Q_PROPERTY and null QVariant

p-himik

I think that this topic might be somehow relevant to my "old one":http://qt-project.org/forums/viewthread/10662/ .

Here is code:
@#include <QDebug>
#include <QtCore>

class Test : public QObject
{
Q_OBJECT
Q_PROPERTY( bool test READ test WRITE setTest )

bool test_;

public:
Test() :
test_( true )
{}

bool test() const { return test_; }
void setTest( bool test ) { test_ = test; }

};

int main( int argc, char* argv[] )
{
Test t;
qDebug() << t.property( "test" ) << QVariant( QString() ).isNull();
qDebug() << "set" << t.setProperty( "test", QVariant( QString() ) );
qDebug() << "test" << t.test();
qDebug() << "set" << t.setProperty( "test", QVariant( QString( "true" ) ) );
qDebug() << "test" << t.test();
qDebug() << "set" << t.setProperty( "test", QVariant( QString() ) );
qDebug() << "test" << t.test();

return 0;

}

#include "main.moc"
@

And here is output:
@QVariant(bool, true) true
set false
test true
set true
test true
set false
test true @

So assigning of a null QVariant is not successful. Why is it so?
It would be very convenient to assign default-constructed values to properties instead.

goetz

setProperty would eventually call convert() on the QVariant. The conversion always fails for a null QVariant.

p-himik

Thanks for the answer.
Documentation says:

bq. bool QVariant::convert ( Type t )
Casts the variant to the requested type, t. If the cast cannot be done, the variant is cleared. Returns true if the current type of the variant was successfully cast; otherwise returns false.
Warning: For historical reasons, converting a null QVariant results in a null value of the desired type (e.g., an empty string for QString) and a result of false.

As I can understand from warning setProperty should set property to a null value of the desired type (and I understand it as a default-constructed value) when the QVariant is null.

andre

I think the behavior is correct, because there is no such thing as a default constructed boolean. There is no standard value for a boolean (which is why you need to initialize it yourself explicitly).

Considder this:
@
bool test;
qDebug() << test;
@
Will the output be 'true' or 'false'? There is no way to tell.

p-himik

Sorry I haven't wrote it before: I meant "these":http://qt-project.org/doc/qt-4.8/containers.html#default-constructed-values default-constructed values.

andre

That documentation refers to the container classes. QVariant isn't such a container class.

p-himik

Yes, I know. But "this":http://qt-project.org/doc/qt-4.8/qvariant.html#value says about default-constructed values as well.
I think it's strange that QVariant::value() returnes default-constructed value if this QVariant is null but QObject::setProperty() returnes false. As I think either QVariant::value() should accept a pointer to a bool (as QVariant::toInt(bool *)) or QObject::setProperty() should set property to a default-constructed value if QVariant is null.

andre

You are right, and it is weird that the QVariant documentation actually points to the documentation for the containers on this. I did not see that before. Sorry, I think you'd best talk with someone more into the details of QVariant than I am. Perhaps you can discuss this on IRC in the #qt-labs channel (freenode network).

p-himik

Thanks for advice!

goetz

The Qt meta object system tries to convert the QVariant passed to setProperty() to the type of the property. So, what's called in the end is

@
QVariant v;
bool ok = v.convert(QVariant::Bool);
if(!ok)
return false; // from setProperty()
@

Thus, if the QVariant passed in cannot be converted to the type of the property, the setProperty call will fail eventually. Also note that QVariant::convert() will return false in case you pass it a Null QVariant - which is the case here:

@
QVariant v1 = QString(); // null - convert fails

QVariant v2 = QString(""); // NOT null, convert succeeds
QVariant v3 = ""; // NOT null, convert succeeds
@

Alltogether, this is good™. You don't want to set a bool property from a QColor or a QVariantMap value, but get an error condition in theses cases.

Also, QVariant::value() is not involved in this game at all. Thus, applying its documentation to setProperty() is invalid.

References:

Implementation of QObject::setProperty()
Implementation of QMetaProperty::write()
API docs of QVariant::convert()

p-himik

Thanks, Volker. I've read that "For historical reasons, converting a null QVariant results in a null value of the desired type (e.g., an empty string for QString) and a result of false." and now everything is clear.
Though I don't like that historical reasons. They forced me to write another three lines of code :)