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From: Hyman Rosen <hyrosen@mail.com>
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Subject: Re: why learn C?
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Date: Thu, 12 Apr 2007 15:47:36 GMT
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Date: 2007-04-12T15:47:36+00:00
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jimmaureenrogers@worldnet.att.net wrote:
> I discovered that C++ template language does not allow the
> specification of a floating point value as a template parameter.

In C++, instantiations are considered the same if their
parameters are the same, and the standards committee did
not want the burden of defining T<3.0*(1.0/3.0)> vis a vis
T<1.0>.

> one cannot write a C++ template to handle range-specified
 > floating point classes.

Of course this is wrong. Every floating point number that you
write in a program is in fact a rational number, and therefore
depending on the purpose those numbers will have in the template,
they be represented using small sets of integers in a variety of
ways. The template itself can then assemble these parts into a
floating-point constant and use it. Or if that's too burdensome,
references to floating-point variables are permitted, so getting
what you want looks rather like Ada generic instantiation.

> But you cannot write a template equivalent to:
> type Voltage_Type is digits 11 range -5.0..5.0;

Using the variable reference approach, we can do

template <typename T, int digits, T const &lo, T const &hi> class FloatRange;
extern double const vtlo = -5.0;
extern double const vthi =  5.0;
typedef FloatRange<double, 11, vtlo, vthi> Voltage_Type;