Qt's runtime connection of signals and slots prints out a warning to the application output when it can't perform the connection. You can also look at the return value from QObject::connect. A side note, not directly related to the question, but I've had issues with QVector and storing local objects in it. If done improperly, the objects will. 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. What the code below does is to create a slot and signal with a function that will emit a signal when called (the function that is) so the signal is “fired” off to what ever is listening to it, to connect a signal to a slot you use the QObject::connect as below.

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.

Signals and slots in Qt

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.

Connecting the signal to the slot

Prior to the fifth version of Qt to connect the signal to the slot through the recorded macros, whereas in the fifth version of the recording has been applied, based on the signs.

Writing with macros:

Writing on the basis of indicators:

The advantage of the second option is that it is possible to determine the mismatch of signatures and the wrong slot or signal name of another project compilation stage, not in the process of testing applications.

An example of using signals and slots

For example, the use of signals and slots project was created, which in the main window contains three buttons, each of which is connected to the slot and these slots already transmit a signal in a single slot with the pressed button number.

Project Structure

Project Structure

According to the tradition of conducting lessons enclosing structure of the project, which is absolutely trivial and defaulted to the disgrace that will not even describe members of her classes and files.

mainwindow.h

Thus, the following three buttons - three slots, one signal at all three buttons, which is fed into the slot button and transmits the number buttons into a single slot that displays a message with the number buttons.

mainwindow.cpp

A file in this logic is configured as described in the preceding paragraphs. Just check the code and go to the video page, there is shown in detail the whole process, demonstrated the application, and also shows what happens if we make coding a variety of errors.

Video

Signals and slots are used for communication between objects. Thesignal/slot mechanism is a central feature of Qt and probably thepart that differs most from other toolkits.

In GUI programming we often want a change in one widget to be notifiedto another widget. More generally, we want objects of any kind to beable to communicate with one another. For example if we were parsingan XML file we might want to notify a list view that we're using torepresent the XML file's structure whenever we encounter a new tag.

Older toolkits achieve this kind of communication using callbacks. Acallback is a pointer to a function, so if you want a processingfunction to notify you about some event you pass a pointer to anotherfunction (the callback) to the processing function. The processingfunction then calls the callback when appropriate. Callbacks have twofundamental flaws. Firstly they are not type safe. We can never becertain that the processing function will call the callback with thecorrect arguments. Secondly the callback is strongly coupled to theprocessing function since the processing function must know whichcallback to call.

An abstract view of some signals and slots connections

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

The signals and slots mechanism is type safe: the signature of asignal must match the signature of the receiving slot. (In fact a slotmay have a shorter signature than the signal it receives because itcan ignore extra arguments.) Since the signatures are compatible, thecompiler can help us detect type mismatches. Signals and slots areloosely coupled: a class which emits a signal neither knows nor careswhich slots receive the signal. Qt's signals and slots mechanismensures that if you connect a signal to a slot, the slot will becalled with the signal's parameters at the right time. Signals andslots can take any number of arguments of any type. They arecompletely typesafe: no more callback core dumps!

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

An example of signals and slots connections

Slots can be used for receiving signals, but they are also normalmember functions. Just as an object does not know if anything receivesits signals, a slot does not know if it has any signals connected toit. This ensures that truly independent components can be created withQt.

You can connect as many signals as you want to a single slot, and asignal can be connected to as many slots as you desire. It is evenpossible to connect a signal directly to another signal. (This willemit the second signal immediately whenever the first is emitted.)

Together, signals and slots make up a powerful component programmingmechanism.

A Small Example

A minimal C++ class declaration might read:

A small Qt class might read:

This class has the same internal state, and public methods to access thestate, but in addition it has support for component programming usingsignals and slots: this class can tell the outside world that its statehas changed by emitting a signal, valueChanged(), and it hasa slot which other objects can send signals to.

All classes that contain signals or slots must mention Q_OBJECT intheir declaration.

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

The line emit valueChanged(v) emits the signalvalueChanged from the object. As you can see, you emit asignal by using emit signal(arguments).

Here is one way to connect two of these objects together:

Calling a.setValue(79) will make a emit a valueChanged()signal, which b will receive in its setValue() slot,i.e. b.setValue(79) is called. b will then, in turn,emit the same valueChanged() signal, but since no slot has beenconnected to b's valueChanged() signal, nothing happens (thesignal is ignored).

Note that the setValue() function sets the value and emitsthe signal only if v != val. This prevents infinite loopingin the case of cyclic connections (e.g. if b.valueChanged()were connected to a.setValue()).

A signal is emitted for every connection you make, so if youduplicate a connection, two signals will be emitted. You can alwaysbreak a connection using QObject::disconnect().

This example illustrates that objects can work together without knowingabout each other, as long as there is someone around to set up aconnection between them initially.

The preprocessor changes or removes the signals, slots andemit keywords so that the compiler is presented with standard C++.

