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Subject: Re: [boost] Boost.Math and Math Constants
From: Lars Viklund (zao_at_[hidden])
Date: 2011-12-07 22:39:18

On Thu, Dec 08, 2011 at 03:23:07AM +0000, Brent Spillner wrote:
> On Wed, 7 Dec 2011 15:33:21 Paul A. Bristow wrote:
> >John Maddock has now added a new cunning feature that allows the vast majority of users who just
> >want a built-in double to get the constants quickly as plain variables. So in non-template code,
> >users can write
> >
> > using boost::math::double_constants;
> >  double area =  pi * r * r;
> >
> >and users can read the equations easily.
> >
> >(Float and long double are similarly available in their own namespaces).
> It's a little bit of work up front, but if you define a custom type
> with separate overloads for each arithmetic operator/operand type
> combination (i.e. *(float), *(double), +(float), +(double), etc.) you
> should be able to offer a single namespace with constants that
> auto-promote (or demote) to the right type in expressions. That's
> also easily extensible to user-defined storage classes such as an
> extended precision float, and I think it's more maintainable since the
> using declaration (which is now untyped) can't get out of sync with
> the usage context. To prevent ambiguous overloads (in contexts where
> the constant itself is supplied as an argument to a polymorphic
> function, not in expressions), you can leave just one type (perhaps
> double) as implicitly castable-to, with explicit casts for the others.

A fundamental problem with proxies and things that try to rely on
correct usage of conversion is that the conversion won't kick in when
dealing with templates, as they do not matter when deducing template
parameters. It's very easy to taint your types in such a way that a lot
of machinery goes very very wrong.

Lars Viklund | zao_at_[hidden]

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