The one thing that confuses the most people in the beginning is the Signal & Slot mechanism of Qt. But it's actually not that difficult to understand. In general Signals & Slots are used to loosely connect classes. Illustrated by the keyword emit, Signals are used to broadcast a message to all connected Slots. If no Slots are connected, the message 'is lost in the wild'. So a connection between Signals & Slots is like a TCP/IP connection with a few exceptions, but this metaphor will help you to get the principle. A Signal is an outgoing port and a Slot is an input only port and a Signal can be connected to multiple Slots.
For me one of the best thins is, that you don't have to bother with synchronization with different threads. For example you have one QObject that's emitting the Signal and one QObject receiving the Signal via a Slot, but in a different thread. You connect them via QObject::connect(...) and the framework will deal with the synchronization for you. But there is one thing to keep in mind, if you have an object that uses implicitly sharing (like OpenCV's cv::Mat) as parameter, you have to deal with the synchronization yourself.The standard use-case of Signals & Slots is interacting with the UI from the code while remaining responsive. This is nothing more than a specific version of 'communicating between threads'.Another benefit of using them is loosely coupled objects. The QObject emitting the Signal does not know the Slot-QObject and vice versa. This way you are able to connect QObjects that are otherwise only reachable via a full stack of pointer-calls (eg. this->objA->...->objZ->objB->recieveAQString()). Alone this can save you hours of work if someone decides to change some structure, eg. the UI.
Right now I only mentioned Signal- & Slot-methods. But you are not limited to methods - at least on the Slots side. You can use lambda functions and function pointers here. This moves some of the convenience from languages like Python or Swift to C++.
For some demonstrations I will use the following classes:
Using Connections
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. This is the old way of using signals and slots. The example below uses the well known clicked signal from a QPushButton.The connect method has a non python-friendly syntax. How Qt Signals and Slots Work - Part 3 - Queued and Inter Thread Connections. Which is called by QMetaObject::activate to prepare a Qt::QueuedConnection slot call. Abstract: A fundamental property of the Qt framework is the so called signals and slot mechanism. It is a generic event mechanism which is easy to use. By providing an interface of provided 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 − QtCore.QObject.connect(widget, QtCore.SIGNAL(‘signalname'), slot.
To connect a Signal to a Slot you can simply call QObject::connect(a, &AObject::signalSometing, b, &BObject::recieveAQString) or QObject::connect(a, SIGNAL(signalSometing(QString), b, SLOT(recieveAQString(QString)) if you want to use the 'old' syntax. The main difference is, if you use the new syntax, you have compile-time type-checking and -converting. But one big advantage of the 'old' method is that you don't need to bother with inheritance and select the most specialized method.Lambdas can be a very efficient way of using Signals & Slots. If you just want to print the value, e.g. if the corresponding property changes, the most efficient way is to use lambdas. So by using lambdas you don't have to blow up your classes with simple methods. But be aware, that if you manipulate any object inside the lambda you have to keep in mind, that synchronization issues (in a multithreaded environment) might occur.
You will get an idea of how to use the different methods in the following example:
As you see, recived a QString: 'Hello' is printed two times. This happens because we connected the same Signals & Slots two times (using different methods). In the case, you don't want that, you see some methods to prohibit that and other options in the next section Connection Types.
One side note: if you are using Qt::QueuedConnection and your program looks like the following example, at some point you will probably wonder, why calling the Signal will not call the Slots until app.exec() is called. The reason for this behavior is that the event queue, the Slot-call is enqueued, will start with this call (and block until program exits).
And before we start with the next section here is a little trick to call a method of another thread inside the context of the other thread. This means, that the method will be executed by the other thread and not by the 'calling' one.
