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From: Sylvain Pion (pion_at_[hidden])
Date: 20020906 17:56:13
On Fri, Sep 06, 2002 at 06:11:25PM 0400, Herve Bronnimann wrote:
> Just occured to me: If you want to take advantage of compiletime,
> template arguments of the form <unsigned int> would work, and
> specializations for various ints would give you the desired rounding.
>
> Example (the constants are bogus, but you'll get my point):
>
> template <unsigned int decimal_precision>
> struct interval_pi {
> static const double lower = 3; // default version
> static const double upper = 4;
> };
>
> #define BOOST_INTERVAL_SPECIALIZE_PI_CONSTANT(n, l, u) \
> template <> struct interval_pi<n> { \
> static const double lower = l; \
> static const double upper = u; \
> }
>
> BOOST_INTERVAL_SPECIALIZE_PI_CONSTANT(1, 3.1 , 3.2 );
> BOOST_INTERVAL_SPECIALIZE_PI_CONSTANT(2, 3.14 , 3.15 );
> BOOST_INTERVAL_SPECIALIZE_PI_CONSTANT(3, 3.141 , 3.142 );
> BOOST_INTERVAL_SPECIALIZE_PI_CONSTANT(4, 3.1415 , 3.1416 );
> BOOST_INTERVAL_SPECIALIZE_PI_CONSTANT(5, 3.14159 , 3.14160 );
> BOOST_INTERVAL_SPECIALIZE_PI_CONSTANT(6, 3.1415926 , 3.1415927 );
> BOOST_INTERVAL_SPECIALIZE_PI_CONSTANT(7, 3.14159265 , 3.14159266 );
> BOOST_INTERVAL_SPECIALIZE_PI_CONSTANT(8, 3.141592653 , 3.141592654 );
> BOOST_INTERVAL_SPECIALIZE_PI_CONSTANT(9, 3.1415926535 , 3.1415926536 );
>
> std::cout << interval_pi<7>::lower << std::endl;
>
> You can go on to provide all the (precomputed) precisions ever needed by
> users. It's a little less flexible than switches ( ? : ) but it's
> perhaps more readable... ?
Doesn't work.
You miss the point of where the problem is, and Gaby as well it seems.
My problem is not to generate compiletime constants alone, nor to generate
correct constants alone. I want _both_, and _this_ is harder.
The fact is that the standard (always this good ol' 2.13.3.1) says that, since
the (decimal) numbers that you use are not representable (*), then the compiler
will have the choice between the two representable numbers enclosing it, and
can pick either one. Hence it can pick the one I don't want. So it doesn't
work.
Go read again my proposed solution (going through representable numbers) once
you've understood the problem. And then try to finally propose me the working
nicer solution of my dreams :)
 Sylvain (*) 3.1 is NOT representable in binary, whatever value "digits" has, since it has infinite representation in base 2. Same for most, if not all the ones you use above.
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