Detecting QTcpSocket disconnect with a pointer to QIODevice - qt

Question

I am writing a simple network application that will be sending and receiving TCP messages to a single listener. I use a context class to wrap much of the communication logic as such:
class context : public QObject
{
Q_OBJECT
public:
void setDevice (unique_ptr<QIODevice> device);
void sendMessage (message msg);
signals:
void newMessage (message msg);
private:
unique_ptr<QIODevice> m_device;
}
A generic QIODevice is given to the object instead of the concrete subtype QTcpConnection so that the logic can be tested without a network connection. I expected that any network errors would be presented as IO errors from the QIODevice. However, if the host disconnects, the QIODevice does not emit a signal. The QTcpConnection does emit a signal (disconnected) but the underlying QIODevice remains open, continues to allow reads and writes, and does not appear to give any notification of the error. My confirmation that the socket does see the disconnect is that the error string in QIODevice changes from "Unknown error" to "Socket is not connected"; but checking for an error string seems like a hack that shouldn't be necessary.
Is there actually some way to detect this kind of error with only the base class? If I am required to pass in the QTcpSocket, is there a way that I can still keep unit tests that do not require a client-server connection?