Slots and signals must have same parameters. Otherwise, the connection will not occur. Not only for connection, slot function must have same parameters with signal. For example, this sample doesn’t work: QObject::connect(ui.comboBox, SIGNAL (activated(int)), this, SLOT (onComboboxActivated)); But it works. Qt Signal Tools. Qt-signal-tools is a collection of utility classes related to signal and slots in Qt. It includes: QtCallback - Package up a receiver and slot arguments into an object for invoking later. QtSignalForwarder - Connect signals and events from objects to QtCallback or arbitrary functions. One key and distinctive feature of Qt framework is the use of signals and slots to connect widgets and related actions. But as powerful the feature is, it may look compelling to a lot of developers not used to such a model, and it may take some time at the beginning to get used to understand how to use signals and slots properly.

Signals and slots are used for communication between objects. The signals and slots mechanism is a central feature of Qt and probably the part that differs most from the features provided by other frameworks. Signals and slots are made possible by Qt's meta-object system.

Introduction

In GUI programming, when we change one widget, we often want another widget to be notified. More generally, we want objects of any kind to be able to communicate with one another. For example, if a user clicks a Close button, we probably want the window's close() function to be called.

Other toolkits achieve this kind of communication using callbacks. A callback is a pointer to a function, so if you want a processing function to notify you about some event you pass a pointer to another function (the callback) to the processing function. The processing function then calls the callback when appropriate. While successful frameworks using this method do exist, callbacks can be unintuitive and may suffer from problems in ensuring the type-correctness of callback arguments.

How Qt Signals and Slots Work - Part 2 - Qt5 New Syntax This is the sequel of my previous article explaining the implementation details of the signals and slots. In the Part 1, we have seen the general principle and how it works with the old syntax.

Signals and Slots

In Qt, we have an alternative to the callback technique: We use signals and slots. A signal is emitted when a particular event occurs. Qt's widgets have many predefined signals, but we can always subclass widgets to add our own signals to them. A slot is a function that is called in response to a particular signal. Qt's widgets have many pre-defined slots, but it is common practice to subclass widgets and add your own slots so that you can handle the signals that you are interested in.

The signals and slots mechanism is type safe: The signature of a signal must match the signature of the receiving slot. (In fact a slot may have a shorter signature than the signal it receives because it can ignore extra arguments.) Since the signatures are compatible, the compiler can help us detect type mismatches when using the function pointer-based syntax. The string-based SIGNAL and SLOT syntax will detect type mismatches at runtime. Signals and slots are loosely coupled: A class which emits a signal neither knows nor cares which slots receive the signal. Qt's signals and slots mechanism ensures that if you connect a signal to a slot, the slot will be called with the signal's parameters at the right time. Signals and slots can take any number of arguments of any type. They are completely type safe.

All classes that inherit from QObject or one of its subclasses (e.g., QWidget) can contain signals and slots. Signals are emitted by objects when they change their state in a way that may be interesting to other objects. This is all the object does to communicate. It does not know or care whether anything is receiving the signals it emits. This is true information encapsulation, and ensures that the object can be used as a software component.

Slots can be used for receiving signals, but they are also normal member functions. Just as an object does not know if anything receives its signals, a slot does not know if it has any signals connected to it. This ensures that truly independent components can be created with Qt.

You can connect as many signals as you want to a single slot, and a signal can be connected to as many slots as you need. It is even possible to connect a signal directly to another signal. (This will emit the second signal immediately whenever the first is emitted.)

Together, signals and slots make up a powerful component programming mechanism.

Signals

Signals are emitted by an object when its internal state has changed in some way that might be interesting to the object's client or owner. Signals are public access functions and can be emitted from anywhere, but we recommend to only emit them from the class that defines the signal and its subclasses.

When a signal is emitted, the slots connected to it are usually executed immediately, just like a normal function call. When this happens, the signals and slots mechanism is totally independent of any GUI event loop. Execution of the code following the emit statement will occur once all slots have returned. The situation is slightly different when using queued connections; in such a case, the code following the emit keyword will continue immediately, and the slots will be executed later.

If several slots are connected to one signal, the slots will be executed one after the other, in the order they have been connected, when the signal is emitted.

Signals are automatically generated by the moc and must not be implemented in the .cpp file. They can never have return types (i.e. use void).

