Getting around Qt's templating restrictions - c++

Question

I want to write a simple flexible ftp server in C++ that can be parametrized with a class to handle the user (check login and password, deliver files etc.) supplied on server initialization.
So I came up with this neat (so I thought) idea:
class FtpDelegate
{
public:
FtpDelegate() {}
virtual ~FtpDelegate() {}
virtual bool login(QString username, QString password) = 0;
// ...
};
class DummyDelegate : public FtpDelegate
{
public:
virtual bool login(QString username, QString password)
{
return true;
}
};
template<class Delegate>
class FtpServer : public QObject, Derived_from<Delegate, FtpDelegate>
{
Q_OBJECT
public:
explicit FtpServer(const QHostAddress &address = QHostAddress::Any,
quint16 port = 21,
QObject *parent = 0);
public slots:
void newConnection();
private:
QTcpServer *server;
QHostAddress address;
};
template <class Delegate>
void FtpServer<Delegate>::newConnection()
{
FtpDelegate *delegate = new Delegate();
new FtpConnection (delegate, server->nextPendingConnection(), address, this);
}
class FtpConnection : public QObject
{
Q_OBJECT
public:
explicit FtpConnection(FtpDelegate *delegate,
QTcpSocket *socket,
const QHostAddress &address,
QObject *parent = 0);
public slots:
void newDataConnection();
private:
QTcpSocket *socket;
QTcpServer *dataServer; // needed to transfer data to user
QTcpSocket *dataSocket;
};
// server initialization
FtpServer<DummyDelegate> ftpServer();
and then (you probably saw that coming) bam!
Error: Template classes not supported by Q_OBJECT
it is likely that there are other errors or misconceptions too as I am only starting to learn the C++ template mechanism (and Qt as well).
My question is: what is the best way to make it work without using ugly hacks like passing function pointers or needing to create a factory implementation for each concrete FtpDelegate's derived class. Maybe there's some clever design pattern I just can't see. Eventually I can rewrite the network mechanism to boost if it is the best option.

Answer 1

It is not possible to create a template Q_OBJECT class (see this and answers).
Instead of using static inheritance, you should use a run-time inheritance, and inject an object inheriting from FtpDelegate class.
It looks like the FtpServer is actually a factory creating connections. From your question, I do not see why it has to be Q_OBJECT class. So you may need to reconsider your design, and simplify that class.
what is the best way to make it work without using ugly hacks like passing function pointers or needing to create a factory implementation for each concrete FtpDelegate's derived class.
The best way may be to have a factory class, which creates instances of FtpDelegate type. But you have so many problems in the posted code, that it is not possible to tell more without knowing all gory details.