QObject::connect只有4个参数吗？其实不是,请看它的定义：
QObject::connect(sender, signal, receiver, method, Qt::ConnectionType type = Qt::AutoConnection)
可见实际它有5个参数，只是最后一个参数有默认值，所以平常使用时很少触及。前4个参数都很熟悉，第五个参数是一个enum Qt::Connection Type类型的,它一共有六个值,下面是Qt Assistant中给出的明确说明：
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::AutoConnection | 0 | (default) Same as DirectConnection, if the emitter and receiver are in the same thread. Same as QueuedConnection, if the emitter and receiver are in different threads. |
| Qt::DirectConnection | 1 | The slot is invoked immediately, when the signal is emitted. |
| Qt::QueuedConnection | 2 | The slot is invoked when control returns to the event loop of the receiver's thread. The slot is executed in the receiver's thread. |
| Qt::BlockingQueuedConnection | 4 | Same as QueuedConnection, except the current thread blocks until the slot returns. This connection type should only be used where the emitter and receiver are in different threads. Note: Violating this rule can cause your application to deadlock. |
| Qt::UniqueConnection | 0x80 | Same as AutoConnection, but the connection is made only if it does not duplicate an existing connection. i.e., if the same signal is already connected to the same slot for the same pair of objects, then the connection will fail. This connection type was introduced in Qt 4.6. |
| Qt::AutoCompatConnection | 3 | The default type when Qt 3 support is enabled. Same as AutoConnection but will also cause warnings to be output in certain situations. |