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Date: Tue, 08 Jul 2003 09:59:15 -0400
From: Hyman Rosen <hyrosen@mail.com>
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Subject: Re: proposal for new assignment operators
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Date: 2003-07-08T09:59:15-04:00
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James Rogers wrote:
> This is interesting. Not too long ago in comp.lang.c++.moderated I was
> told floats cannot be used as template parameters. Was I told correctly?

A "typename" template parameter must be matched by an actual argument
which is a type, and this can be any type, including any primitive
type. Template parameters may also be constant values rather than types,
and in that case, those parameters cannot be floating point.

> I was surprised when I was told that. I have wondered if it is true, and
> if so, why.

Template parameters are used in the linkage of instantiated templates
(loosely speaking, the parameters are interpolated into the name of the
instantiation), and template arguments can be constant expressions.
Allowing floating-point parameters would lead to questions of equivalence
of instantiations. In my opinion that's not such a terrible burden, and I
would have allowed it, but the standard doesn't. Some compilers offer it
as an extension.

As a sample of the problem, consider
     template <float X> char *f() { static char c; return &c; }
     bool b = f<1./3.>() == f<1./6.+1./6.>();
How do you specify what the value of 'b' should be?