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

Subject: Re: [Boost-users] [TypeTraits] Is it possible to detect that typeis not exist and use default type?
From: Sakharuk, Vlad \(GMI Development\) (Vlad_Sakharuk_at_[hidden])
Date: 2009-06-30 15:53:21

Thanks!

        -----Original Message-----
        From: boost-users-bounces_at_[hidden]
[mailto:boost-users-bounces_at_[hidden]] On Behalf Of Roman
Perepelitsa
        Sent: Tuesday, June 30, 2009 3:32 PM
        To: boost-users_at_[hidden]
        Subject: Re: [Boost-users] [TypeTraits] Is it possible to detect
that typeis not exist and use default type?
        
        
        2009/6/30 Sakharuk, Vlad (GMI Development)
<Vlad_Sakharuk_at_[hidden]>
        

                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?
                 

        #include <iostream>
        #include <boost/mpl/has_xxx.hpp>
        #include <boost/mpl/eval_if.hpp>
        #include <boost/mpl/identity.hpp>
        
        
        struct someclass {
          static void hello() {
            std::cout << "someclass" << std::endl;
          }
        };
        
        
        struct one {
         public:
          typedef someclass internal;
        };
         
        struct two {};
         
        struct default_internal {
          static void hello() {
            std::cout << "default_internal" << std::endl;
          }
        };
        
        
        BOOST_MPL_HAS_XXX_TRAIT_DEF(internal)
        
        
        
        // Here Internal is either T::internal
        // if it exists or default_internal.
        template <class T, class Internal>
        struct proc_impl {
          static void hi() {
            Internal::hello();
          }
        };
        
        
        template <class T>
        struct get_internal {
          typedef typename T::internal type;
        };
        
        
        template <class T>
        struct proc :
          proc_impl<
            T,
            typename boost::mpl::eval_if<
              has_internal<T>,
              get_internal<T>,
              boost::mpl::identity<default_internal>
	    >::type
	  > {
        };
        
        
        int main() {
          proc<one>::hi();
          proc<two>::hi();
        }

        Roman Perepelitsa.

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