A note about arguments: Our experience shows that signals and slots are more reusable if they do not use special types. If QScrollBar::valueChanged() were to use a special type such as the hypothetical QScrollBar::Range, it could only be connected to slots designed specifically for QScrollBar. Connecting different input widgets together would be impossible.

Slots

A slot is called when a signal connected to it is emitted. Slots are normal C++ functions and can be called normally; their only special feature is that signals can be connected to them.

Since slots are normal member functions, they follow the normal C++ rules when called directly. However, as slots, they can be invoked by any component, regardless of its access level, via a signal-slot connection. This means that a signal emitted from an instance of an arbitrary class can cause a private slot to be invoked in an instance of an unrelated class.

You can also define slots to be virtual, which we have found quite useful in practice.

Compared to callbacks, signals and slots are slightly slower because of the increased flexibility they provide, although the difference for real applications is insignificant. In general, emitting a signal that is connected to some slots, is approximately ten times slower than calling the receivers directly, with non-virtual function calls. This is the overhead required to locate the connection object, to safely iterate over all connections (i.e. checking that subsequent receivers have not been destroyed during the emission), and to marshall any parameters in a generic fashion. While ten non-virtual function calls may sound like a lot, it's much less overhead than any new or delete operation, for example. As soon as you perform a string, vector or list operation that behind the scene requires new or delete, the signals and slots overhead is only responsible for a very small proportion of the complete function call costs. The same is true whenever you do a system call in a slot; or indirectly call more than ten functions. The simplicity and flexibility of the signals and slots mechanism is well worth the overhead, which your users won't even notice.

Note that other libraries that define variables called signals or slots may cause compiler warnings and errors when compiled alongside a Qt-based application. To solve this problem, #undef the offending preprocessor symbol.

A Small Example

A minimal C++ class declaration might read:

A small QObject-based class might read:

The QObject-based version has the same internal state, and provides public methods to access the state, but in addition it has support for component programming using signals and slots. This class can tell the outside world that its state has changed by emitting a signal, valueChanged(), and it has a slot which other objects can send signals to.

All classes that contain signals or slots must mention Q_OBJECT at the top of their declaration. They must also derive (directly or indirectly) from QObject.

Slots are implemented by the application programmer. Here is a possible implementation of the Counter::setValue() slot:

The emit line emits the signal valueChanged() from the object, with the new value as argument.

In the following code snippet, we create two Counter objects and connect the first object's valueChanged() signal to the second object's setValue() slot using QObject::connect():

Calling a.setValue(12) makes a emit a valueChanged(12) signal, which b will receive in its setValue() slot, i.e. b.setValue(12) is called. Then b emits the same valueChanged() signal, but since no slot has been connected to b's valueChanged() signal, the signal is ignored.

Note that the setValue() function sets the value and emits the signal only if value != m_value. This prevents infinite looping in the case of cyclic connections (e.g., if b.valueChanged() were connected to a.setValue()).

By default, for every connection you make, a signal is emitted; two signals are emitted for duplicate connections. You can break all of these connections with a single disconnect() call. If you pass the Qt::UniqueConnectiontype, the connection will only be made if it is not a duplicate. If there is already a duplicate (exact same signal to the exact same slot on the same objects), the connection will fail and connect will return false.

This example illustrates that objects can work together without needing to know any information about each other. To enable this, the objects only need to be connected together, and this can be achieved with some simple QObject::connect() function calls, or with uic's automatic connections feature.

A Real Example

The following is an example of the header of a simple widget class without member functions. The purpose is to show how you can utilize signals and slots in your own applications.

LcdNumber inherits QObject, which has most of the signal-slot knowledge, via QFrame and QWidget. It is somewhat similar to the built-in QLCDNumber widget.

The Q_OBJECT macro is expanded by the preprocessor to declare several member functions that are implemented by the moc; if you get compiler errors along the lines of 'undefined reference to vtable for LcdNumber', you have probably forgotten to run the moc or to include the moc output in the link command.

After the class constructor and public members, we declare the class signals. The LcdNumber class emits a signal, overflow(), when it is asked to show an impossible value.

If you don't care about overflow, or you know that overflow cannot occur, you can ignore the overflow() signal, i.e. don't connect it to any slot.

If on the other hand you want to call two different error functions when the number overflows, simply connect the signal to two different slots. Qt will call both (in the order they were connected).

