我們這裡不講訊號和槽的概念，請各自網路上搜索。但是很多人可能沒想過，emit一個訊號後，訊號的引數是如何傳遞到槽裡面。當然，支援訊號和槽，物件就必須要有元系統，通過元系統來傳遞引數。不過這裡有兩個問題需要考慮到，否則引數是無法正確傳遞的。

1，如果傳遞的是自定義資料型別

對於自定義資料型別，需要宣告該資料型別為元資料，qRegisterMetaType

2，我們知道，在connect訊號和槽的時候，可以指定是連線型別，比如，direct和queue，當然還有其他的。如果是採用佇列方式，引數是會被放在佇列的。那麼這裡就涉及到資料的拷貝，於是我們需要實現自定義資料型別的拷貝函式，否則資料將無法正確地拷貝到佇列，也就無法正確的傳遞了。

個人覺得這兩點是必須要知道的，否則訊號與槽就無法正確的使用。不過我這裡並沒有詳細描述。之後有時間再補充。謝謝

附Qt文件：

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.


ConstantValueDescription
Qt::AutoConnection0(Default) If the receiver lives in the thread that emits the signal, Qt::DirectConnection is used. Otherwise, Qt::QueuedConnection is used. The connection type is determined when the signal is emitted.
Qt::DirectConnection1The slot is invoked immediately when the signal is emitted. The slot is executed in the signalling thread.
Qt::QueuedConnection2The 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::BlockingQueuedConnection3Same as Qt::QueuedConnection, except that the signalling thread blocks until the slot returns. This connection must not be used if the receiver lives in the signalling thread, or else the application will deadlock.
Qt::UniqueConnection0x80This is a flag that can be combined with any one of the above connection types, using a bitwise OR. When Qt::UniqueConnection is set, QObject::connect() will fail if the connection already exists (i.e. if the same signal is already connected to the same slot for the same pair of objects). This flag was introduced in Qt 4.6.
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.


When using signals and slots with multiple threads, see Signals and Slots Across Threads.