To learn more about that here is your source of truth: https://doc.qt.io/qt-5/qmetamethod.html#invoke
Connection Types
Qt::AutoConnection
Qt::AutoConnection is the default value for any QObject::connect(...) call. If both QObjects that are about to be connected are in the same thread, a Qt::DirectConnection is used. But if one is in another thread, a Qt::QueuedConnection is used instead to ensure thread-safety. Please keep in mind, if you have both QObjects in the same thread and connected them the connection type is Qt::DirectConnection, even if you move one QObject to another thread afterwards. I generally use Qt::QueuedConnection explicitly if I know that the QObjects are in different threads.
Qt::DirectConnection
A Qt::DirectConnection is the connection with the most minimal overhead you can get with Signals & Slots. You can visualize it that way: If you call the Signal the method generated by Qt for you calls all Slots in place and then returns.
Qt::QueuedConnection
The Qt::QueuedConnection will ensure that the Slot is called in the thread of the corresponding QObject. It uses the fact, that every thread in Qt (QThread) has a Event-queue by default. So if you call the Signal of the QObject the method generated by Qt will enqueue the command to call the Slot in the Event-queue of the other QObjects thread. The Signal-method returns immediately after enqueuing the command. To ensure all parameters exist within the other threads scope, they have to be copied. The meta-object system of Qt has to know all of the parameter types to be capable of that (see qRegisterMetaType).
Qt::BlockingQueuedConnection
A Qt::BlockingQueuedConnection is like a Qt::QueuedConnection but the Signal-method will block until the Slot returns. If you use this connection type on QObjects that are in the same thread you will have a deadlock. And no one likes deadlocks (at least I don't know anyone).
Qt::UniqueConnection
Qt::UniqueConnection is not really a connection type but a modifier. If you use this flag you are not able to connect the same connection again. But if you try it QObject::connect(...) will fail and return false.
This is not everything you will ever need to know about Signals & Slots but with this information you can cover about 80% of all use-cases (in my opinion).If it happens and you need the other 20% of information, I'll give you some good links to search your specific problem on:
The Qt documentation:
Very deep understanding:
Part1: https://woboq.com/blog/how-qt-signals-slots-work.html
Part2: https://woboq.com/blog/how-qt-signals-slots-work-part2-qt5.html
Part3: https://woboq.com/blog/how-qt-signals-slots-work-part3-queuedconnection.html
< previous step | index | next step >
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.
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.
Older 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. Callbacks have two fundamental flaws: Firstly, they are not type-safe. We can never be certain that the processing function will call the callback with the correct arguments. Secondly, the callback is strongly coupled to the processing function since the processing function must know which callback to call.
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. 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.
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.
Building the Example
The C++ preprocessor changes or removes the signals, slots, and emit keywords so that the compiler is presented with standard C++.
By running the moc on class definitions that contain signals or slots, a C++ source file is produced which should be compiled and linked with the other object files for the application. If you use qmake, the makefile rules to automatically invoke moc will be added to your project's makefile.
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. Only the class that defines a signal and its subclasses can emit the signal.
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. On an i586-500, you can emit around 2,000,000 signals per second connected to one receiver, or around 1,200,000 per second connected to two receivers. 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.
Meta-Object Information
The meta-object compiler (moc) parses the class declaration in a C++ file and generates C++ code that initializes the meta-object. The meta-object contains the names of all the signal and slot members, as well as pointers to these functions.
The meta-object contains additional information such as the object's class name. You can also check if an object inherits a specific class, for example:
The meta-object information is also used by qobject_cast(), which is similar to QObject::inherits() but is less error-prone:
See Meta-Object System for more information.
A Real Example
Here is a simple commented example of a widget.
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.
It's not obviously relevant to the moc, but if you inherit QWidget you almost certainly want to have the parent argument in your constructor and pass it to the base class's constructor.
Some destructors and member functions are omitted here; the moc ignores member functions.
LcdNumber emits a signal 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.
Some irrelevant member functions have been omitted from this example.
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() 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.
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.
The QSignalMapper class is provided for situations where many signals are connected to the same slot and the slot needs to handle each signal differently.
Qt Signal Emitted But Slot Not Called
Suppose you have three push buttons that determine which file you will open: 'Tax File', 'Accounts File', or 'Report File'.