A slot is a receiving function used to get information about state changes in other widgets. LcdNumber uses it, as the code above indicates, to set the displayed number. Since display() is part of the class's interface with the rest of the program, the slot is public.

Several of the example programs connect the valueChanged() signal of a QScrollBar to the display() slot, so the LCD number continuously shows the value of the scroll bar.

Qt Signal Slot Not Working

Note that display() is overloaded; Qt will select the appropriate version when you connect a signal to the slot. With callbacks, you'd have to find five different names and keep track of the types yourself.

Signals And Slots With Default Arguments

The signatures of signals and slots may contain arguments, and the arguments can have default values. Consider QObject::destroyed():

When a QObject is deleted, it emits this QObject::destroyed() signal. We want to catch this signal, wherever we might have a dangling reference to the deleted QObject, so we can clean it up. A suitable slot signature might be:

To connect the signal to the slot, we use QObject::connect(). There are several ways to connect signal and slots. The first is to use function pointers:

There are several advantages to using QObject::connect() with function pointers. First, it allows the compiler to check that the signal's arguments are compatible with the slot's arguments. Arguments can also be implicitly converted by the compiler, if needed.

You can also connect to functors or C++11 lambdas:

In both these cases, we provide this as context in the call to connect(). The context object provides information about in which thread the receiver should be executed. This is important, as providing the context ensures that the receiver is executed in the context thread.

The lambda will be disconnected when the sender or context is destroyed. You should take care that any objects used inside the functor are still alive when the signal is emitted.

The other way to connect a signal to a slot is to use QObject::connect() and the SIGNAL and SLOT macros. The rule about whether to include arguments or not in the SIGNAL() and SLOT() macros, if the arguments have default values, is that the signature passed to the SIGNAL() macro must not have fewer arguments than the signature passed to the SLOT() macro.

All of these would work:

But this one won't work:

...because the slot will be expecting a QObject that the signal will not send. This connection will report a runtime error.

Note that signal and slot arguments are not checked by the compiler when using this QObject::connect() overload.

Advanced Signals and Slots Usage

For cases where you may require information on the sender of the signal, Qt provides the QObject::sender() function, which returns a pointer to the object that sent the signal.

Lambda expressions are a convenient way to pass custom arguments to a slot:

Using Qt with 3rd Party Signals and Slots

It is possible to use Qt with a 3rd party signal/slot mechanism. You can even use both mechanisms in the same project. Just add the following line to your qmake project (.pro) file.

It tells Qt not to define the moc keywords signals, slots, and emit, because these names will be used by a 3rd party library, e.g. Boost. Then to continue using Qt signals and slots with the no_keywords flag, simply replace all uses of the Qt moc keywords in your sources with the corresponding Qt macros Q_SIGNALS (or Q_SIGNAL), Q_SLOTS (or Q_SLOT), and Q_EMIT.

See also QLCDNumber, QObject::connect(), Digital Clock Example, Tetrix Example, Meta-Object System, and Qt's Property System.

© 2020 The Qt Company Ltd. Documentation contributions included herein are the copyrights of their respective owners. The documentation provided herein is licensed under the terms of the GNU Free Documentation License version 1.3 as published by the Free Software Foundation. Qt and respective logos are trademarks of The Qt Company Ltd. in Finland and/or other countries worldwide. All other trademarks are property of their respective owners.

Signals and slots were one of the distinguishing features that made Qt an exciting and innovative tool back in time. But sometimes you can teach new tricks to an old dog, and QObjects gained a new way to connect between signals and slots in Qt5, plus some extra features to connect to other functions which are not slots. Let’s review how to get the most of that feature. This assumes you are already moderately familiar with signals and slots.

One simple thought about the basics

I am not going to bore you with repeating basic knowledge you already have, but I want you to look at signals and slots from a certain angle, so it will be easier to understand the design of the feature I will cover next. What’s the purpose of signals and slots? It’s a way in which “one object” makes sure that when “something happened”, then “other object” “reacts to something happened”. As simple as that. That can be expressed in pseudocode like this:

Notice that the four phrases that are into quotes in the previous paragraph are the four arguments of the function call in the pseudocode. Notice also that one typical way to write the connect statement is aligning the arguments like this, because then the first column (first and third arguments) are object instances that answer “where?” and the second column (second and fourth arguments) are functions that answer “what?”.

