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. -- Dave Abrahams Boost Consulting http://www.boost-consulting.com

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