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Subject: Re: [boost] Boost.Math and Math Constants
From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2011-12-07 23:30:49


AMDG

On 12/07/2011 07:23 PM, 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.
>
> That seems more Boost-y to me, at any rate.
>

Don't go that route please. We did it in
the Units library, and it causes way too
many problems, because it's too clever.
The only thing that acts exactly like
a double is a double. When your custom
type fails to act like a double it's going
to cause surprises.

In Christ,
Steven Watanabe


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