QT learning 16 parent-child relationships between QT objects

Qt Study 16 Qt The parent-child relationship between objects

problem

In the last section QCalculatorUI Class object Create... In heap space A text box and a button , but There is no counterpart delete Code , Whether it is a memory leak ？

Qt The relationship between objects

• Qt Objects can There's a father son relationship
  • Every object is preservation Have it Pointers to all child objects
  • Every object has a Pointer to its parent object

• When specifying Qt When the parent object of the object
  • Its parent object will add the pointer of the object to the linked list of child objects
  • The object holds a pointer to its parent object

QObject* p = new QObject();
QObject* c1 = new QObject();
QObject* c2 = new QObject();
c1->setParent(p);
c2->setParent(p);

Code experiments

#include <QCoreApplication>
#include <QDebug>

void fcTest()
{
    
    QObject* p = new QObject();
    QObject* c1 = new QObject();
    QObject* c2 = new QObject();

    c1->setParent(p);
    c2->setParent(p);
    qDebug() << "c1: " << c1;	// c1: QObject(0x767550)
    qDebug() << "c2: " << c2;	// c2: QObject(0x7675a8)

    const QObjectList& list = p->children();
    for (int i = 0; i < list.length(); i++) {
    
        qDebug() << list[i];	// QObject(0x767550) QObject(0x7675a8)
    }
    qDebug() << "p: " << p;		// p: QObject(0x7674f8)
    qDebug() << "c1 parent: " << c1->parent();	// c1 parent: QObject(0x7674f8)
    qDebug() << "c2 parent: " << c1->parent();	// c2 parent: QObject(0x7674f8)
}

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

    return a.exec();
}

• When Qt When an object is destroyed

  • Remove yourself from the parent object's Children List remove
  • To his own Children List Destroy all objects in

  Use Qt When developing , Not only should we always pay attention to Memory leak The problem of , Also, always pay attention to whether the object can be Multiple destruction The problem of ！

• utilize Qt object The father son relationship Can form an object tree

• Deleting a node in the tree will Cause the corresponding subtree to be destroyed

Code experiments

#include <QCoreApplication>
#include <QDebug>

class MObj : public QObject
{
    
    QString m_name;
public:
    MObj(const QString& name) {
    
        m_name = name;
        qDebug() << "Constructor: " << m_name;
    }
    ~MObj() {
    
        qDebug() << "Destructor: " << m_name;
    }
};

void delTest()
{
    
    MObj *obj1 = new MObj("obj1");  // Constructor: "obj1"
    MObj *obj2 = new MObj("obj2");  // Constructor: "obj2"
    MObj *obj3 = new MObj("obj3");  // Constructor: "obj3"
    MObj *obj4 = new MObj("obj4");  // Constructor: "obj4"

    obj2->setParent(obj1);
    obj3->setParent(obj1);
    obj4->setParent(obj3);
    delete obj3;    // Destructor: "obj3" Destructor: "obj4"

    const QObjectList& list = obj1->children();
    qDebug() << "obj2: " << obj2;   // obj2: QObject(0x107a920)
    for (int i = 0; i < list.length(); i++) {
    
        qDebug() << list[i];    // QObject(0x107a920)
    }
}

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

    return a.exec();
}

Summary

• Qt Objects can exist Father and son
• Through a father son relationship Can get Qt Object tree
• Qt object Remove when destroyed And the parent object Father and son
• Qt object At the time of destruction The colleague destroys all child objects