Re: [boost] RE: math constant - generic function circle_area example.

From: Joerg Walter (jhr.walter_at_[hidden])
Date: 2003-01-23 19:27:05

Hi Paul,

you wrote:

> > I've been looking into an earlier version of the proposed math constants
> > before and asked myself how to implement a generic function like
> >
> > template<class T>
> > T circle_area (const T &radius) {
> >     return math_constants<T>::pi * radius * radius;
> > }
> >
> > How should this be done?
> >
> > Thanks,
> >
> > Joerg
>
> Attached is an example using Michael Kenniston's Kunning Function
constants.
>
> Briefly
>
> template <typename T>
> T circle_area(const T& radius)
> {
>   // Usage example: circle_area<double>( 2.) // Explicit type double.
>   // or             circle_area(2.F) // Implicit type float.
> return boost::math::constant< T, boost::math::pi_tag >() * radius *
radius;
> }

OK. I've tried to compile this with GCC 3.2.1 and after applying some
remedies like

<for example>
#ifdef LATER
  // Define constant is namespaces to hold three builtin floating-point
representations.
  namespace float_constants
  {
   constant< float, pi_tag > const pi;
  }
  namespace double_constants
  {
   constant< double, pi_tag > const pi;
  }
  namespace long_double_constants
  {
   constant< long double, pi_tag > const pi;
  }
#endif
</for example>

it compiled and executed giving the expected results. How is this solution
related to your latest (Dec. 12) upload at groups.yahoo.com/group/boost?

> It compiles with MSVC 7.0 (but not 6 - see MK's original example for why
not).
> (long double == double for MSVC, so long double not fully
testable/useful).
>
> It seems to do the trick, without too many surprises.  I have displayed
the
> output using the 17 significant digits for double so one can see the
difference
> between a float pi 3.1415927410125732 and a double pi 3.1415926535897931
(even
> though only 9 are really significant for float).
>
> cout << "circle_area<float>(1.) = "  << circle_area<float>(1.) << endl; //
> Explicit type float.
> cout << "circle_area<double>(1.) = "  << circle_area<double>(1.) << endl;
//
> Explicit type double.
> cout << "circle_area(1.F) = "  << circle_area(1.F) << endl; // Implicit
type
> float.
> cout << "circle_area(1.) = "  << circle_area(1.) << endl; // Implicit type
> double.
> cout << "circle_area(1.L) = "  << circle_area(1.L) << endl; // implicit
type
> long double.
> cout << "circle_area<double>(1.F) = "  << circle_area<double>(1.F) <<
endl; //
> Explicit over-rides implicit.
>
> And silly types like int fail helpfully.
>   // cout << "circle_area(1) = "  << circle_area(1)<< endl; // Implicit
int -
> does not link!
>   // cout << "circle_area<int>(1.) = "  << circle_area<int>(1.)<< endl; //
> Explicit int - does not compile!
>
> Output is
>
> Test test_circle_area.cpp Thu Jan 23 00:06:28 2003
> float pi  = 3.14159274
> double pi = 3.1415926535897931
> float pi  = 3.1415927410125732
> circle_area<float>(1.) = 3.1415927410125732
> circle_area<double>(1.) = 3.1415926535897931
> circle_area(1.F) = 3.1415927410125732
> circle_area(1.) = 3.1415926535897931
> circle_area(1.L) = 3.1415926535897931
> circle_area<double>(1.F) = 3.1415926535897931
> circle_area<long double>(1.) = 3.1415926535897931
> circle_area<float>(2.) = 12.566370964050293
> circle_area<double>(2.) = 12.566370614359172
> circle_area(2.F) = 12.566370964050293
> circle_area(2.) = 12.566370614359172
> circle_area(2.L) = 12.566370614359172
> circle_area<double>(2.F) = 12.566370614359172
> boost::math::constant< float, boost::math::pi_tag >() 3.1415927410125732
>
> I haven't looked at any assembler to check on efficiency, but believe/hope
from
> previous examples that it will be optimal if inlined.
>
> Does this look sensible/useful?

Only if I learn to see the relation to your latest upload (which I'm
assuming your review request is related to).

Best,

Joerg