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

16 Oct 2007


      on Tue Oct 16 2007, "Peng Yu"  wrote:
...
On 10/16/07, David Abrahams  wrote:
...
on Sat Oct 13 2007, "Peng Yu"  wrote:
...
On 10/13/07, John Maddock  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::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 
#include <iostream>
struct fixed_point {
};
template <typename T>
struct my_is_arithmetic : public boost::is_arithmetic<T> {
};
template <>
struct my_is_arithmetic : 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().doit();
}
/* output
1
0
*/
Then should implement a new class that uses my_is_arithmetic in terms
of the library_class_which_I_can_not_change.  If you can't do that, I
think there are no good answers, so use whatever hack gets the job
done. :-)
-- 
Dave Abrahams
Boost Consulting
http://www.boost-consulting.com