[SOLVED] Qt Meta-Obect System: QObject::setProperty() with QVariant user defined types - fails

genC

Hello all,

Here is my case: I have a class, MyClass, that has references to other objects (composition). I want to dynamically initialize an instance of this class using @QObject::setProperty(const char * fieldName, QVariant &value;)@
The properties are pointers to objects that inherit QObjects. I have those pointers stored in a QObject* array (let's call it objVals) and I also have the field names in an array fieldNames.
To do this, I create a QVariant (let's call it value) for each QObject* in the array, then call setProperty() with fieldNames[i] and value.
For example

@MyClass mc;
for (int i = 0; i < numObjFields; ++i) {
QVariant value;
value.setValue(objVals[i]);
mc.setProperty(fieldNames[i],value);
}@

if the properties' real type is not QObject*, but a derived class, setProperty fails.
Now if I cast objVals[i] to it's true type, it succeeds (assuming I know the type at compile time of course)

@MyClass mc;
for (int i = 0; i < numObjFields; ++i) {
QVariant value;
value.setValue((trueType*)objVals[i]);
mc.setProperty(fieldNames[i],value);
}@

I have declared all the types with Q_DECLARE_METATYPE, Q_OBJECT and the properties with Q_PROPERTY with the proper getters and setters.

Why is this problem happennig? Is there a way to overcome it? Your help will be very much appreciated.

Thanks!

steno

Do do you register the meta type with the call qRegisterMetaType("your custom meta type")?

genC

Yes I tried with qRegisterMetaType, it didn't work either.

dangelog

What's the (apparent) type of objVals?

[quote]I have declared all the types with Q_DECLARE_METATYPE[/quote]

Which types are you talking about exactly?

[quote author="steno" date="1311784107"]Do do you register the meta type with the call qRegisterMetaType("your custom meta type")?[/quote]

Not needed.

steno

[quote author="steno" date="1311784107"]Do do you register the meta type with the call qRegisterMetaType("your custom meta type")?[/quote]

Not needed.[/quote]

Doh, true. Only needed for queued signals/slots and the property system.

andre

For the sake of trying to pinpoint what happens: if you try to set a property by directly entering its name instead of retreiving it from your array, and then calling setProperty on the QObject* pointer, does that work?

I mean: what happens if you use:
@
myQObjectPtr->setProperty('ThePropertyOfTheSubclass', QVariant("a value"));
@

genC

bq. peppe wrote:
What’s the (apparent) type of objVals?

The type of objVals is QObject** (array of QObject pointers).

bq. peppe wrote:
Which types are you talking about exactly?

I have created my own classes, that inherit QObject. I'm registering the pointers to those classes.
For example
@class Class1 : public QObject {
Q_OBJECT

public:
Class1() {
}

virtual ~Class1() {
}

};

Q_DECLARE_METATYPE(Class1*)@

Then I have another class Class2 containing a reference to Class1
@class MyClass : public QObject {
Q_OBJECT
Q_PROPERTY (Class1* cl READ getCl WRITE setCl)
Class1* cl;

public:
MyClass() {
cl = NULL;
}

Class1* getCl() const {
    return cl;
}
void setCl(Class1 *obj) {
    cl = obj;
}

};@

bq. Andre wrote:
I mean: what happens if you use:
@myQObjectPtr->setProperty('ThePropertyOfTheSubclass', QVariant("a value"));@

It doesn't work either

Here is a sample code that you can compile and run to troubleshoot the problem (Qt console application)

main.cpp:
@#include <QtCore/QCoreApplication>

#include <iostream>
#include "MyClasses.h"

using namespace std;

int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);

MyClass mc;
QObject* c = new Class1;

QVariant value;
value.setValue(c);

if (mc.setProperty("cl",value) == false)
    cout<<"setProperty failed"<<endl;

return a.exec&#40;&#41;;

}
@

header file (MyClasses.h)
@#ifndef MYCLASSES_H
#define MYCLASSES_H

#include <QObject>
#include <QVariant>
#include <QMetaObject>

class Class1: public QObject {
Q_OBJECT
public:
virtual ~Class1() {}
};

Q_DECLARE_METATYPE(Class1*)

class MyClass: public QObject {
Q_OBJECT
Q_PROPERTY (Class1* cl READ getCl WRITE setCl)
Class1* cl;

public:
MyClass() {
cl = NULL;
}

Class1* getCl() const {
    return cl;
}
void setCl(Class1 *obj) {
    cl = obj;
}

virtual ~MyClass() {}

};

#endif // MYCLASSES_H
@

dangelog

No, that code is wrong -- the type of the QVariant is QObject*, which does not match the type of the property (Class1*). Therefore the setProperty will fail. Create your QVariant from a Class1* value and it will work.

Yes, it's a kind of limitation of the whole QMetaType stuff...

genC

Ok that's what I assumed, but I hoped there was something else.

Thank you all for your help!

genC

Oh, one last thing: why can't the QVariant of type QObject* be converted to a Class1* (using QVariant::convert(Type t))?

Thanks

alexisdm

[quote author="genC" date="1311851619"]Oh, one last thing: why can't the QVariant of type QObject* be converted to a Class1* (using QVariant::convert(Type t))?[/quote]Because QVariant::type() returns 2 different type ids for Class1* and QObject*, and you can see in the source code of QVariant::canConvert(Type) that if the two type ids are different the functions returns false except for a few hard-coded conversions between Qt builtin types or between numeric types.

But if it's really a composition and not an aggregation, you could just add the QObjects as children, and give them names (QObject::setObjectName) to find them later with yourContainerObject->findChild<Class1*>("yourObjectName");.

genC

I see.
Well in fact, I'm using both composition and aggregation, so QObjects as children might not really work for me...

Thank you