#include <boost/variant.hpp>
#include <boost/lexical_cast.hpp>

#include <string>

typedef boost::variant<int, double, std::string> Variant;

Variant foo();

struct ToString : boost::static_visitor<std::string> {
    template <typename T>
    std::string operator()( const T & t ) const {
        return boost::lexical_cast<std::string>( t );
    }
};

int main() {
    const std::string str = apply_visitor( ToString(), foo() );
}