Re: Using Global Variables

[caen!uvaarpa!software.org!smithd@uunet.uu.net (Doug Smith)]

In article <16864C1E7.M19481@mwvm.mitre.org> M19481@mwvm.mitre.org writes:
> In article <1992Sep16.152620.9286@nosc.mil>
> sampson@nosc.mil (Charles H. Sampson) writes:
>  
> >
> >In article <15390@suns5.crosfield.co.uk> pdg@crosfield.co.uk (paul goffin)
> >writes (in an article on Ada compiler pricing):
> >
> >>So what did I mean by cost?  Well, apart from the training costs
> >>and the cost of setting up development systems that act>>Ada, (New compiler
 driving scripts, new way of addressing hardware,
> >>new way of managing "global variables" - Yes, nasty as they are,
> >>you do sometimes have to use global variables in the real world!)
> >>there was a nasty shock in the price of the compilers.
> >
> >     I'd like to hear more about this need to use global variables in
> >the real world, from Mr. Goffin and anyone else.  I can't remember the
> >last time I used one in an Ada project, and I think what I do is pretty
> >real world stuff.  (Well, I do contract to the U. S. Navy, but that's
> >close.)
> >
> >                                Charlie
>  
> The main reason I've heard used to justify the use of global variables is
> performance of a real-time system.  The argument goes that the overhead of
> parameter passing is just too high to meet the performance requirements on
> the target machine (which cannot be changed), hence global variables.  Now
> you may argue that the target machine should have been chosen more wisely,
> or the choice delayed until after the software was totally developed so as
> to be able to pick the right size machine -- but I imagine that that's what
> Mr. Goffin had in mind when he used the term "real world."
>  
> David - dhite@mitre.org

OK.  Let's get specific.  Assuming that having to `with' a package does
not violate the definition of global, the following is definitely the
declaration of a global variable:

    package Contains_Global is
       Global_Variable : Integer;
    end Contains_Global;

Now I can effectively convert this to declare the variable inside the
package body, but instead of assignment (a predefined procedure with
special semantics), I must provide my own procedures to access it:

   package Convert_Global is
      procedure Put (Value : in     Integer);
      procedure Get (Value :    out Integer);

      pragma Inline (Put, Get);
   end Convert_Global;

   package body Convert_Global is
      Is_This_A_Global_Variable : Integer; ----------- What do you say?

      procedure Put (Value : in     Integer) is
      begin
         Is_This_A_Global_Variable := Value;
      end Put;

      procedure Get (Value :    out Integer) is
      begin
         Value := Is_This_A_Global_Variable;
      end Get;

   end Convert_Global;

In an attempt to limit the potential explosion of discussion about
why one is better than the other--let's try to define the term `Global'
first!  What is the Integer declared inside the package body?

Doug
smithd@software.org