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From: dewar@gnat.com (Robert Dewar)
Newsgroups: comp.lang.ada
Subject: Re: Localized Variable Declaration
Date: 31 May 2002 17:58:43 -0700
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David Rasmussen <pinkfloydhomer@yahoo.com> wrote in message news:<3CF78D3D.3030400@yahoo.com>...
> You can do the same thing in C++ by just doing
> 
> int whatever(int foo)
> {
>      ...
>      ...
>      { // local scope starts here
>           int local = 42;
>           ....
>      } // and ends here
>      ...
> }

Yes, sure, and the interesting thing is that in my experience it is far
more common to see extensive use of local declare blocks in Ada than it
is in C. I don't know why this is, because indeed narrowing the scope of
variables is an important issue for readability.

Personally I like a local declarative block being announced clearly with
declare, for me the use of { is too overloaded. For example:

   while (*a++ = *b++)
   {
      stuff
   }

and

   while (*a++ = *b++);
   {  
      stuff
   }

are uncomfortably close, and I prefer having a very definite keyword that
announces a local declare block.

As for not allowing declarations in the middle of statements, this is
very deliberate for several reasons.

Two significant reasons are:

The semantics of embedded declarations is nasty with respect to whether they
are allowed within control structures, what happens if a goto skips over them,
etc.

Forcing the use of a declare forces the writer to think about the scope of
the declarations that are introduced and end the scopes appropriately. The
C++ approach makes it easier for the programmer to ignore this, and by default
let the scope extend to the end.

Note that it is definitely not the case that C++ can do more than Ada here,
just that you have to be noisier about it in Ada. That's a choice we often
make, because we unconditionally favor the reader over the writer (for
example, consider unchecked conversion vs a cast).