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. 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. Important: Please read the Qt Code of Conduct. And it seems to be a multi-thread signal/slot connect issue. 1 Reply Last reply.
The connection mechanism uses a vector indexed by signals. But all the slots waste space in the vector and there are usually more slots than signals in an object. So from Qt 4.6, a new internal signal index which only includes the signal index is used. While developing with Qt, you only need to know about the absolute method index.
published at 20.08.2015 15:28 by Jens Weller
This is the 7th blog post in my series about writing applications with C++ using Qt and boost. This time it is about how to notify one part of our application that something has happened somewhere else. I will start with Qt, as it brings with signals and slots a mechanism to do exactly that. But, as I have the goal not to use Qt mainly in the UI Layer, I will also look on how to notify other parts of the application, when things are changing. The last episode was about QWidgets and data.
The video for this episode:
Signals and Events in Qt
But lets start with Qt. Qt offers two different systems for our needs, Qt signal/slot and QEvents. While Qt signal/slot is the moc driven signaling system of Qt (which you can connect to via QObject::connect), there is a second Event interface informing you about certain system-like events, such as QMouseEvent, QKeyEvent or QFocusEvent. Usually you have to overwrite a method to receive such events, or use an event filter, like I showed in my last post for QFocusEvents. Some classes translate QEvents to signals, such as the TreeView, which has a signal for displaying context menus. But as this blog post is more on signaling then system events...
Qt has had its own signaling mechanism for a long time now, so when you use Qt, you also will use QSignals. Qt also uses its own keywords for this: signals, slots and emit. There is an option to turn this of, and use the macros Q_SIGNAL/S,Q_SLOT/S and Q_EMIT instead: CONFIG += no_keywords. This allows to use 3rd party libraries which use these terms, e.g. boost::signal. Qt signal/slot implementation is thread safe, so that you can use it to send messages between different QThreads, this is especially important, as anything UI related should run in the main thread of Qt, anything that could block your UI should not run in this thread, so running jobs in a QThreadPool and emitting the finished result as a signal is a common pattern. Maybe I will touch this in a later post...
For now, lets see the basics of using signals and slots in Qt. This is the code from my MainWindow class constructor, connecting several signals to slots:
So, the traditional, moc driven connect method is QObject* derived sender, the SIGNAL macro defining the signal to connect to, followed by the QObject* derived receiver, then SLOT(...) is the last argument, naming the slot to connect to. There is a fifth defaultet parameter: the ConnectionType. The last line contains the new, lambda based connection option, where you again have the sender and its slot, this time as a method-pointer, and then followed by a lambda acting as the receiving slot.
This syntax can lead to a rare error, when ever a signal is overloaded, like QComboBox::currentIndexChanged, which is available with an int or QString parameter. Then you'll need an ugly static_cast to tell the compiler which version you'd like:
In this case I didn't even needed the argument from the slot. It is fairly easy to use your own signals and slots, all you need is a QObject derived class, which is processed by the moc. Mostly of course you already have classes derived from QObject indirectly, which then use signals and slots, like the page panel class:
So, slots and signals are normal member functions, declared after the qt-specific keyword signals/slots. When you want to emit a signal, its enough to just write 'emit my_signal();', and all observers on this signal will get notified. Slots are often used to react to certain events in the UI, like the currentIndexChanged signal in this case. In the widget editor of QtCreator you get an overview of available signals when right clicking and selecting 'go to slot...', this will create a slot for this signal in your QWidget derived class.
There is also the option to map certain widgets to certain values when a signal fires, this is done via QSignalMapper. I use this in a different program to have one widget for editing flag like settings, where each flag is a bit in a settings value:
The constructor only takes a QStringList for the option names, and an int for how many columns of check boxes the current use case should have. The QSignalMapper is a member variable, and each QCheckBox connects its clicked signal to the map() slot of QSignalMapper. With setMapping the connection between the sender and the value is set up. QSignalMapper offers int, QObject*, QWidget* and QString as mapping values. QVariant or a generic interface is not provided by Qt. In the clicked slot I simply toggle the bit for the corresponding flag.
When working in Qt, most of it types provide support for signals and slots through deriving from QObject, which offers connect/disconnect methods to manage your slot connections. This brings again the disadvantages of QObject and the moc, as templates can't be used in this context, all classes using signal/slot must be concrete classes. Deriving your classes from templates (CRTP e.g.) can help here to mix in a generic layer.
