Qt No Such Slot In Derived Class

The function of the interface is to provide a way to interact with other systems. Other systems don't need (or can't) know the details inside, but can only communicate with each other through the interface provided outside.

Make sure the QOBJECT macro is present in the definition of all QObject-derived classes. Make sure you declare your QObject-derived classes in your header files ONLY. Make sure all of your header files are listed in your.pro file in the HEADERS= list. Run qmake every time you add QOBJECT to one of your classes or modify your.pro file. In such programming, what if the interaction between signal slots is to be done? To be able to interact, most people may write code similar to the following: Regrettably, Qt issued a warning: warning: Signals cannot be declared virtual. So, how to solve this problem? Here are three solutions: Derived from QObject, signals do not use. The QGraphicsTransform class is an abstract base class for building advanced transformations on QGraphicsItems. As an alternative to QGraphicsItem::transform, QGraphicsTransform lets you create and control advanced transformations that can be configured independently using specialized properties. Qt Development General and Desktop. And a derived class. QObject::connect: No such slot QCcmStatusPanel::DistributorDepthSlot(int) Reply Quote 0. QT no such slot. 0 Hi everyone, I'm trying to modify the sources codes of wireshark QT but apparently I can't add new slots. I mean i added in mainwindows.h my.

Pure virtual functions (slots are no exception) are easy to understand, so what about signals?

Is it valid to define a pure virtual signal in Qt?

Indeed, this topic is very interesting.. Usually, we will define some pure virtual slot functions, but the topic of pure virtual signal is seldom discussed. So, can the signal be pure and imaginary?

  • Solution

Copyright: Once you go to Meng San Li, please indicate the source of the reprint. http://blog.csdn.net/liang19890820

Qt No Such Slot In Derived Class

Some understandings about signal and pure virtual:

  • Signals will never be implemented (that is, signals are defined in. h and not required in. cpp)
  • The main purpose of declaring a pure virtual function is to force inherited classes to provide an implementation.

Signals are not implemented. If they are declared to be pure virtual, they need inherited classes to provide an implementation, which directly conflicts with the 'signal is not implemented'. It's like having one person in two places at the same time. It's impossible. Therefore, it seems to be a mistake to declare a pure false signal.

In such programming, what if the interaction between signal slots is to be done? To be able to interact, most people may write code similar to the following:

Regrettably, Qt issued a warning:

warning: Signals cannot be declared virtual

So, how to solve this problem?

Here are three solutions:

  • Derived from QObject, signals do not use virtual
  • Define 'signal' as a pure virtual function
  • Define the connection mode of the signal slot in the interface

The test results are as follows:

Derived from QObject, signals do not use virtual

Let's not use virtual, well, that's not necessary, this is the simplest solution!

Specific implementation is also independent of the signal, only need to achieve other interfaces can:

But this approach is not ideal, because in general, the interface is a simple class and does not need to be derived from QObject.

Define 'signal' as a pure virtual function

'Signal' is defined as a pure virtual function:

Note: For IClock, alarm() is a pure virtual function, not a 'signal' (but can be used as a signal), because IClock is not a QObject.

Specific implementations need to be derived from QObject (because signals are needed), and alarm() needs to be defined as a signal:

At this time, the specific implementation of alarm() is accomplished by moc internally.

To test the validity, define a class - monitor.h:

Note: When connecting signals, you need to convert them to QObject.

Testing in main.cpp: Poker tournaments near me uk map.

Define the connection mode of the signal slot in the interface

In addition to the above, the connection mode of the signal slot can also be defined in the interface.

First, define an interface to connect the signal slot - connectToAlarm():

Then, the specific connection of the signal slot is realized in the derived class:

Since then, the connection has become much simpler.

Added by ednark on Mon, 20 May 2019 20:57:11 +0300

Class
  • PyQt Tutorial
  • PyQt Useful Resources
  • Selected Reading

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.

Qt No Such Slot In Derived Classroom

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

Qt No Such Slot In Derived Class Example

The above code produces the following output −

Output