Re: [Boost-users] [variant] ideas needed

From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2008-02-20 17:31:31

AMDG

ope wrote:
> 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

Let me make sure that I understand what you want.
If the variant contains a double the result should be true.
If the variant contains the string "NA" the result should be false.
The variant cannot contain a string that is not "NA"

struct available
      : public boost::static_visitor<bool>
{
      bool operator()( const double&) const
      {
          return true;
      }

      bool operator()( const string& lhs) const
      {
          assert(lhs == "NA");

          return false; //???
      }
};

In Christ,
Steven Watanabe