[Qt-interest] How to create object instance knowing only its class name in a QString?

[Jason H]

This came up for my team recently too.

What are the differences between using templates and using an approach where all classes in the factory use common base class (minimally QObject) and qobject_cast<>()?



________________________________
 From: Mandeep Sandhu <mandeepsandhu.chd at gmail.com>
To: frares at gmail.com 
Cc: QT Interest List <qt-interest at trolltech.com> 
Sent: Monday, January 30, 2012 8:08 AM
Subject: Re: [Qt-interest] How to create object instance knowing only its class name in a QString?
 
On Mon, Jan 30, 2012 at 6:27 PM,  <frares at gmail.com> wrote:
> Hi, all.
>
> How do I instantiate an object knowing its class name in a QString and that
> all similar classes are derived from a "facade" super class?
>

You can use the factory pattern here. The QMetaObject class is your
friend in such cases.
Eg: You can keep  a map of class names to their corresponding static
meta-objects and then invoke QMetaObject::newInstance() to create an
object of that class.

Here's some sample code:

class ObjectFactory
{
public:
    template<class T>
    void registerClass(const QString &className)
    {
        m_factoryObjects.insert(className, &(T::staticMetaObject));
    }

    template<class T>
    T* createObject(const QString &className, QObject *parent = 0)
    {
        const QMetaObject *meta = m_factoryObjects.value(className);

        return (meta ?
                qobject_cast<T *>(meta->newInstance(Q_ARG(QObject *, parent)))
                    : 0);
    }

private:
    QHash<QString, const QMetaObject *> m_factoryObjects;
};

HTH,
-mandeep
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