Example Slot Signal C++

Qt5 alpha has been released. One of the features which I have been working on is a new syntax for signals and slot.This blog entry will present it.

  1. 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 − QtCore.QObject.connect(widget, QtCore.SIGNAL(‘signalname’), slot.
  2. In this video iam going to show you how you can create Signal And Slots in Qt5 C with Practical Examples, in this we are going to introduce Signal And Slot.

Here is how you would connect a signal to a slot:

What really happens behind the scenes is that the SIGNAL and SLOT macros will convert their argument to a string. Then QObject::connect() will compare those strings with the introspection data collected by the moc tool.

Slotkey - This is class responsible for assigning ID to any newly created instance of class slot. Implementation of this class is something I really don't like. This class is used as a base class of class slot. Slotkey::slotid is used in class signal as a key to store said slot and to be able to disconnect slot from signal. (although in my.

What's the problem with this syntax?

While working fine in general, we can identify some issues:

  • No compile time check: All the checks are done at run-time by parsing the strings. That means if you do a typo in the name of the signal or the slot, it will compile but the connection will not be made, and you will only notice a warning in the standard output.
  • Since it operates on the strings, the type names of the slot must match exactly the ones of the signal. And they also need to be the same in the header and in the connect statement. This means it won't work nicely if you want to use typedef or namespaces

In the upcoming Qt5, an alternative syntax exist. The former syntax will still work. But you can now also use this new way of connecting your signals to your slots:

Which one is the more beautiful is a matter of taste. One can quickly get used to the new syntax.

So apart from the aesthetic point of view, let us go over some of the things that it brings us:

Compile-time checking

You will get a compiler error if you misspelled the signal or slot name, or if the arguments of your slot do not match those from the signal.
This might save you some time while you are doing some re-factoring and change the name or arguments of signals or slots.

An effort has been made, using static_assert to get nice compile errors if the arguments do not match or of you miss a Q_OBJECT

Arguments automatic type conversion

Not only you can now use typedef or namespaces properly, but you can also connect signalsto 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. It works because QVariant has an implicit constructor that takes a QString


Connecting to any function

As you might have seen in the previous example, the slot was just declared as publicand not as slot. Qt will indeed call directly the function pointer of the slot, andwill not need moc introspection anymore. (It still needs it for the signal)

But what we can also do is connecting to any function or functor:

This can become very powerful when you associate that with boost or tr1::bind.

C++11 lambda expressions

Everything documented here works with the plain old C++98. But if you use compiler that supportsC++11, I really recommend you to use some of the language's new features.Lambda expressions are supportedby at least MSVC 2010, GCC 4.5, clang 3.1. For the last two, you need to pass -std=c++0x asa flag.

You can then write code like:

This allows you to write asynchronous code very easily.


Update: Also have a look what other C++11 features Qt5 offers.

It is time to try it out. Check out the alpha and start playing. Don't hesistate to report bugs.

In a new Qt project, it is often desirable to mix C++ and QML code. At least inour experience, it is rare that a project is either pure C++ or pure QML. TheQt Documentation has some overview of the available options of mixing thetwo. This documentation was somewhat lacking in the past, but has vastlyimproved for the recently introduced version 5.4.

In particular, we look at an example of integrating a C++ object as a contextproperty into QML. See Embedding C++ Objects into QML with ContextProperties in the official documentation that discusses this topic.

The sample code from this project is available on Github.

Project Setup

In Qt Creator, chooce a project of type Qt Quick Application. This allows one to use C++ together with QML.

General Concept

There are different ways to send signals from C++ to QML and back. In this article, we show how to do this by embedding a C++ class directly into QML. This has the advantage that no Qt::connect connections need to be set-up manually.

In our example, we have a Receiver class that is implemented in C++. Thisclass defines a signal sendToQml and a slot receiveFromQml. Both have an integerparameter. The signal is sent to QML, and the slot is invoked from QML.

Example Slot Signal C++ Jammers

To invoke the C++ slot, a QML program can then simply call the respectivefunction on the embedded object. In order to receive the C++ signal, the QMLprogram can use the Connections class.

Example Slot Signal C++ Jammer

Embedding the Object

We have seen how to connect C++ and QML, but what is left is the task ofactually embedding the C++ object into QML. This is done by setting a contextproperty. In order to do this, one needs to get the rootContext from the QMLengine, and set its property.

As mentioned in the overview, there is no need to manually connect the C++receiver objects signals and slots to QML. Setting the context property isenough for embedding.

Naming Conventions

Example Slot Signal C++ Booster

Please note that Qt has a specific naming convention that needs to be respectedfor the connection to work. In our example, the Receiver class defines asignal that is called sendToQml. The Connections object can then accessthis signal with the name onSendToQml. The signal name must be lower case.The other end can then connect the signal by prepending the word on andcontinue with camel case.

Full Example

Qt Signal Slot Example C

The complete source code is on Github. Please clone the code and try torun it. Instructions how to run it can be found in the project’s readme.

Example Slot Signal C++ Signal

When everything is setup correctly, the program outputs a message at startupthat it received a value from C++, and when you click in the window, anothermessage should appear that a value was sent to C++. Both messages are sent tothe debug console.