• Qt Slot Parameter
• Qt Signal Slot Parameter

New-style Signal and Slot Support¶ This section describes the new style of connecting signals and slots introduced in PyQt4 v4.5. One of the key features of Qt is its use of signals and slots to communicate between objects. Their use encourages the development of reusable components. A signal is emitted when something of potential interest. Not only you can now use typedef or namespaces properly, but you can also connect signals to slots that take arguments of different types if an implicit conversion is possible. In the following example, we connect a signal that has a QString as a parameter to a slot that takes a QVariant. The Qt signals/slots and property system are based on the ability to introspect the objects at runtime. Introspection means being able to list the methods and properties of an object and have all kinds of information about them such as the type of their arguments. QtScript and QML would have hardly been possible without that ability.

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Unlike a console mode application, which is executed in a sequential manner, a GUI based application is event driven. Functions or methods are executed in response to user’s actions like clicking on a button, selecting an item from a collection or a mouse click etc., called events.

Widgets used to build the GUI interface act as the source of such events. Each PyQt widget, which is derived from QObject class, is designed to emit ‘signal’ in response to one or more events. The signal on its own does not perform any action. Instead, it is ‘connected’ to a ‘slot’. The slot can be any callable Python function.

In PyQt, connection between a signal and a slot can be achieved in different ways. Following are most commonly used techniques −

A more convenient way to call a slot_function, when a signal is emitted by a widget is as follows −

Suppose if a function is to be called when a button is clicked. Here, the clicked signal is to be connected to a callable function. It can be achieved in any of the following two techniques −

or

Example

In the following example, two QPushButton objects (b1 and b2) are added in QDialog window. We want to call functions b1_clicked() and b2_clicked() on clicking b1 and b2 respectively.

When b1 is clicked, the clicked() signal is connected to b1_clicked() function

When b2 is clicked, the clicked() signal is connected to b2_clicked() function

Example

The above code produces the following output −

Output

How often is a an object copied, if it is emitted by a signal as a const reference and received by a slot as a const reference? How does the behaviour differ for direct and queued signal-slot connections? What changes if we emit the object by value or receive it by value?
Nearly every customer asks this question at some point in a project. The Qt documentation doesn’t say a word about it. There is a good discussion on stackoverflow, which unfortunately leaves it to the reader to pick the right answer from all the answers and comments. So, let’s have a systematic and detailed look at how arguments are passed to signals and slots.

Setting the Stage

For our experiments, we need a copyable class that we will pass by const reference or by value to signals and slots. The class – let’s call it Copy – looks as follows.

The copy constructor and the assignment operator simply perform a member-wise copy – like the compiler generated versions would do. We implement them explicitly to set breakpoints or to print debugging messages. The default constructor is only required for queued connections. We’ll learn the reason later.
We need another class, MainView, which ultimately derives from QObject. MainView provides the following signals and slots.

MainView provides four signal-slot connections for each connection type.

The above code is used for direct connections. For queued connections, we comment out the first line and uncomment the second and third line.
The code for emitting the signals looks as follows:

Direct Connections

sendConstRef => receiveConstRef

We best set breakpoints in the copy constructor and assignment operator of the Copy class. If our program only calls emit sendConstRef(c), the breakpoints are not hit at all. So, no copies happen. Why?
The result is not really surprising, because this is exactly how passing arguments as const references in C++ works and because a direct signal-slot connection is nothing else but a chain of synchronous or direct C++ function calls.
Nevertheless, it is instructive to look at the chain of function calls executed when the sendConstRef signal is emitted.

The meta-object code of steps 2, 3 and 4 – for marshalling the arguments of a signal, routing the emitted signal to the connected slots and de-marshalling the arguments for the slot, respectively – is written in such a way that no copying of the arguments occurs. This leaves us with two places, where copying of a Copy object could potentially occur: when passing the Copy object to the functions MainView::sendConstRef or MainView::receiveConstRef.
These two places are governed by standard C++ behaviour. Copying is not needed, because both functions take their arguments as const references. There are also no life-time issues for the Copy object, because receiveConstRef returns before the Copy object goes out of scope at the end of sendConstRef.

sendConstRef => receiveValue

Based on the detailed analysis in the last section, we can easily figure out that only one copy is needed in this scenario. When qt_static_meta_call calls receiveValue(Copy c) in step 4, the original Copy object is passed by value and hence must be copied.

sendValue => receiveConstRef

One copy happens, when the Copy object is passed by value to sendValue by value.

sendValue => receiveValue

This is the worst case. Two copies happen, one when the Copy object is passed to sendValue by value and another one when the Copy object is passed to receiveValue by value.

Queued Connections

A queued signal-slot connection is nothing else but an asynchronous function call. Conceptually, the routing function QMetaObject::activate does not call the slot directly any more, but creates a command object from the slot and its arguments and inserts this command object into the event queue. When it is the command object’s turn, the dispatcher of the event loop will remove the command object from the queue and execute it by calling the slot.
When QMetaObject::activate creates the command object, it stores a copy of the Copy object in the command object. Therefore, we have one extra copy for every signal-slot combination.
We must register the Copy class with Qt’s meta-object system with the command qRegisterMetaType('Copy'); in order to make the routing of QMetaObject::activate work. Any meta type is required to have a public default constructor, copy constructor and destructor. That’s why Copy has a default constructor.
Queued connections do not only work for situations where the sender of the signal and the receiver of the signal are in the same thread, but also when the sender and receiver are in different threads. Even in a multi-threaded scenario, we should pass arguments to signals and slots by const reference to avoid unnecessary copying of the arguments. Qt makes sure that the arguments are copied before they cross any thread boundaries.

Conclusion

The following table summarises our results. The first line, for example, reads as follows: If the program passes the argument by const reference to the signal and also by const reference to the slot, there are no copies for a direct connection and one copy for a queued connection.

Qt Slot Parameter

Qt Signal Slot Parameter

The conclusion from the above results is that we should pass arguments to signals and slots by const reference and not by value. This advice is true for both direct and queued connections. Even if the sender of the signal and the receiver of the slot are in different threads, we should still pass arguments by const reference. Qt takes care of copying the arguments, before they cross the thread boundaries – and everything is fine.
By the way, it doesn’t matter whether we specify the argument in a connect call as const Copy& or Copy. Qt normalises the type to Copy any way. This normalisation does not imply, however, that arguments of signals and slots are always copied – no matter whether they are passed by const reference or by value.