// In header: <boost/log/attributes/function.hpp>
template<typename R> 
class function : public attribute {
public:
  // types
  typedef R value_type;  // The attribute value type. 
  // member classes/structs/unions
  // Base class for factory implementation.
  class impl : public attribute::impl {
  };
  // Factory implementation.
  template<typename T> 
  class impl_template : public function< R >::impl {
  public:
    // construct/copy/destruct
    explicit impl_template(T const &);
    // public member functions
    virtual attribute_value get_value();
  };
  // construct/copy/destruct
  template<typename T> explicit function(T const &);
  explicit function(cast_source const &);
};