Qt::QueuedConnection
The Qt::QueuedConnection will ensure that the Slot is called in the thread of the corresponding QObject. It uses the fact, that every thread in Qt (QThread) has a Event-queue by default. So if you call the Signal of the QObject the method generated by Qt will enqueue the command to call the Slot in the Event-queue of the other QObjects thread. The Signal-method returns immediately after enqueuing the command. To ensure all parameters exist within the other threads scope, they have to be copied. The meta-object system of Qt has to know all of the parameter types to be capable of that (see qRegisterMetaType).
Qt::BlockingQueuedConnection
A Qt::BlockingQueuedConnection is like a Qt::QueuedConnection but the Signal-method will block until the Slot returns. If you use this connection type on QObjects that are in the same thread you will have a deadlock. And no one likes deadlocks (at least I don't know anyone).
Qt::UniqueConnection
Qt::UniqueConnection is not really a connection type but a modifier. If you use this flag you are not able to connect the same connection again. But if you try it QObject::connect(...) will fail and return false.
This is not everything you will ever need to know about Signals & Slots but with this information you can cover about 80% of all use-cases (in my opinion).If it happens and you need the other 20% of information, I'll give you some good links to search your specific problem on:
The Qt documentation:
Very deep understanding:
Part1: https://woboq.com/blog/how-qt-signals-slots-work.html
Part2: https://woboq.com/blog/how-qt-signals-slots-work-part2-qt5.html
Part3: https://woboq.com/blog/how-qt-signals-slots-work-part3-queuedconnection.html
< previous step | index | next step >
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.
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.
Older 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. Callbacks have two fundamental flaws: Firstly, they are not type-safe. We can never be certain that the processing function will call the callback with the correct arguments. Secondly, the callback is strongly coupled to the processing function since the processing function must know which callback to call.
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. 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.
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.
Building the Example
The C++ preprocessor changes or removes the signals, slots, and emit keywords so that the compiler is presented with standard C++.
By running the moc on class definitions that contain signals or slots, a C++ source file is produced which should be compiled and linked with the other object files for the application. If you use qmake, the makefile rules to automatically invoke moc will be added to your project's makefile.
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. Only the class that defines a signal and its subclasses can emit the signal.
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. On an i586-500, you can emit around 2,000,000 signals per second connected to one receiver, or around 1,200,000 per second connected to two receivers. 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.
Meta-Object Information
The meta-object compiler (moc) parses the class declaration in a C++ file and generates C++ code that initializes the meta-object. The meta-object contains the names of all the signal and slot members, as well as pointers to these functions.
The meta-object contains additional information such as the object's class name. You can also check if an object inherits a specific class, for example:
The meta-object information is also used by qobject_cast(), which is similar to QObject::inherits() but is less error-prone:
See Meta-Object System for more information.
A Real Example
Here is a simple commented example of a widget.
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.
It's not obviously relevant to the moc, but if you inherit QWidget you almost certainly want to have the parent argument in your constructor and pass it to the base class's constructor.
Some destructors and member functions are omitted here; the moc ignores member functions.
LcdNumber emits a signal 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.
Some irrelevant member functions have been omitted from this example.
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() 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.
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.
The QSignalMapper class is provided for situations where many signals are connected to the same slot and the slot needs to handle each signal differently.
Qt Signal Emitted But Slot Not Called
Suppose you have three push buttons that determine which file you will open: 'Tax File', 'Accounts File', or 'Report File'.
In order to open the correct file, you use QSignalMapper::setMapping() to map all the clicked() signals to a QSignalMapper object. Then you connect the file's QPushButton::clicked() signal to the QSignalMapper::map() slot.
Then, you connect the mapped() signal to readFile() where a different file will be opened, depending on which push button is pressed.
Note: The following code will compile and run, but due to signature normalization, the code will be slower.
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.
Qt Signal Slot Not Called One
See also Meta-Object System and Qt's Property System.
Qt Signal Slot Not Called The Day
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