Re: some questions re. Ada/GNAT from a C++/GCC user

[John G. Volan <John_Volan@ccmail.dayton.saic.com>]

In article <315AC5E7.3A77@escmail.orl.mmc.com>
Ted Dennison, dennison@escmail.orl.mmc.com writes:

>If you want to declare variables later in the code just 
>use a declare block:
>   declare
>      Var1 : My_Type;
>      ...
>   begin
>      Var1 := My_Value;
>      ...
>   end;
>
>This has the added benefit of limiting the scope of Var1 to the
>begin..end  block of code.

There is another benefit, but it isn't obvious unless you take into
account the concurrency aspects of Ada:

A C programmer would complain about having to introduce an extra
begin/end block in order to introduce variables.  Isn't the scope
of a variable well-defined in C, even without this extra baggage?
It just extends from the declaration to the end of whatever block
you're already in.

An Ada programmer would counter that, in Ada, some variables might be
instances of _task_ types.  By definition, a task object gets created
when its declaration is elaborated, but it does not get _activated_
(i.e., it doesn't start executing its statements) until you hit the
"begin" of the enclosing block.  This guarantees that everything in the
enclosing declarative region is elaborated before any a task starts up.
In fact, _all_ the tasks in a declarative region are activated by the
"begin", simultaneously.  (Or virtually simultaneously. The point is,
Ada does not prescribe any particular order of activation, and does not
preclude truly simultaneous activation if the underlying run time
environment can support it.)  In effect, if a declarative region
contains tasks, then there is a synchronization point at the "begin"
which the Ada programmer can rely on.

C, of course, has no concurrency abstractions built in (nor does C++,
last time I checked), so, gee, I guess this isn't an issue in C. :-)

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