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Newsgroups: comp.lang.ada
Subject: Re: why learn C?
From: Georg Bauhaus <bauhaus@futureapps.de>
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Date: Mon, 02 Apr 2007 12:54:06 +0200
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Date: 2007-04-02T12:49:40+02:00
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On Mon, 2007-04-02 at 00:43 -0700, kevin cline wrote:
> On Mar 30, 9:41 pm, "jimmaureenrog...@worldnet.att.net"

> > type My_Int is range -10..10;
> >
> > You can provide run time checking in any language. In C++ you would
> > need to define a class for My_Int.
> 
> Actually, you would define a template class RangedInt, and then write
> something like:
> 
>   typedef RangedInt<-10,10> My_Int;
> 
> > The assignment operator would need
> > to be re-defined to check for the range, and throw an exception when
> > the range limits are violated.
> 
> Right.  One eight-line class to replicate the Ada ranged integer type.

I'd add a few more lines to a RangedInt in order to have iteration
support, explicit range test, 'Valid, using integer operations with
compatible subtypes, computing in the base type, and having template
compile/link time checks for array-style data structures built on
top of the type properties of RangedInt. (Type casts?)

And then, unfortunately, C++ doesn't have dynamic types
(unless I'm missing something):

template <int left>
int foo(int n)
{
  typedef class RangedInt<left, n> Int;
  Int x(12);
  // ...
  return *x;
}

Int would have to be visible only from within foo(),
its values would have to be bound by n.
Objects of type Int would only exist within foo,
and be accessible while foo executes (no & allowed).
With Ada 2005, this is true of all locally extensible types.

But OTOH, is there any good reason to replace the handful
of machine oriented base types that C++ has (inherited
from C) and instead use something that is more predictable
and more expressive?
I think, yes, when apparently students can do their homework
significantly more successfully when they can choose names, values,
named values and types in the ways that have inspired the Ada type
system.
(According at least to a seemingly unbiased report presented here
some weeks ago, about an embedded systems class.)