// In header: <boost/log/attributes/counter.hpp>
template<typename T> 
class counter : public attribute {
public:
  // types
  typedef T value_type;  // A counter value type. 
  // member classes/structs/unions
  // Factory implementation.
  class impl : public attribute::impl {
  public:
    // construct/copy/destruct
    impl(value_type, value_type) noexcept;
    // public member functions
    virtual attribute_value get_value();
  };
  // construct/copy/destruct
  explicit counter(value_type = (value_type) 0, value_type = (value_type) 1);
  explicit counter(cast_source const &);
};