enum Qt::ConnectionType

This enum describes the types of connection that can be used between signals and slots. In particular, it determines whether a particular signal is delivered to a slot immediately or queued for delivery at a later time.

Constant	Value	Description
Qt::DirectConnection	1	When emitted, the signal is immediately delivered to the slot.
Qt::QueuedConnection	2	When emitted, the signal is queued until the event loop is able to deliver it to the slot.
Qt::BlockingQueuedConnection	4	Same as QueuedConnection, except that the current thread blocks until the slot has been delivered. This connection type should only be used for receivers in a different thread. Note that misuse of this type can lead to deadlocks in your application.
Qt::AutoConnection	0	If the signal is emitted from the thread in which the receiving object lives, the slot is invoked directly, as with Qt::DirectConnection; otherwise the signal is queued, as with Qt::QueuedConnection.

With queued connections, the parameters must be of types that are known to Qt's meta-object system, because Qt needs to copy the arguments to store them in an event behind the scenes. If you try to use a queued connection and get the error message

QObject::connect: Cannot queue arguments of type 'MyType'

call qRegisterMetaType() to register the data type before you establish the connection.