Passing a QLocalSocket* to a method expecting a QIODevice*

Question

Perhaps I'm being over-ambitious, but I'm trying to write a server program which can accept connections over both QLocalSockets and QTcpSockets. The concept is to have a 'nexus' object with both a QLocalServer and QTcpServer listening for new connections:

    Nexus::Nexus(QObject *parent)
        : QObject(parent)
    {
        // Establish a QLocalServer to deal with local connection requests:
        localServer = new QLocalServer;

        connect(localServer, SIGNAL(newConnection()),
                this,        SLOT(newLocalConnection()));
        localServer -> listen("CalculationServer");


        // Establish a UDP socket to deal with discovery requests:
        udpServer = new QUdpSocket(this);
        udpServer -> bind(QHostAddress::Any, SERVER_DISCOVERY_PORT);
        connect(udpServer, SIGNAL(readyRead()),
                this,      SLOT(beDiscovered()));

        // Establish a QTcpServer to deal with remote connection requests:
        tcpServer = new QTcpServer;

        connect(tcpServer, SIGNAL(newConnection()),
                this,      SLOT(newTcpConnection()));
        tcpServer -> listen(QHostAddress::Any, SERVER_COMMAND_PORT);
    }

... and then separate slots which establish a server object, whose constructor takes a pointer to a QIODevice. In theory, this ought to work because both QLocalSocket and QTcpSocket inherit QIODevice. Here is the newLocalConnection slot, for example:

void Nexus::newLocalConnection()
{
    // Create a new CalculationServer connected to the newly-created local socket:
    serverList.append(new CalculationServer(localServer -> nextPendingConnection()));

    // We don't allow more than one local connection, so stop listening on the server:
    localServer -> close();
}

The problem is that this won't compile, giving an error:

error C2664: 'CalculationServer::CalculationServer(QIODevice *,QObject *)' : cannot convert parameter 1 from 'QLocalSocket *' to 'QIODevice *' 1> Types pointed to are unrelated; conversion requires reinterpret_cast, C-style cast or function-style cast

Now the types pointed to are clearly not unrelated, and elsewhere in my code I have no problems at all with actions like:

QLocalSocket *socket = new QLocalSocket;
QIODevice    *server = new QIODevice;

server = socket;

... so can anyone tell me why the compiler has a problem with this? Is there a way that I can make the constructor accept the QLocalServer*? I suppose there is the nuclear option of getting the constructor to take a void pointer plus an extra variable to tell it what it's being sent, so it can then recast the void pointer to either a QLocalSocket or QTcpSocket, but I feel uncomfortable resorting to reinterpret_cast on what looks like it ought to be a straightforward bit of C++ polymorphism.

Regards,

Stephen.

Answer 1

The most likely reason is that you have forgotten to #include <QLocalSocket> in the source file where the error occurs.