message: Add value_type member type to stream
diff: @@ -34,6 +34,12 @@ struct invalid_stream_t {};
  template <class T>
  class stream {
  public:
  + // -- member types -----------------------------------------------------------
  +
  + using value_type = T;
  +
  + // -- constructors and destructors -------------------------------------------
  +
  stream() = default;
  stream(stream&&) = default;
  stream(const stream&) = default;
  @@ -64,6 +70,8 @@ public:
  // nop
  }
  + // -- accessors --------------------------------------------------------------
  +
  /// Returns the unique identifier for this stream.
  inline const stream_id& id() const {
  return id_;
  @@ -74,12 +82,16 @@ public:
  return ptr_;
  }
  + // -- serialization support --------------------------------------------------
  +
  template <class Inspector>
  friend typename Inspector::result_type inspect(Inspector& f, stream& x) {
  return f(meta::type_name("stream"), x.id_);
  }
  private:
  + // -- member variables -------------------------------------------------------
  +
  stream_id id_;
  stream_handler_ptr ptr_;
  };
  