Run the moc on class definitions that containsignals or slots. This produces a C++ source file which should be compiledand linked with the other object files for the application. If you useqmake, the makefile rules toautomatically invoke the moc will be added toyour makefile for you.

Signals

Signals are emitted by an object when its internal state has changedin 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 thesignal.

A list box, for example, emits both clicked() andcurrentChanged() signals. Most objects will probably only beinterested in currentChanged() which gives the current list itemwhether the user clicked it or used the arrow keys to move to it. Butsome objects may only want to know which item was clicked. If thesignal is interesting to two different objects you just connect thesignal to slots in both objects.

When a signal is emitted, the slots connected to it are executedimmediately, just like a normal function call. The signal/slotmechanism is totally independent of any GUI event loop. Theemit will return when all slots have returned.

If several slots are connected to one signal, the slots will beexecuted one after the other, in an arbitrary order, when the signalis emitted.

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

A note about arguments. Our experience shows that signals and slotsare more reusable if they do not use special types. If QScrollBar::valueChanged() were to use a special type such as thehypothetical QRangeControl::Range, it could only be connected toslots designed specifically for QRangeControl. Something as simple asthe program in Tutorial #1 part 5would be impossible.

Slots

A slot is called when a signal connected to it is emitted. Slots arenormal C++ functions and can be called normally; their only specialfeature is that signals can be connected to them. A slot's argumentscannot have default values, and, like signals, it is rarely wise touse your own custom types for slot arguments.

Since slots are normal member functions with just a little extraspice, they have access rights like ordinary member functions. Aslot's access right determines who can connect to it:

A public slots section contains slots that anyone can connectsignals to. This is very useful for component programming: you createobjects that know nothing about each other, connect their signals andslots so that information is passed correctly, and, like a modelrailway, turn it on and leave it running.

A protected slots section contains slots that this class and itssubclasses may connect signals to. This is intended for slots that arepart of the class's implementation rather than its interface to therest of the world.

A private slots section contains slots that only the class itselfmay connect signals to. This is intended for very tightly connectedclasses, where even subclasses aren't trusted to get the connectionsright.

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

The signals and slots mechanism is efficient, but not quite as fast as'real' callbacks. Signals and slots are slightly slower because of theincreased flexibility they provide, although the difference for realapplications is insignificant. In general, emitting a signal that isconnected to some slots, is approximately ten times slower thancalling the receivers directly, with non-virtual function calls. Thisis the overhead required to locate the connection object, to safelyiterate over all connections (i.e. checking that subsequent receivershave not been destroyed during the emission) and to marshall anyparameters in a generic fashion. While ten non-virtual function callsmay 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 avery small proportion of the complete function call costs. The same istrue whenever you do a system call in a slot; or indirectly call morethan ten functions. On an i586-500, you can emit around 2,000,000signals per second connected to one receiver, or around 1,200,000 persecond connected to two receivers. The simplicity and flexibility ofthe signals and slots mechanism is well worth the overhead, which yourusers won't even notice.

Meta Object Information

The meta object compiler (moc) parses the classdeclaration in a C++ file and generates C++ code that initializes themeta object. The meta object contains the names of all the signal andslot members, as well as pointers to these functions. (For moreinformation on Qt's Meta Object System, see Whydoesn't Qt use templates for signals and slots?.)

The meta object contains additional information such as the object's class name. You can also check if an objectinherits a specific class, for example:

A Real Example

Here is a simple commented example (code fragments from qlcdnumber.h ).

QLCDNumber inherits QObject, which has most of the signal/slotknowledge, via QFrame and QWidget, and #include's the relevantdeclarations.

Q_OBJECT is expanded by the preprocessor to declare several memberfunctions that are implemented by the moc; if you get compiler errorsalong the lines of 'virtual function QButton::className not defined'you have probably forgotten to run the moc or toinclude the moc output in the link command.

Qt Signal Emitted But Slot Not Called One

It's not obviously relevant to the moc, but if you inherit QWidget youalmost certainly want to have the parent and namearguments in your constructors, and pass them to the parentconstructor.

Some destructors and member functions are omitted here; the mocignores member functions.

QLCDNumber emits a signal when it is asked to show an impossiblevalue.

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

If, on the other hand, you want to call two different error functionswhen the number overflows, simply connect the signal to two differentslots. Qt will call both (in arbitrary order).

A slot is a receiving function, used to get information about statechanges in other widgets. QLCDNumber uses it, as the code aboveindicates, to set the displayed number. Since display() is partof the class's interface with the rest of the program, the slot ispublic.

Qt Signal Emitted But Slot Not Called The Most

Several of the example programs connect the newValue() signal of aQScrollBar to the display() slot, so the LCD number continuously showsthe value of the scroll bar.

Qt Signal Emitted But Slot Not Called Two

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

Qt Signal Emitted But Slot Not Called Light

Some irrelevant member functions have been omitted from thisexample.