[Boost-users] [variant] ideas needed

20 Feb 2008

      Hi,

I've the following code:

---8<---
#include <boost/variant.hpp>

#include <iostream>
#include <string>

struct available
      : public boost::static_visitor<bool>
{
      template <typename T, typename U>
      bool operator()( const T&, const U& ) const
      {
          return false; // cannot compare different types
      }

      template <typename T>
      bool operator()( const T& lhs, const T& rhs ) const
      {
          static const std::string NA( "NA" );

          return lhs != NA; //???
      }
};

using namespace boost;
using namespace std;

int main()
{
      variant< double, std::string > R;
      variant< double, std::string > L;

      R = 3.14;
      L = "NA";

      cout << R << endl;
      cout << L << endl;

      bool r = boost::apply_visitor(available(), R );
      cout << ( r ? "true" : "false" ) << endl;

      bool l = boost::apply_visitor(available(), L );
      cout << ( l ? "true" : "false" ) << endl;
}

--->8---

which does not compile. Anyway, from a data parser I get a double or a
"NA" (not available) string. This test shall handle both cases. To
process using the double I have to check if it available (than use
defaults). Unfortunately, the static_visitor<bool> needs two arguments.
How can I get the syntax (or simpler) as used in the example?

Thanks
Olaf