Developer Notes for Qt Code

Contents

1. Backward Compatibility
2. QObject Subclassing Style
3. Debugging Tips

Backward Compatibility

The source code must be compatible with the minimum required Qt version which is set in configure.ac:

BITCOIN_QT_CONFIGURE([5.9.5])

If an optional feature requires a higher version of Qt, or if a feature was replaced by another one, use the QT_VERSION and QT_VERSION_CHECK macros:

#include <QDate>
#include <QDateTime>
#include <QtGlobal>  // For QT_VERSION and QT_VERSION_CHECK macros.

QDateTime StartOfDay(const QDate& date)
{
#if (QT_VERSION >= QT_VERSION_CHECK(5, 14, 0))
    return date.startOfDay();  // QDate::startOfDay was introduced in Qt 5.14.0.
#else
    return {date};
#endif
}

Do not compare versions of QT_VERSION directly using 0xMMNNPP (MM = major, NN = minor, PP = patch), as this approach is less readable and more error-prone.

Every time the minimum required Qt version is bumped, grep or git grep for all of the QT_VERSION instances and adjust/remove them accordingly.

QObject Subclassing Style

When sublassing the QObject class follow the pattern below:

#include <QObject>

class MyWidget : public QObject
{
    Q_OBJECT

public:
    // Public members.

public Q_SLOTS:
    // Public slots.

Q_SIGNALS:
    // Signals (inherently public).

private:
    // Private members.

private Q_SLOTS:
    // Private slots.
};

Use the Q_SIGNALS and Q_SLOTS macros instead of the signals and slots keywords of the Qt moc (Meta-Object Compiler). This prevents potential conflicts with 3rd party signal/slot mechanisms.

Debugging Tips

For debugging, including signal-to-slot connection issues, you can use the QT_FATAL_WARNINGS environment variable:

$ QT_FATAL_WARNINGS=1 src/qt/bitcoin-qt -printtoconsole -debug=qt

This tip can be combined with a debugger.

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More notes come soon...