Re: templated type and numeric_limits?

From:

Alan Woodland <ajw05@aber.ac.uk>

Newsgroups:

comp.lang.c++

Date:

Mon, 16 Mar 2009 17:51:58 +0000

Message-ID:

<56g296xc1h.ln2@news.aber.ac.uk>

James Kanze wrote:

On Mar 11, 10:15 pm, David Johansen <davejohan...@gmail.com> wrote:

On Mar 11, 12:22 pm, SG <s.gesem...@gmail.com> wrote:

On 11 Mrz., 19:46, Dave Johansen <davejohan...@gmail.com> wrote:

Is it possible to specialized numeric_limits with a templated type?

Sure.

  template<unsigned int DIGITS>
  class BigInt;

  template<unsigned int DIGITS>
  class std::numeric_limits<BigInt<DIGITS> >
  {
    // ...
  };

Thanks for the tip because it worked, but I guess I'm missing
something and I don't get how/why this works. Is the syntax
for a explicit specialization of a class when using a
templated typed different? Or is something else going on that
I'm not realizing?

Because if I do something like this:

class test
{
};

class std::numeric_limits<test>
{
};

Then it doesn't compile without the "template<>", so what am I
not getting?

In order to explicitly specialize a template with a single type
parameter (like std::numeric_limits), you need a type. A class
is a type; a class template is *not*. What SG provided is not
an explicit specialization, but a partial specialization,
something different, which obeys different rules. You could
also use an explicit specialization (using template<>), but only
for a specific type, e.g.:

    template<>
    class std::numeric_limits< BigInt< 200 > >
    {
        // ...
    } ;

SG's suggestion, using partial specialization, is probably what
you want, however.

I'd always assumed that specialising something counted as a declaration,
and hence because the namespace in question is std would be considered
UB. Is that not the case and specialising is permitted or is this an
exception to that?

Alan