[Boost-users] Looking for lazy if_

From: Agoston Bejo (gusz1_at_[hidden])
Date: 2004-12-21 18:46:46

Hi!
I'm looking for some "lazy" version of boost::mpl::if_. I'm not very much
into MPL, so I

don't really know either if such a function exists in boost or whether it
can be easily

constructed.
Platform: VC++7.1

Thx in advance,
Agoston

This example shows you how it may cause problems that both "branches" of if_
get evaluated:

//------------------------------------------------------------
#include <iostream>
#include <vector>
#include <boost/mpl/if.hpp>
#include <boost/type_traits.hpp>

using namespace std;
using namespace boost;
using namespace mpl;

template<typename T>
struct t_type
{
 typedef typename if_<
  is_arithmetic<T>,
  T, typename T::value_type>::type  // ERROR!
 type;
};

int _tmain(int argc, _TCHAR* argv[])
{
 cout << typeid(t_type<int>::type).name() << endl; // 'int'
 cout << typeid(t_type<vector<double> >::type).name()
   << endl; // 'double'
 return 0;
}

//------------------------------------------------------------
Error message:

error C2825: 'T::value_type': cannot form a qualified name
see reference to class template instantiation 't_type<T>' being compiled
        with
        [
            T=int
        ]