Equality in QMap - operator== doesn't get invoked?

tmladek

I have a custom class that I serialize into QVariant, which I then store in a QMap.
QMap's documentation states:

Returns true if other is equal to this map; otherwise returns false.
Two maps are considered equal if they contain the same (key, value) pairs.
This function requires the value type to implement operator==().

So I assumed that the operator== of my type would be called, but when I put breakpoints into it, or debug statements, the debugger doesn't stop there, nor do the debug statements get printed out.

edit: And this is even though I've called QMetaType::registerEqualsComparator() on my custom type.

Any ideas? Thank you!

kshegunov

@tmladek
Hi,
Here's the implementation. It should be working normally. Can you show a minimal snippet that reproduces the behavior you're experiencing?

Kind regards.

tmladek

Hm, it seems like it's actually in the fact that it's a QList. Anyway.

#include <QCoreApplication>
#include <QVariant>
#include <QMap>
#include <QMetaType>
#include <QDataStream>
#include <QDebug>

struct Cat {
    QString name;
    QString color;
    bool operator==(const Cat &other) const;
}; Q_DECLARE_METATYPE(Cat)

bool Cat::operator==(const Cat &other) const {
    if (this->name != other.name) return false;
    if (this->color != other.color) return false;
    return true;
}

QDataStream &operator<<(QDataStream &out, const Cat &obj) {
    out << obj.name << obj.color;
    return out;
}
QDataStream &operator>>(QDataStream &in, Cat &obj) {
    in >> obj.name >> obj.color;
    return in;
}

int main(int argc, char *argv[])
{
    qRegisterMetaType<Cat>("Cat");
    qRegisterMetaTypeStreamOperators<Cat>("Cat");
    QMetaType::registerEqualsComparator<Cat>();

    Cat cat_1;
    cat_1.name = "Bob";
    cat_1.color = "black";
    QList<Cat> list_1;
    list_1.append(cat_1);
    QVariant variant_1 = QVariant::fromValue(list_1);
    QMap<QString, QVariant> map_1;
    map_1.insert("cat", variant_1);

    Cat cat_2;
    cat_2.name = "Bob";
    cat_2.color = "black";
    QList<Cat> list_2;
    list_2.append(cat_2);
    QVariant variant_2 = QVariant::fromValue(list_2);
    QMap<QString, QVariant> map_2;
    map_2.insert("cat", variant_2);

    qDebug() << (list_1 == list_2) << (map_1 == map_2);
}

Outputs true false.

kshegunov

@tmladek
You are comparing QVariants that contain QList<Cat>, so you need to provide registered comparison operator for QList<Cat> (or conversions might take place), which you don't do.

PS.
Also, this:

qRegisterMetaType<Cat>("Cat");

should be:

qRegisterMetaType<Cat>();

as specifically mentioned in the documentation.