Re: [boost] Initilialising const static floats in template & MSVC C2864 (forintervals of math constants)

From: Guillaume Melquiond (gmelquio_at_[hidden])
Date: 2002-10-16 04:41:38

On Wed, 16 Oct 2002, Paul A. Bristow wrote:

> Herve Bronnimann [hbr_at_[hidden]] and Sylvain Pion suggested using a template
> method for presenting the smallest interval containing math constants:
>
> > The template solution has advantages : it can simply refuse to compile if you
> > try to ask a digits/radix pair of values which you have not planned.
> > With the initial switch I proposed, you can only get a run-time error.
> >
> > The template solution is also more easily extensible as said : you don't
> > need to have only one big statement in only one file, you can spread the
> > various versions over several files.
> >
> > So I guess you could write your constants like :
> >
> > template <class T, int radix  = std::numeric_limits<T>::radix,
> >                    int digits = std::numeric_limits<T>::digits>
> > class pi;
> >
> > template <>
> > class pi<float, 2, 24>
> > {
> 	static const float lower =   3.1415925025939941406250F;
> 	static const float upper =   3.1415927410125732421875F;
> 	static const float nearest =
> 3.14159265358979323846264338327950288419716939937510F;
>
> };
>
> Sadly this seems not supported by MSVC 7.0:
>
> "error C2864: 'lower' : only const static integral data members can be
> initialized inside a class or struct"  - NO FLOATING POINT!
>
> though it works OK for GCC 2.95 and 3.2.

I'm surprised it worked with gcc 2.95 and 3.2 since I did try such a
solution and it didn't work. I needed to put them out of the classes and
to use functions inside the class to return the global constants. It was
ugly but sufficient.

I seem to remember that it isn't allowed by the Standard to define
floating-point constants inside a class.

> Can any language lawyers advise if this is really a language restriction, or
> simply not implemented by MSVC 7.0?
>
> Is it supported by MSVC 7.1?
>
> Workarounds?
>
> Thanks for any advice.
>
> Dr Paul A Bristow, hetp Chromatography
> Prizet Farmhouse, Kendal, Cumbria, LA8 8AB  UK
> [phone]  Mobile [phone]
> mailto:pbristow_at_[hidden]

Guillaume