Signals and Slots

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

In most GUI toolkits widgets have a callback for each action they can trigger. This callback is a pointer to a function. In Qt, signals and slots have taken over from these messy function pointers.

Signals and slots can take any number of arguments of any type. They are completely 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 by objects when they change their state in a way that may be interesting to the outside world. This is all the object does to communicate. It does not know if anything is receiving the signal at the other end. 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 normal member functions. A slot does not know if it has any signal(s) connected to it. Again, the object does not know about the communication mechanism and can be used as a true software component.

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 desire. 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:

    class Foo
    {
    public:
        Foo();
        int  value() const { return val; }
        void setValue( int );
    private:
        int  val;
    };

A small Qt class might read:

    class Foo : public QObject
    {
        Q_OBJECT
    public:
        Foo();
        int  value() const { return val; }
    public slots:
        void setValue( int );
    signals:
        void valueChanged( int );
    private:
        int  val;
    };

This class has the same internal state, and 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 may send signals to.

All classes that contain signals and/or slots must mention Q_OBJECT in their declaration.

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

    void Foo::setValue( int v )
    {
        if ( v != val ) {
            val = v;
            emit valueChanged(v);
        }
    }

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

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

    Foo a, b;
    connect(&a, SIGNAL(valueChanged(int)), &b, SLOT(setValue(int)));
    b.setValue( 11 );
    a.setValue( 79 );
    b.value();          // this would now be 79, why?

Calling a.setValue(79) will make a emit a signal, which b will receive, i.e. b.setValue(79) is invoked. b will in turn emit the same signal, which nobody receives, since no slot has been connected to it, so it disappears into hyperspace.

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

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

The preprocessor changes or removes the signals, slots and emit keywords so the compiler won't see anything it can't digest.

Run the moc on class definitions that contains signals or slots. This produces a C++ source file which should be compiled and linked with the other object files for the application.

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.

A list box, for instance, emits both highlighted() and activated() signals. Most object will probably only be interested in activated() but some may want to know about which item in the list box is currently highlighted. If the signal is interesting to two different objects you just connect the signal to slots in both objects.

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

If several slots are connected to one signal, the slots will be executed one after the other, in an arbitrary order, 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 word 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 QRangeControl::Range, it could only be connected to slots designed specifically for QRangeControl. Something as simple as the program in Tutorial 5 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. A slot's arguments cannot have default values, and as for signals, it is generally a bad idea to use custom types for slot arguments.

Since slots are normal member functions with just a little extra spice, they have access rights like everyone else. A slot's access right determines who can connect to it:

A public slots: section contains slots that anyone can connect signals to. This is very useful for component programming: You create objects that know nothing about each other, connect their signals and slots so information is passed correctly, and, like a model railway, turn it on and leave it running.

A protected slots: section contains slots that this class and its subclasses may connect signals to. This is intended for slots that are part of the class' implementation rather than its interface towards the rest of the world.

A private slots: section contains slots that only the class itself may connect signals to. This is intended for very tightly connected classes, where even subclasses aren't trusted to get the connections right.

Of course, you can also define slots to be virtual. We have found this to be very useful.

Signals and slots are fairly efficient. Of course there's some loss of speed compared to "real" callbacks due to the increased flexibility, but the loss is fairly small, we measured it to approximately 10 microseconds on a i586-133 running Linux (less than 1 microsecond when no slot has been connected) , so the simplicity and flexibility the mechanism affords is well worth it.

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 names of all 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:

  if ( widget->inherits("QButton") ) {
        // yes, it is a push button, radio button etc.
  }

A Real Example

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

    #include "qframe.h"
    #include "qbitarray.h"

    class QLCDNumber : public QFrame

QLCDNumber inherits QObject, which has most of the signal/slot knowledge, via QFrame and QWidget, and #include's the relevant declarations.

    {
        Q_OBJECT

Q_OBJECT 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 "virtual function QButton::className not defined" you have probably forgotten to run the moc or to include the moc output in the link command.

    public:
        QLCDNumber( QWidget *parent=0, const char *name=0 );
        QLCDNumber( uint numDigits, QWidget *parent=0, const char *name=0 );

It's not obviously relevant to the moc, but if you inherit QWidget you almost certainly want to have parent and name arguments to your constructors, and pass them to the parent constructor.

Some destructors and member functions are omitted here, the moc ignores member functions.

    signals:
        void    overflow();

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

"But I don't care about overflow," or "But I know the number won't overflow." Very well, then you don't connect the signal to any slot, and everything will be fine.

"But I want to call two different error functions when the number overflows." Then you connect the signal to two different slots. Qt will call both (in arbitrary order).

    public slots:
        void    display( int num );
        void    display( double num );
        void    display( const char *str );
        void    setHexMode();
        void    setDecMode();
        void    setOctMode();
        void    setBinMode();
        void    smallDecimalPoint( bool );

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

Several of the example program connect the newValue 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 more irrelevant member functions have been omitted from this example.

    };

Moc output

This is really internal to Qt, but for the curious, here is the meat of the resulting mlcdnum.cpp:

    const char *QLCDNumber::className() const
    {
        return "QLCDNumber";
    }

    QMetaObject *QLCDNumber::metaObj = 0;

    void QLCDNumber::initMetaObject()
    {
        if ( metaObj )
            return;
        if ( !QFrame::metaObject() )
            QFrame::initMetaObject();

That last line is because QLCDNumber inherits QFrame. The next part, which sets up the table/signal structures, has been deleted for brevity.

    }

    // SIGNAL overflow
    void QLCDNumber::overflow()
    {
        activate_signal( "overflow()" );
    }

One function is generated for each signal, and at present it almost always is a single call to the internal Qt function activate_signal(), which finds the appropriate slot or slots and passes on the call. It is not recommended to call activate_signal() directly.