64 bit integers in QML

Question

How do I deal with 64 bit integers in QML? I know that useless Javascript can't handle them normally as it uses doubles for everything, and if I try to use them in a signal, everything gets set to undefined. Is there a way around this? Perhaps I have to use a QVariant or something?

Answer 1

If you do not need methods on your 64-bit object, use Q_GADGET to create a type that you can store in a QVariant:

struct Int64Data
{
    Q_GADGET
public:
    int64_t value;
};

Register using:

qRegisterMetaType<Int64Data>();

You can pass this type to QML:

Q_INVOKABLE QVariant getMax() {
    Int64Data idx;
    idx.value = INT64_MAX;
    return QVariant::fromValue(idx);
}

You cannot manipulate it directly in QML, but you can pass to slots:

Q_INVOKABLE void dumpIndex(const QVariant idx) const {
    if (idx.canConvert<Int64Data>()) {
        auto data = qvariant_cast<Int64Data>(idx);
        qDebug() << "Index value:" << data.value;
    }
}

Answer 2

You can include qglobal.h and use qint64/quint64, or qlonglong/qulonglong, which are typedefs for long long int and unsigned long long int respectively. These are recognised by the QML engine. However, creating your own typedefs, or using long long explicitly, seems not to work.

Answer 3

Create your own Integer64 class derived from QObject and give it the functions you need, like

class Integer64 : public QObject
{
    Q_OBJECT

public:
    // ...
    Q_PROPERTY(QString value READ value WRITE setValue NOTIFY valueChanged)
    QString value() const;
    void setValue(QString value);

    Q_INVOKABLE void add(Integer64 * other);
    Q_INVOKABLE void subtract(Integer64 * other);
    Q_INVOKABLE void multiply(Integer64 * other);
    Q_INVOKABLE void divide(Integer64 * other);
    // ...

where you use value() to get a string representation in QML and setValue() to set a value from a string representation.

An additional constructor can be used to efficiently create Integer64 from C++

public:
    explicit Integer64(QObject *parent = 0);
    explicit Integer64(qint64 value, QObject *parent);
    // ...

Then register the type to QML

qmlRegisterType<Integer64>("MyModules", 1, 0, "Integer64");