In C++ instead of pseudocode, and using real life objects and classes, this would look like this in Qt4 or earlier:

That could be a typical statement from a “Hello World” tutorial, where a button is created and shown, and when it’s pressed the whole window closes and the application terminates.

Now to the main point that I want you to notice here. This has a very subtle advantage over a typical mechanism used in standard C or C++ 11 like callbacks with function pointers and lambda functions wrapped in std::function, and is subtle only because is so nice we often forget about it when we have used signals and slots for a while. If the sender object is destroyed, it obviously can not emit any signal because it is a member function of its class. But for the sender to call the receiver, it needs a pointer to it, and you as a user, don’t need to worry at all about the receiver being destroyed and becoming invalid (that is done automatically by the library), so you very rarely need to call QObject::disconnect.

So signals and slots are very safe by default, and in an automatic way.

The new versus the old way to use connect

The previous example shows one way that works across old versions of Qt published so far (Qt 1 to 5). Recently a blog post about porting a tutorial application from Qt 1 to Qt 5.11 has been published, and no porting was needed at all for signals, slots, or the connections! That doesn’t mean the feature is perfect, since a new way to make connections was added, keeping all the previous functionality.

The main problem with the example above is that (as you probably knew, or guessed from being all uppercase) is that SIGNAL and SLOT are macros, and what those macros do is convert to a string the argument passed. This is a problem because any typo in what gets passed to those means that the call to connect would fail and return false. So since Qt 5.0, a new overload to QObject::connect exists, and supports passing as second and fourth arguments a function pointer to specify which member function should be called. Ported to the new syntax, the above example is:

Now any typo in the name will produce a compile time error. If you misspelled “click” with “clik” in the first example, that would only fail printing a warning in the console when that function gets called. If you did that in some dialog of an application you would have to navigate to that dialog to confirm that it worked! And it would be even more annoying if you were connecting to some error handling, and is not that easy to trigger said error. But if you did the same typo in the last example, it would be a compile time error, which is clearly much better.

This example is usually said to be using the “new syntax”, and the previous example the “old syntax”. Just remember that the old is still valid, but the new is preferred in most situations.

Qt Signal Slot Parameter

Since this is an exciting new feature added to a new major version, which has received some extra polishing during the minor releases, many blog posts from other members of the Qt community have been published about it (for example covering implementation details or the issues that could arise when there are arguments involved). I won’t cover those topics again, and instead I will focus on the details that in my experience would be most beneficial for people to read on.

No need to declare members as slots anymore (or almost)

The new syntax allows to call not just a member function declared as slot in the header with public slots: (or with protected or private instead of public), but any kind of function (more on that in the next section). There is still one use case where you would want to declare functions as slots, and that is if you want to make that function usable by any feature that happens at run time. That could be QML, for example.

Connecting to anything callable

Now we can connect to any “callable”, which could be a free standing function, a lambda function or a member function of an object that doesn’t derive from QObject. That looks in code like the following:

But wait, where is that nice symmetry with 2 rows and two columns now?

When you connect to a lambda, there is a receiver object, the lambda itself, but there is no signature to specify since it’s the function call operator (the same would happen to a function object or “functor”, by the way). And when there is a free standing function there is a signature, but there is no instance, so the third and the fourth arguments of the first two calls are somewhat merged. Note that the arguments are still checked at compile time: the signal has no arguments, and the lambda has no arguments either. Both sender and receiver are in agreement.

The example using std::bind requires a bit more explanation if you are not familiar with it. In this case we have the two objects and the two function pointers, which is to be expected for what is wanted. We don’t often think about it like this, but we always need a pointer to call a member function (unless it is static). When it is not used, it is because this is implicit, and this->call() can be shortened to call(). So what std::bind does here is create a callable object that glues together the particular instance that we want with one member function. We could do the same with a lambda:

Note that std::bind is actually much more powerful, and can be very useful when the number of arguments differ. But we will leave that topic to another article.

One common use of the above pattern with std::bind is when you have a class implemented through a data pointer (private implementation or pimpl idiom). If you need a button or a timer to call a member function of the private class that is not going to be a QObject, you can write something like this:

Recovering symmetry, safety and convenience

With the previous examples that nice balance of the four arguments is gone. But we are missing something more important.

