Qt Signals: QueuedConnection vs. DirectConnection

Understanding the difference between DirectConnection and QueuedConnection is crucial for effective signal usage in Qt. These connection types govern how signals are emitted and received, particularly when dealing with objects residing in different threads.

DirectConnection

When using a DirectConnection, signal emission triggers an immediate invocation of the connected slot. This means the slot method will be executed in the thread of the object emitting the signal. This can be problematic if the slot method is not thread-safe, potentially leading to subtle bugs that are difficult to identify.

QueuedConnection

In contrast, QueuedConnection employs a different approach. When a signal is emitted using this connection type, an event is posted to the event loop of the object receiving the signal. This event is subsequently queued and executed whenever control returns to the event loop. This method ensures proper synchronization between threads and guarantees that slot methods are invoked in a thread-safe manner.

When to Use

The choice between DirectConnection and QueuedConnection primarily depends on the thread affinity of the objects involved.

• DirectConnection: Suitable for objects residing in the same thread, especially when thread-safety is ensured.
• QueuedConnection: Preferred for objects in different threads to avoid potential thread-safety issues.

Implementation Example

Consider two QObjects, A and B, which are located on distinct threads.QObject A:

class A : public QObject {
    Q_OBJECT
public:
    void emitSignal() {
        emit somethingChanged();
    }
};

QObject B:

class B : public QObject {
    Q_OBJECT
public:
    void handleChange() {
        // Implement slot logic
    }
};

If A and B are on different threads, the following code establishes a QueuedConnection:

QObject::connect(A, SIGNAL(somethingChanged()), B, SLOT(handleChange()), Qt::QueuedConnection);

This ensures that the handleChange slot will be executed in its own event loop, providing a safe and synchronized mechanism for inter-thread communication.