Boost Exception

#include <boost/exception/exception.hpp>

namespace
boost
    {
    template <class Tag,class T>
    class
    error_info
        {
        public:
    
        typedef T value_type;    
    
        error_info( value_type const & v );    
        value_type const & value() const;
        value_type & value();    
        };    
    
    template <class E, class Tag, class T>
    E const & operator<<( E const & x, error_info<Tag,T> const & v );
    }