Qt Tutorial 7 - One Thing Leads to Another

t7/lcdrange.h

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

    #ifndef LCDRANGE_H
    #define LCDRANGE_H

This, together with the #endif at the end of the file, is the standard 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.

    #include <QWidget>

<QWidget> is included since our LCDRange class inherits QWidget. The header file of a parent class must always be included - we cheated a bit in the previous chapters, and we let <QWidget> be included indirectly via other header files.

    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 the compiler usually spends most of its time parsing header files, not the actual source code. This trick alone can often speed up compilations by a factor of two or more.

    class LCDRange : public QWidget
    {
        Q_OBJECT

    public:
        LCDRange(QWidget *parent = 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 value);

    signals:
        void valueChanged(int newValue);

These three members make up an interface between this widget and other components in a program. Until now, LCDRange didn't really have an API 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 (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 valueChanged() signal is used when the LCDRange's value has changed.

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

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

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

        LCDRange *previousRange = 0;

        for (int row = 0; row < 3; ++row) {
            for (int column = 0; column < 3; ++column) {
                LCDRange *lcdRange = new LCDRange;
                grid->addWidget(lcdRange, row, column);
                if (previousRange)
                    connect(lcdRange, SIGNAL(valueChanged(int)),
                            previousRange, SLOT(setValue(int)));
                previousRange = lcdRange;
            }
        }

    qmake -project
    qmake