[Interest] QVariant cannot take custom type

[Pritam Ghanghas]

QObjects are not supposed to be copiable, providing a copy constructor violates Qt's principle that QObject is an identity type. If you want to store QObjects in QVariant store pointers to be them instead.
________________________________
From: interest-bounces+pritam_ghanghas=infosys.com at qt-project.org [interest-bounces+pritam_ghanghas=infosys.com at qt-project.org] on behalf of Aekold Helbrass [helbrass at gmail.com]
Sent: Friday, April 05, 2013 1:31 PM
To: Samuel Gaist
Cc: Qt-interest Interest
Subject: Re: [Interest] QVariant cannot take custom type

I've provided copy-constructor, as was suggested in that article.


On Fri, Apr 5, 2013 at 10:50 AM, Samuel Gaist <samuel.gaist at edeltech.ch<mailto:samuel.gaist at edeltech.ch>> wrote:
Hi,

IIRC, you can't have Item or Holder in a QVariant because there QObject so the copy constructor is disabled for them.

This thread might be of interest http://stackoverflow.com/questions/7872578/how-to-properly-use-qregistermetatype-on-a-class-derived-from-qobject

Hope it helps

On 5 avr. 2013, at 09:37, Aekold Helbrass wrote:

> Hi All!
>
> Short story is: I'm trying to put custom type into QVariant and it doesn't work, on debug I see QVariant with value <not accessible> and trying to read it I'm getting segfault.
>
> Long story, I'm new to Qt/C++ so I'm trying to do everything very carefully, but I'm almost sure it's some rookie mistake in my code. This code started as attempt to bind JSON tree to object tree, and stopped on reading of QList, because QMetaProperty wants QVariant.
>
> Source code:
> ================ main.cpp ====================
> #include <QCoreApplication>
> #include <QObject>
> #include <QList>
> #include <QMetaType>
> #include <QMetaObject>
> #include <QMetaProperty>
> #include <QDebug>
>
> #include "item.h"
> #include "holder.h"
>
> Q_DECLARE_METATYPE(Item)
> Q_DECLARE_METATYPE(Item*)
> Q_DECLARE_METATYPE(Holder)
>
> int main(int argc, char *argv[]) {
>     QCoreApplication a(argc, argv);
>
>     int typeItem = qRegisterMetaType<Item>("Item");
>     int typeHolder = qRegisterMetaType<Holder>("Holder");
>     int typeItemList = qRegisterMetaType<QList<Item*> >("QList<Item*>>");
>     int typeItemPointer = qRegisterMetaType<Item*>("Item*");
>
>     QList<QVariant> list;
>     for (int i = 0; i < 10; i++) {
>         void* created = QMetaType::create(typeItem);
>         Item* item = static_cast<Item*>(created);
>         item->setName("item " + i);
> // attempt 1:
>         QVariant var(typeItem, created);
> // attempt 2:
>         //QVariant var;
>         //var.setValue(item);
> // attempt 3:
>         //QVariant var = QVariant::fromValue(item);
>         list.append(var);
>     }
>
>     Holder* holder = new Holder();
>     int index = holder->metaObject()->indexOfProperty("items");
>     QMetaProperty property = holder->metaObject()->property(index);
>     property.write(holder, list);
>
>     QList<Item*> items = holder->items();
>     for (int i = 0; i < items.size(); i++) {
>         Item* item = items.at<http://items.at>(i);
>         qDebug() << "name: " << item->name();
>     }
>
>     return a.exec();
> }
>
> ======================================= holder.h ===============================
> #ifndef HOLDER_H
> #define HOLDER_H
>
>
> #include <QObject>
>
> #include <QList>
> #include "item.h"
>
> class Holder : public QObject {
>
>     Q_OBJECT
>     Q_PROPERTY(QList<Item*> items READ items WRITE setItems)
>
> public:
>     Holder(QObject* parent=0) : QObject(parent) {}
>
>     Holder(const Holder& copy, QObject* parent=0) : QObject(parent) {
>
>         pItems = copy.items();
>
>     }
>     Holder& operator =(const Holder& value) {
>
>         pItems = value.items();
>
>     }
>     QList<Item*> items() const {
>
>         return pItems;
>
>     }
> public slots:
>
>     void setItems(const QList<Item*>& value) {
>
>         pItems = value;
>
>     }
> private:
>
>     QList<Item*> pItems;
>
> };
>
> #endif // HOLDER_H
>
>
>
> ============================== item.h ================================
> #ifndef ITEM_H
>
> #define ITEM_H
>
> #include <QObject>
>
> class Item : public QObject {
>
>     Q_OBJECT
>     Q_PROPERTY(QString name READ name WRITE setName)
>
> public:
>     Item(QObject* parent=0) : QObject(parent) {}
>
>     Item(const Item& copy, QObject* parent=0) : QObject(parent) {
>
>         pName = copy.name<http://copy.name>();
>
>     }
>     Item& operator =(const Item& value) {
>
>         pName = value.name<http://value.name>();
>
>     }
>     QString name() const {
>
>         return pName;
>
>     }
>
> public slots:
>
>     void setName(const QString& value) {
>
>         pName = value;
>
>     }
> private:
>
>     QString pName;
>
> };
>
> #endif // ITEM_H
>
>
>
> ======================== end ===================================
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