Compare QVariant in Qt6

autoantwort

In Qt5 is was possible to compare QVariants, but that functionality is now deprecated: Doc.
But there is the function bool QMetaType::compare(const void *lhs, const void *rhs, int typeId, int *result) you can use. But the function always return false (no comparison possible). My code:

    QVariant left  = ...;
    QVariant right = ...;
    int res;
    if (!QMetaType::compare(left.data(), right.data(), left.type(), &res)) {
        qWarning() << "Can not compare " << left << " and " << right; // prints: Can not compare  QVariant(int, 0)  and  QVariant(int, 2)
        return 0;
    }
    return res < 0;

What am I doing wrong?

YanisFrt

@autoantwort

You can Use

return  QPartialOrdering::Less == QVariant::compare(left, right);

SGaist

Hi,

Do you mean compare them for equality ?

autoantwort

No I want to know whether one is smaller than the other. The old code was simply:

return left < right;

Christian Ehrlicher

See https://lists.qt-project.org/pipermail/interest/2020-April/034945.html for a discussion on why it was deprecated.

autoantwort

@Christian-Ehrlicher I understand the reasons why they deprecate it. I have the same use-case as described there. So a solution seems to be:

bool SortedModelVectorView::lessThan(const QModelIndex &source_left, const QModelIndex &source_right) const{
    const auto left = ....;
    const auto right = ....;
    switch (left.type()) {
    case QVariant::Int: return left.value<int>() < right.value<int>();
    case QVariant::Bool: return left.value<bool>() < right.value<bool>();
    case QVariant::String: return left.value<QString>() < right.value<QString>();
    }
    qWarning() << "Can not compare " << left << " and " << right << ". Type " << left.type() << " not handled";
    return 0;

Where you have to implement all types that you will handle. Ok this is one solution, but still why says QMetaType::compare that a comparison is not possible?

YanisFrt

@autoantwort

You can Use

return  QPartialOrdering::Less == QVariant::compare(left, right);