Question

Say i create a derived class as below,

    class CHIProjectData : public QObject 
{ 
CHIProjectData(QMap<QString,QString> aProjectData,
                           CHIMetaData* apMetaData = 0,
                           QObject* parent = 0); 
    private:
            QMap<QString,QString> m_strProjectData;
            CHIAkmMetaData* m_pMetaData; 
};

and i implement like,

CHIProjectData::CHIProjectData(QMap<QString,QString> aProjectData,
                               CHIMetaData* apMetaData,
                               QObject* aParent)
    :m_strProjectData(aProjectData),
    m_pMetaData(apMetaData),
    QObject(aParent)
{

}

i know i initiate the member variables m_strProjectData, m_pMetaData in the constructor. but what does the last part “QObject(aParent)” do? does it create an object of base class and consider that as a member variable?

Answer 1

QObject(aParent) calls QObject‘s constructor with the aParent parameter. QObject is not a member variable in this case. It may seem like a subtle point, but its an important one because the way you access the properties and methods of a subobject requires different syntax than as for a member variable.

Here’s an analogy to try to understand the difference between a subobject and a member variable.

In the movie “Batman: The Dark Night” there is a scene where Batman is pursuing the bad guy in his car. But the car becomes damaged and unusable, and he has to escape. At that point Batman pushes a button and part of the car detatches from the rest, becoming a motorcycle. This is kind of like a subobject. The car is a motorcycle.

Now consider the case of an RV towing a smaller vehicle, the likes of which are frequently seen on the highways of America. In this case, the RV has a vehicle. The vehicle is a member variable of the RV.