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From: "jimmaureenrogers@worldnet.att.net" <jimmaureenrogers@worldnet.att.net>
Newsgroups: comp.lang.ada
Subject: Re: why learn C?
Date: 13 Apr 2007 18:20:22 -0700
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Date: 2007-04-13T18:20:22-07:00
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On Apr 12, 9:47 am, Hyman Rosen <hyro...@mail.com> wrote:
> jimmaureenrog...@worldnet.att.net wrote:
> > one cannot write a C++ template to handle range-specified
>
>  > floating point classes.
>
> Of course this is wrong. Every floating point number that you
> write in a program is in fact a rational number, and therefore
> depending on the purpose those numbers will have in the template,
> they be represented using small sets of integers in a variety of
> ways. The template itself can then assemble these parts into a
> floating-point constant and use it. Or if that's too burdensome,
> references to floating-point variables are permitted, so getting
> what you want looks rather like Ada generic instantiation.
>
> > But you cannot write a template equivalent to:
> > type Voltage_Type is digits 11 range -5.0..5.0;
>
> Using the variable reference approach, we can do
>
> template <typename T, int digits, T const &lo, T const &hi> class FloatRange;
> extern double const vtlo = -5.0;
> extern double const vthi =  5.0;
> typedef FloatRange<double, 11, vtlo, vthi> Voltage_Type;

Your example relies upon passing pre-defined constants for the
parameters
lo and hi. It also allows you to specify whatever type is aliased by
typename T. How does this template ensure that typename T is an alias
for
a floating point type? It appears that T could be instantiated with
any
type, including for instance the String type.

C++ template syntax provides no way to pass float or double literals
for the parameters lo and hi. It also provides no syntax to ensure
that
typename T is a floating point type. These deficiencies render your
example not equivalent to the Ada example. Furthermore, they are
weaknesses that are not a problem in the Ada generic formal parameter
syntax.

Jim Rogers