Question

I need to create several custom event classes for a Qt application.

Right now, it looks like I will need to implement the following event type registration code for each event class:

class MyEvent : public QEvent
{
public:
    MyEvent() : QEvent(registeredType())
    {
    }

    static QEvent::Type eventType;

private:
    static QEvent::Type registeredType();
}

QEvent::Type MyEvent::eventType = QEvent::None;

QEvent::Type MyEvent::registeredType()
{
    if (eventType == QEvent::None)
    {
        int generatedType = QEvent::registerEventType();
        eventType = static_cast<QEvent::Type>(generatedType);
    }
    return eventType;
}

Any suggestions on how I can simplify this, or at least hide it with a macro?

Answer 1

That’s what templates are for. They can be used with constant integral parameters, which need to be known at compile time too:

enum EventNames { UpdateEvent,... }

template<EventNames E>
class MyEvent : public QEvent
{
public:
    MyEvent() : QEvent(registeredType())
    {
    }

    static QEvent::Type eventType;

private:
    static QEvent::Type registeredType();
}

The common code lokes like this:

template<EventNames E>
QEvent::Type MyEvent<E>::registeredType()
{
    if (eventType == QEvent::None)
    {
        int generatedType = QEvent::registerEventType();
        eventType = static_cast<QEvent::Type>(generatedType);
    }
    return eventType;
}

Static initialization (beware!) looks like this:

QEvent::Type MyEvent<UpdateEvent>::eventType = QEvent::None;

The code specific for each event type can be implemented as template specialization then.