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From: taashlo@sandia.gov (Tad Ashlock)
Newsgroups: comp.lang.ada
Subject: Re: How to Emulate C++ Macro with Static Local Variable?
Date: 25 Sep 2003 10:23:44 -0700
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Stephen Leake <Stephe.Leake@nasa.gov> wrote in message news:<uhe310yvm.fsf@nasa.gov>...
> taashlo@sandia.gov writes:
[snip]
> Just declare each "hidden static" variable directly, and call 'bar'. 
> 
> Yes, it's a few more lines of code, but it's worth it!
> 
> > for (int i = 0; i < 55; ++i) {
> >    switch (wozit()) {
> >       case 1: BAR(10); break;
> >       case 2: BAR(20); break;
> >       case 3: BAR(30); break;
> >    }
> > }
> 
> This would become:
> 
> package foo
>     t_1 : s;
>     t_2 : s;
>     t_3 : s;
> 
> procedure Something is
> begin
>     for i in 1 .. 54 loop
>        case wozit is
>        when 1 =>
>            bar (t_1, 10);
>        when 2 =>
>            bar (t_2, 20);
>        when 3 =>
>            bar (t_3, 30);
>        end case;
>     end loop;
> end Something;
> 
> end foo;
> 
> You may complain this is using "ugly global variables", and it is. So is the
> C code. Using macros to hide doesn't really make it prettier :).

If the problem with this was just the ugliness, then I wouldn't have a
problem! :-)  The real problem with this is that a typo (e.g. t_2
instead of t_3) would be impossible for the compiler to catch, and
would produce a subtle bug where the subsequent behavior of a call
to bar() would depend on which call executed first.

The C++ implementation doesn't have *this* problem.

> A better way is to make 't' part of a state object that is passed in
> to Something, but that's left as an exercise for the reader.

This (I believe) would have the same problem I noted above.

Thank you,
Tad