// In header: <boost/log/expressions/predicates/has_attr.hpp>

class has_attribute<void> {
public:
  // types
  typedef bool result_type;  // Function result_type. 
  typedef void value_type;   // Expected attribute value type. 
  // construct/copy/destruct
  explicit has_attribute(attribute_name const &);
  // public member functions
  result_type operator()(attribute_value_set const &) const;
  result_type operator()(boost::log::record_view const &) const;
};