Re: [Boost-users] How to add a type to is_arithmetic

From: Peng Yu (pengyu.ut_at_[hidden])
Date: 2007-10-16 15:43:36

On 10/16/07, David Abrahams <dave_at_[hidden]> wrote:
>
> on Sat Oct 13 2007, "Peng Yu" <pengyu.ut-AT-gmail.com> wrote:
>
> > On 10/13/07, John Maddock <john_at_[hidden]> wrote:
> >> Peng Yu wrote:
> >> > Hi,
> >> >
> >> > Suppose I define a class fixed_point as
> >> >
> >> > class fixed_point{
> >> > ....some operations are defined here
> >> >
> >> > private:
> >> > int _integer_part;
> >> > unsigned _fractional_part
> >> > }
> >> >
> >> > I want is_arithmetic<fixed_point>::type to be true_
> >>
> >> The aim of is_arithmetic is that it returns true only for built-in
> >> arithmetic types: in other words it's a type introspection tool.  If you
> >> specialise it for UDT's you will very likely break other code (for example
> >> has_trivial_XXX assumes that all arithmetic types have trivial
> >> construct/asign/copy/destroy operations).  Probably the correct traits to be
> >> specialising (and using to detect "numbers") is std::numeric_limits.
> >
> > Unfortunately, there are not types that contain constants in
> > std::numeric_limits as in the boost type_traits library. If I want do
> > metaprogramming based the types, I can not used numeric_limits. You
> > have a point that specialize is_arithmetic might break other code. But
> > are there any other walk around for this problem of adding user
> > defined types?
>
> Yes.
>
>         template <class T>
>         struct my_is_arithmetic : boost::is_arithmetic<T> {};
>
> now use my_is_arithmetic and specialize it for fixed_point.

I assume that you suggest me to use  my_is_arithmetic in the library
class. However, my question was that I do not have the freedom to
change the library (as discussed in the later message in this thread,
dated Oct 16, 2007 11:20 AM). The following code shows the fact.

#include <boost/type_traits.hpp>
#include <iostream>

struct fixed_point {
};

template <typename T>
struct my_is_arithmetic : public boost::is_arithmetic<T> {
};

template <>
struct my_is_arithmetic<fixed_point> : public boost::is_arithmetic<int> {
};

template <typename T>
struct library_class_which_I_can_not_change {
  void doit() const {
    std::cout << boost::is_arithmetic<T>::type::value << std::endl;
  }
};

int main() {
  library_class_which_I_can_not_change<int>().doit();
  library_class_which_I_can_not_change<fixed_point>().doit();
}

/* output
1
0
*/