What would happen if the lambda of the previous examples would use an invalid pointer? In the very first C++ example we showed a button wanting to close the application. Imagine that the button required to close a dialog, or stop some network request, etc. If the object is destroyed because said dialog is already closed or the request finished long ago, we want to manage that automatically so we don’t use an invalid pointer.

An example. For some reason you show some widget and you need to do some last minute update after it has been shown. It needs to happen soon but not immediately, so you use a timer with a short timeout. And you write

That works, but has a subtle problem. It could be that the widget gets shown and immediately closed. The timer under the scenes doesn’t know that, and it will happily call you, and crash the application. If you made the timer connect to a slot of the widget, that won’t happen: as soon as the dialog goes away, the connection gets broken.

Since Qt 5.2 we can have the best of both worlds, and recover that nice warm feeling of having a well balanced connect statement with two objects and two functions. 🙂

In that Qt version an additional overload was added to QObject::connect, and now there is the possibility to pass as third argument a so called “context object”, and as fourth argument the same variety of callables shown previously. Then the context object serves the purpose of automatically breaking the connection when the context is destroyed. That warranties the problem mentioned is now gone. You can easily handle that there are no longer invalid captures on a lambda.

Qt Signal Slot Performance

The previous example is almost as the previous:

Now it is as if the lambda were a slot in your class, because to the timer, the context of the connection is the same.

The only requirement is that said context object has to be a QObject. This is not usually a problem, since you can create and ad-hoc QObject instance and even do simple but useful tricks with it. For example, say that you want to run a lambda only on the first click:

This will delete the ad-hoc QObject guard on the first invocation, and the connection will be automatically broken. The object also has the button as a parent, so it won’t be leaked if the button is never clicked and goes away (it will be deleted as well). You can use any QObject as context object, but the most common case will be to shut down timers, processes, requests, or anything related to what your user interface is doing when some dialog, window or panel closes.

Tip: There are utility classes in Qt to handle the lifetime of QObjects automatically, like QScopedPointer and QObjectCleanupHandler. If you have some part of the application using Qt classes but no UI tightly related to that, you can surely find a way to leverage those as members of a class not based on QObject. It is often stated as a criticism to Qt, that you can’t put QObjects in containers or smart pointers. Often the alternatives do exist and can be as good, if not better (but admittedly this is a matter of taste).

Bonus point: thread safety by thread affinity

The above section is the main goal of this article. The context object can save you crashes, and having to manually disconnect. But there is one additional important use of it: making the signal be delivered in the thread that you prefer, so you can save from tedious and error prone locking.

Again, there is one killer feature of signals and slots that we often ignore because it happens automatically. When one QObject instance is the receiver of a signal, its thread affinity is checked, and by default the signal is delivered directly as a function call when is the same thread affinity of the sender. But if the thread affinity differs, it will be delivered posting an event to the object. The internals of Qt will convert that event to a function call that will happen in the next run of the event loop of the receiver, so it will be in the “normal” thread for that object, and you often can forget about locks entirely. The locks are inside Qt, because QCoreApplication::postEvent (the function used to add the event to the queue) is thread-safe. In case of need, you can force a direct call from different threads, or a queued call from the same thread. Check the fifth argument in the QObject::connect documentation (it’s an argument which defaults to Qt::AutoConection).

Let’s see it in a very typical example.

This shows a class that derives from QRunnable to reimplement the run() function, and that derives from QObject to provide the finished() signal. An instance is created after the user activates a button, and then we show some progress bar and run the task. But we want to notify the user when the task is done (show some message, hide some progress bar, etc.).

In the above example, the third argument (context object) might be forgotten, and the code will compile and run, but it would be a serious bug. It would mean that you would attempt to call into the UI thread from the thread where the task was run (which is a helper thread pool, not the UI thread). This is wrong, and in some cases Qt will nicely warn you that you are using some function from the wrong thread, but if you are not lucky, you will have a mysterious crash.

Wrap up

Hopefully now you’ve understood why that odd point was made in the introduction section. You don’t have to agree that it is aesthetically pleasing to write the arguments to connect in two rows and two columns, but if you understood the importance of using a context object as a rule of thumb, you probably will find your preferred way to remember if that third argument is needed when you write (or review other’s) code using connect.