Signal slot connection does not work with Move Semantics

Kofr

Here I present simple code, which represents a problem

#include <QCoreApplication>
#include <QList>

struct Data {
    int data;
};

class Obj: public QObject {
    Q_OBJECT
public:
    Obj() : QObject(nullptr) {
        connect(this, &Obj::startMove, this, &Obj::receiveData);
        emitMoveOp();
    }
    void emitMoveOp() {
        QList<Data> data;
        data.append({1});
        emit startMove(std::move(data));
    }


public slots:
    void receiveData(QList<Data> && data) {
        dataList = data;
    }
signals:
    void startMove(QList<Data> && data);
private:
    QList<Data> dataList;

};

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    Obj obj;
    return a.exec();
}

I am getting an error caused by some problem with connection D:\dev\tools\Qt\5.9.1\mingw53_32\include\QtCore\qobjectdefs_impl.h:136: error: forming pointer to reference type 'QtPrivate::RemoveRef<QList<Data>&&>::Type {aka QList<Data>&&}' (o->*f)((*reinterpret_cast<typename RemoveRef<SignalArgs>::Type *>(arg[II+1]))...), ApplyReturnValue<R>(arg[0]); ^
How to make signal slot connection work with rvalues?

m.sue

Hi @Kofr

I would guess that move semantics is not supported by the signal slot mechanism.

-Michael.

VRonin

QList uses shared data, there is basically no advantage in using move semantic. Switch to the good old const QList<Data>& data

Kofr

@m.sue the question is why?

VRonin

@Kofr said in Signal slot connection does not work with Move Semantics:

the question is why?

if you check the moc output it's pretty clear. Arguments are "stored" as void* and then casted to the correct type (QList<Data>) and passed to the slot that can't receive it as it only accepts QList<Data>&&

See https://bugreports.qt.io/browse/QTBUG-60339

m.sue

@VRonin
@Kofr

Yeah. The gerrit commits listed in the bug report you quoted show that move semantics are already implemented and even used in the development branch of Qt: https://codereview.qt-project.org/#/c/192417/6/src/widgets/widgets/qmenu.h

So we will not have to wait for long.

-Michael.