[Solved] Q_RETURN_ARG for QObject-derived type

pierssen

I have an object that I receive as a QObject pointer. I can get and set properties and invoke void methods without any problem. However, getting a return value from a method is a problem. If the type is a standard type such as bool, I can grab it this way:

@
bool bReturn;
qMethod.invoke(pqObj, Q_RETURN_ARG(bool, bReturn));
@

but if the return type is a QObject-derived class, I just can't get that value. QVariant doesn't work, and neither does QObject *. The type in question is registered with the QML engine. Is there a way I can get that type and use it in Q_RETURN_ARG?

sierdzio

Q_RETURN_ARG should work with QObject pointers. Since you say it does not in your case, however, you may try to use void * and then cast it to QObject.

pierssen

Nope. Doesn't like void * either.

SGaist

Hi and welcome to devnet,

@
MyWidget.h

class MyWidget : public QWidget
{
Q_OBJECT
public:
MyWidget(QWidget parent = 0);
Q_INVOKABLE QWidget me() { return this; }
};

main.cpp
MyWidget widget;
QWidget retVal = Q_NULLPTR;
QMetaObject::invokeMethod(&widget, "me", Qt::DirectConnection,
Q_RETURN_ARG(QWidget, retVal));
qDebug() << retVal;
@

Returns the right value.

Can you show your current function code and how you call it exactly ?

pierssen

Hi, sorry for the delay in replying [I must have missed the email].

@
parrot.h:

class Parrot : public QObject
{
Q_OBJECT
Q_PROPERTY(bool isDead READ isDead)
Q_PROPERTY(bool canVoom READ canVoom)

public:
Parrot(QObject *parent = 0);
bool isDead() const { return true; }
bool canVoom() const { return false; }
Q_INVOKABLE int queryVolts() { return 4000; }

};

boutique.h:

class Boutique : public QObject
{
Q_OBJECT
public:
explicit Boutique(QObject parent = 0);
Q_INVOKABLE bool isOpen();
Q_INVOKABLE Parrot purchaseParrot();
Q_INVOKABLE bool registerComplaint(Parrot *polly);

};

boutique.cpp:

bool Boutique::isOpen()
{
return true;
}

Parrot* Boutique::purchaseParrot()
{
Parrot *polly = new Parrot();
return polly;
}

bool Boutique::registerComplaint(Parrot *polly)
{
bool bResult = polly->isDead();
qDebug("He's not dead: he's resting.");
bResult = polly->canVoom();
return bResult;
}

main.cpp:

auto topLevelObject = engine.rootObjects().value(0);
QObject * pqTop = qobject_cast<QObject *>(topLevelObject);
QObject *pqBoutique = pqTop->findChild<QObject *>("myBoutique");
if (!pqBoutique)
{
    qCritical("Error: QML Object \"myBoutique\" not found.");
    return -1;
}
int iMethod;
QMetaMethod qMethod;
bool bResult, bIsOpen, bIsDead;
QObject *pqPolly;
const QMetaObject *pqClass = pqBoutique->metaObject();
iMethod = pqClass->indexOfMethod("isOpen()");
qMethod = pqClass->method(iMethod);
bResult = qMethod.invoke(pqBoutique, Q_RETURN_ARG(bool, bIsOpen)); // bResult = true
iMethod = pqClass->indexOfMethod("purchaseParrot()");
qMethod = pqClass->method(iMethod);
bResult = qMethod.invoke(pqBoutique, Q_RETURN_ARG(QObject *, pqPolly)); // bResult = false

@

pierssen

Got it!

@
pqClass = pqBoutique->metaObject();
iMethod = pqClass->indexOfMethod("purchaseParrot()");
qMethod = pqClass->method(iMethod);
int iType = qMethod.returnType(); // some int
const char name = qMethod.typeName(); // "Parrot"
QVariant qv(iType, NULL);
void* data = qv.data();
bResult = qMethod.invoke(pqBoutique, QGenericReturnArgument(name, data));
bResult = qv.canConvert(QMetaType::QObjectStar); // true
QObject pqPolly = qv.value<QObject>();
bResult = pqPolly->property("isDead").toBool();
bResult = pqPolly->property("canVoom").toBool();
@

SGaist

Wouldn't it be simpler to call
@
QObject pqPolly;
QMetaObject::invokeMethod(pqBoutique, "purchaseParrot", Qt::DirectConnection,
Q_RETURN_ARG(QObject, pqPolly));
@

?

pierssen

I tried that and it didn't work. That was the -problem I originally ran into- reason I started this discussion.

SGaist

Ok, strange...