// In header: <boost/log/attributes/constant.hpp>
template<typename T> 
class constant : public attribute {
public:
  // types
  typedef T value_type;  // Attribute value type. 
  // member classes/structs/unions
  // Factory implementation.
  class impl : public attribute_value_impl< value_type > {
  public:
    // construct/copy/destruct
    explicit impl(value_type const &);
    explicit impl(value_type &&);
  };
  // construct/copy/destruct
  explicit constant(value_type const &);
  explicit constant(value_type &&);
  explicit constant(cast_source const &);
  // public member functions
  value_type const & get() const;
};