7. Parent-child relationship between QT-Qt objects

A parent-child relationship can exist between Qt objects

Objects that inherit from the QObject class or its subclasses are called Qt objects.

When specifying the parent of a Qt object

• You need to set the parent-child relationship between objects through the setParent() member function
• The child object will save its own pointer address in the children List list of the parent object (because the parent object can have multiple child objects)
• There will be a pointer to its parent object inside the child object, which can be viewed through the parent () member function

 

code experiment

Functions that need to be used:

void QObject::setParent ( QObject * parent );  　　  // Set the parent object as its own parent object

const   QObjectList & QObject::children ();  　　　　// Return a QObjectList (child object linked list), which stores the address of the child object. 

QObject * QObject::parent ();             　　　　  // Return a pointer to the parent object

 

code show as below:

#include <QtCore/QCoreApplication>
#include <QDebug>

class MObj : public QObject
{
};

void func()
{
    MObj *obj1 = new MObj;
    MObj *obj2 = new MObj;
    MObj *obj3 = new MObj;

    qDebug()<<"obj1 ="<<obj1;
    qDebug()<<"obj2 ="<<obj2;
    qDebug()<<"obj3 ="<<obj3;
    qDebug();

    obj2 ->setParent(obj1);       // Set obj1 as the parent object of obj2 
    obj3 ->setParent(obj1);       // Set obj1 as the parent object of obj3

    const QObjectList& list =obj1->children();       //获取obj1 的children list
    for(int i=0; i<list.length();i++)
    {
        qDebug()<<"children list:"<<list[i]<<endl;
    }

    qDebug()<<"obj2 parent:"<<obj2->parent();
    qDebug()<<"obj3 parent:"<<obj3->parent();

}

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    func();
    return a.exec();
}

run print:

 

 

 

 

When a Qt object is destroyed

• Remove your own address from the parent object's linked list children List
• Destroy (delete) all child objects in its own child object linked list children Li s t

 

Therefore, when deleting a Qt object, you also need to pay attention to whether it has child objects, as shown in the following figure:

 

 

 

code test delete

code show as below:

#include <QDebug>
#include <QString>
class MObj : public QObject
{
private:
    QString mvalue;
public:
    MObj(QString i=0):mvalue(i)
    {}
    ~MObj()
    {
        qDebug()<<"~Mobj : "<<mvalue;
    }
};
void func()
{
    MObj *obj1 = new MObj("obj1");
    MObj *obj2 = new MObj("obj2");
    MObj *obj3 = new MObj("obj3");
    MObj *obj4 = new MObj("obj4");
    obj2->setParent(obj1);
    obj3->setParent(obj1);
    obj4->setParent(obj3);
    delete obj3;
}
int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    func();
    return a.exec();
}

run print: