Cool, so
> it isn't inherited. (This is discussed in section 14.6 of my book.)
>
> -----------------------------------------------------------------
>  John English              | mailto:je@brighton.ac.uk
>  Senior Lecturer           | http://www.it.bton.ac.uk/staff/je
>  Dept. of Computing        | ** NON-PROFIT CD FOR CS STUDENTS **
>  University of Brighton    |    -- see http://burks.bton.ac.uk
> -----------------------------------------------------------------