Re: [Boost-users] [TypeTraits] Is it possible to detect that type is not exist and use default type?

30 Jun 2009


      Thanks!

-----Original Message-----
From: boost-users-bounces@lists.boost.org
[mailto:boost-users-bounces@lists.boost.org] On Behalf Of Steven
Watanabe
Sent: Tuesday, June 30, 2009 3:40 PM
To: boost-users@lists.boost.org
Subject: Re: [Boost-users] [TypeTraits] Is it possible to detect that
type is not exist and use default type?


AMDG

Sakharuk, Vlad (GMI Development) wrote:
...
Here is example:
class one {
    public:
    typedef someclass internal;
};
class two {
// No internal class
};
class default_internal{
};
I would like to be able to write something if it possible:
template<class T, class B = if_exist(T::internal)  T::internal else 
default_internal > class proc { };
So I can use proc with both
proc<one> One;
proc<two> Two;
without specifying derived type?
Use BOOST_MPL_HAS_XXX_TRAIT_DEF with boost::mpl::eval_if.

#include <boost/mpl/has_xxx.hpp>
#include <boost/mpl/eval_if.hpp>

BOOST_MPL_HAX_XXX_TRAIT_DEF(internal)

template<class T>
struct get_internal {
    typedef typename T::internal type;
};

template<
    class T,
    class B = typename boost::mpl::eval_if<has_internal<T>,
        get_internal<T>,
        boost::mpl::identity<default_internal>
    >::type
...
class proc;

In Christ,
Steven Watanabe

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