[Boost-users] boost.variant as argument

From: Olaf Peter (ope-devel_at_[hidden])
Date: 2008-04-09 14:39:26

Hi,

I've wrapped a variant<double,bool> into a tuple. Unpacking the tuple 
works as far I can see. I want to print the tuple for doubles with one 
pretty print functor and for booleans with another functor (not shown here).

Anyway, variant_print takes a long argument list as the error message 
shows. How can I pass it convinient?
I guess, inside the variant_print I have to use the vistor to dispatch 
the print functors, isn't?

BTW, fusion could be used? On debian/linux 1.35 is not released yet, so 
I use 1.34.1.

Thanks,
Olaf

---8<---
#include <boost/variant.hpp>
#include <boost/tuple/tuple.hpp>
#include <boost/tuple/tuple_io.hpp>
#include <iostream>

struct pretty_print
{
     pretty_print( double d ) : m_d( d ) { }

     std::ostream& print_on( std::ostream& os ) const
     {
         os << m_d;
         return os;
     }

     double m_d;
};

std::ostream& operator<<( std::ostream& os, const pretty_print& p )
{
     return p.print_on( os );
}

struct variant_print
{
     // ?????????
     variant_print(::boost::variant<double, bool>& v)
     { }

};

using namespace std;
using namespace boost::tuples;
using namespace boost;

int main()
{
     typedef variant<double, bool>  double_variant;
     typedef tuple<double_variant>  double_tuple;

     double_tuple v1;
     double_tuple v2;

     get<0>( v1 ) = 3.14;
     get<0>( v2 ) = true;

     // This works
     cout << pretty_print( get<double>( get<0>( v1 ) ) ) << endl;
     // This not
     cout << variant_print( get<0>( v1 ) ) << endl;
}

--->8---

$ LANG=en g++ -Wall tuple.cpp -o tuple && ./tuple
tuple.cpp: In function 'int main()':
tuple.cpp:55: error: no match for 'operator<<' in 'std::cout << 
variant_print(((boost::variant<double, bool, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, 
boost::detail::variant::void_>&)((boost::variant<double, bool, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, 
boost::detail::variant::void_>*)boost::tuples::get [with int N = 0, HT = 
boost::variant<double, bool, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_>, TT = 
boost::tuples::null_type](((boost::tuples::cons<boost::variant<double, 
bool, boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_>, 
boost::tuples::null_type>&)(& 
v1.boost::tuples::tuple<boost::variant<double, bool, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_>, 
boost::tuples::null_type, boost::tuples::null_type, 
boost::tuples::null_type, boost::tuples::null_type, 
boost::tuples::null_type, boost::tuples::null_type, 
boost::tuples::null_type, boost::tuples::null_type, 
boost::tuples::null_type>::<anonymous>))))))'
...