Qt Tutorial - Chapter 7: One Thing Leads to Another

This file is mainly lifted from main.cpp in Chapter 6; only the changes are noted here.

    #ifndef LCDRANGE_H
    #define LCDRANGE_H

This is the classic C construction to avoid errors if a header file happens to be included more than once. If you don't use it already, it is a very good habit to develop. The #ifndef should enclose all of the header file.

    #include <qvbox.h>

qvbox.h is included. LCDRange inherits QVBox, and the header file of a parent class must always be included. We cheated a bit in the previous chapters, and we let qwidget.h be included indirectly via other header files such as qpushbutton.h.

    class QSlider;

This is another classic trick, but one that's much less used often. Because we don't need QSlider in the interface of the class, only in the implementation, we use a forward declaration of the class in the header file and include the header file for QSlider in the .cpp file.

This makes the compilation of big projects much faster, because when a header file has changed, fewer files need to be recompiled. It can often speed up big compilations by a factor of two or more.

    class LCDRange : public QVBox
    {
        Q_OBJECT
    public:
        LCDRange( QWidget *parent=0, const char *name=0 );

Note the Q_OBJECT. This macro must be included in all classes that contain signals and/or slots. If you are curious, it defines the functions that are implemented in the meta object file.

        int value() const;
    public slots:
        void setValue( int );

    signals:
        void valueChanged( int );

These three members make up an interface between this widget and other components in a program. Until now, LCDRange didn't really have an interface at all.

value() is a public function for accessing the value of the LCDRange. setValue() is our first custom slot and valueChanged() is our first custom signal.

Slots must be implemented in the normal way (remember that a slot is also a C++ member function). Signals are automatically implemented in the meta object file. Signals follow the access rules of protected C++ functions (i.e., they can be emitted only by the class they are defined in or by classes inheriting from it).

The signal valueChanged() is used when the LCDRange's value has changed - just as you guessed from the name. This is not the last signal you'll see called somethingChanged().

t7/lcdrange.cpp

        connect( slider, SIGNAL(valueChanged(int)),
                 lcd, SLOT(display(int)) );
        connect( slider, SIGNAL(valueChanged(int)),
                 SIGNAL(valueChanged(int)) );

    int LCDRange::value() const
    {
        return slider->value();
    }

    void LCDRange::setValue( int value )
    {
        slider->setValue( value );
    }

        LCDRange *previous = 0;
        for( int r = 0 ; r < 4 ; r++ ) {
            for( int c = 0 ; c < 4 ; c++ ) {
                LCDRange* lr = new LCDRange( grid );
                if ( previous )
                    connect( lr, SIGNAL(valueChanged(int)),
                             previous, SLOT(setValue(int)) );
                previous = lr;
            }
        }

qmake -project
qmake