QObjects Child Managment

QObject s Child Managment

Example 9.1 shows a QObject derived class.

Example 9.1. src/qobject/person.h

[ . . . . ]
class Person : public QObject {

 public:
 Person(QObject* parent, QString name);
 virtual ~Person();
};
[ . . . . ]

The complete implementation is shown in Example 9.2 to show that there is no explicit object deletion done in ~Person().

Example 9.2. src/qobject/person.cpp

#include "person.h"
#include 
static QTextStream cout(stdout, QIODevice::WriteOnly);

Person::Person(QObject* parent, QString name)
 : QObject(parent) {
 setObjectName(name);
 cout << QString("Constructing Person: %1").arg(name) << endl;
}

Person::~Person() {
 cout << QString("Destroying Person: %1").arg(objectName()) <<
 endl;
}

main(), shown in Example 9.3, creates some objects, adds them to other objects, and then exits. All heap objects were implicitly destroyed.

Example 9.3. src/qobject/main.cpp

#include 
#include "person.h"

static QTextStream cout(stdout, QIODevice::WriteOnly);
int main(int , char**) {
 cout << "First we create a bunch of objects." << endl;
 Person bunch(0, "A Stack Object"); <-- 1
 /* other objects are created on the heap */
 Person *mike = new Person(&bunch, "Mike");
 Person *carol = new Person(&bunch, "Carol");
 new Person(mike, "Greg"); <-- 2
 new Person(mike, "Peter");
 new Person(mike, "Bobby");
 new Person(carol, "Marcia");
 new Person(carol, "Jan");
 new Person(carol, "Cindy");
 new Person(0, "Alice"); <-- 3
 cout << "
Display the list using QObject::dumpObjectTree()"
 << endl;
 bunch.dumpObjectTree();
 cout << "
Program finished - destroy all objects." << endl;
 return 0;
}

 

(1)not a pointer

(2)We do not need to remember pointers to children, since we can reach them via object navigation.

(3)Alice has no parentmemory leak?

Here is the output of this program:

First we create a bunch of objects.
Constructing Person: A Stack Object
Constructing Person: Mike
Constructing Person: Carol
Constructing Person: Greg
Constructing Person: Peter
Constructing Person: Bobby
Constructing Person: Marcia
Constructing Person: Jan
Constructing Person: Cindy
Constructing Person: Alice

Display the list using QObject::dumpObjectTree()
QObject::A Stack Object
 QObject::Mike
 QObject::Greg
 QObject::Peter
 QObject::Bobby
 QObject::Carol
 QObject::Marcia
 QObject::Jan
 QObject::Cindy
Program finished - destroy all objects.
Destroying Person: A Stack Object
Destroying Person: Mike
Destroying Person: Greg
Destroying Person: Peter
Destroying Person: Bobby
Destroying Person: Carol
Destroying Person: Marcia
Destroying Person: Jan
Destroying Person: Cindy

Notice that Alice is not part of the dumpObjectTree() and does not get destroyed.

Exercise: QObject's Child Managment

Add the function

void showTree(QObject* theparent)

to main.cpp. The output of this function, after all objects have been created, should look like this:

Member: Mike - Parent: A Stack Object
Member: Greg - Parent: Mike
Member: Peter - Parent: Mike
Member: Bobby - Parent: Mike
Member: Carol - Parent: A Stack Object
Member: Marcia - Parent: Carol
Member: Jan - Parent: Carol
Member: Cindy - Parent: Carol

Composite Pattern Parents and Children