While Qt is fairly well prepared to manage its own messaging needs, what alternatives exist, that could be used in the non Qt related code? The C++ standard offers currently only std::function, which can be used to implement a callback mechanism. But this has its limitations, of a 1:1 or 1:many connection this is a viable option. I use it to notify my MainWindow class that a node in the tree has changed its name. Also its useful to implement classes which execute a callback in a certain context, like EventFilter in the last blog post in this series. But std::function is not an implementation of the observer pattern, and implementing your own with it would be reinventing the wheel. Boost has had for a long time a signal library, which now is available as version 2: boost::signals2.
Using boost::signals2
Honestly, if I could avoid using signals2, I would, as it has one certain disadvantage: build times increase. So far my project is kind of small, has only a few classes, which most of are less then 100 loc. Adding boost::signals2 to a class makes it hard to build a project quickly for debugging or just seeing if the work of the past hour still compiles.
The need for signals2 came in my application, when I began to understand, that there are some events, which go from the Qt layer into the boost/standard C++ layer, and then need to travel back into the Qt layer. Each Page has a shared_ptr to a layout object, which is part of a LayoutItem holding the list of layouts for a document. There is one LayoutPanel to edit, create and delete layouts in LayoutItem, and each PagePanel has a QComboBox, so that the user can select the layout for the page. Now, when a user creates/renames a layout, each PagePanel needs to be notified, but when it gets deleted, also page needs to change. This could be implemented in the Qt layer, each Qt class involved has access to the boost/C++ layer, and can make the necessary changes. But then, this important business logic of removing a layout will only work through the UI. When I use boost::signals2, it can be done in the boost/standard C++ layer.
boost::signals2 has a signal template, which has the signature as the argument, this signal type also then has the typedef for the slot type, signal::connect returns a connection object:
When ever an object subscribes to the layout signals, it must to so for all three, the vector should invoke RVO. Currently, PagePanel is the only subscriber, it simply connects to the signals using boost::bind:
One detail here is, that I do use scoped_connection, which will call disconnect() on its destruction, while the default boost::signals2::connection class does not. scoped_connection can be moved, but not copied. But once it is in the vector, it will stay there. Also, you should forward declare the connection classes, so that you don't have to include the boost/signals2.hpp headers, this prevents leaking into other sources.
But boost::signals2 can do far more. I have no use for code that depends on the order of slots called, but you can specify this with signal::contect(int group, slot):
In some context it is interesting to handle the return value of a signal, for this boost::signal2 offers a combiner, which is the second template parameter to signal: signal<float(float,float), aggregate_combiner<std::vector<float> > >. This combiner then also overwrites the return value of the signal, which is now std::vector instead of float. Another feature is that you can block a connection with shared_connection_block.
boost::signal2 is currently header only, thread safe and offers a few more customization points, for example you can change the mutex, but also the signature type, which currently is boost::function.
Alternatives to boost::signals2
If you know very well what you are doing, you could use boost::signal instead of its new version, signals2. This might improve your compile times, but boost::signals is not any more maintained. Also, while signals2 is header-only, signals is not. The thread safety is a key feature of signals2, which at some time sooner or later will come into play in your code base. I don't want to introduce a 3rd party library into my project just to have signaling/observer pattern, but you should know, that there are a few alternatives (I googled that too):
- libsigslot
- has open bugs from 2003 - 2011, memory leaks and other issues. But seems to do the job.
- libsigc++
- a standard C++ implementation, inspired by Qt, you (might) have to derive your objects from a base class. Virtual function calls are the base of this library it seems, at least for method slots, which the call has to be derived from sigc::trackable.
- gtkmm and glibmm seem to use this for their signaling needs.
- the 5 open bugs seem to be feature requests mostly (and nil is a keyword in Object-C, well...)
- the library has been rewritten using modern C++ idioms (claims the site)
- This codeproject article from 2005 gives some insights, but C++11 changes some of them I think.
- slimsig
- seems to be a header only alternative to boost::signals2
- 2 open bugs, no change in one year
- boost::synapse
- this library is proposed for boost, but has not yet been reviewed.
- I think it could be a more lightweight alternative to signals2
- Currently its not threadsafe.
The only disadvantage of boost::signal2 is really its impact on compile and link time, which can be reduced through pimple and other isolation techniques, so that a recompilation is only triggered when really needed. One idea which came in my mind during this blog post is a std_signal2 header, which replaces the boost types (function, mutex etc.) with the corresponding std types. I'm not sure how this would work out, but boost::signals2 seems to be pretty well build to do this, a lot of template parameters have default values which then configure the library, and are hidden from the day to day usage.
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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.
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.
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.
Qt Connect Signal Slot Threads
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.
Qt Signals And Slots Tutorial
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.
Qt Signal Slot Performance
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