Using pointers for QObject attributes

Question

Since I learned Qt, I've been confused by the fact that in the documentations, and books I've read, they use pointers for attributes that are instances of QObject subclasses, such as widgets.

I know that QObjects delete their children, but shouldn't we avoid using pointers except when it's really necessary?

here is a working example in which I don't use pointers:

Widget.h file:

#include <QSlider> #include <QLabel> #include <QVBoxLayout> class Widget : public QWidget { Q_OBJECT public: Widget(QWidget *parent = 0); public slots: void change(int); private: QSlider m_slider; QLabel m_label; QVBoxLayout m_layout; };

and Widget.cpp file:

#include "Widget.h" Widget::Widget(QWidget *parent) : QWidget(parent) , m_slider(Qt::Horizontal) , m_label("0") , m_layout(this) { m_layout.addWidget(&m_slider); m_layout.addWidget(&m_label); connect(&m_slider, SIGNAL(valueChanged(int)), this, SLOT(change(int))); } void Widget::change(int n){ m_label.setText(QString::number(n)); }

the only difference here is that I have to include the header in Widget.h file, would that be the reason for using pointers?

And I want to add that I've seen some similar question on StackOverflow but the answer was that widgets should live between function calls, but that's achieved here when using them as attributes.

Why do you feel you should avoid pointers though?

jdi
– jdi

2013-01-02 19:04:19 +00:00
Commented Jan 2, 2013 at 19:04 — jdi
– jdi, Commented Jan 2, 2013 at 19:04

jdi · Accepted Answer · 2013-01-02 19:14:53Z

There seems to be a pretty lengthy discussion on this same question here:
Class member: pointer or variable?

Some of the points from this link talk about:

Long lived vs Short lived objects
Heap vs Stack
Guaranteed order of destructors based on the parents set on the objects

It seems that the take away here is there isn't a hard and fast rule, but rather a case-by-case basis. As I am currently also migrating from PyQt -> Qt and picking up my C++, my limited opinion so far is that I always use pointers for class members that are QObject subclasses. I just feel like I know they will be destroyed only by the Qt parent->child destruction process.

Again, this is a limited perspective. I am sure someone with strong C++ experience can offer a better answer. But the link I referred to does include very experienced Qt programmers.

Thanks for the link. I've understood that it may be unsafe to make QObject member variables, if we are going to change their parent from the containing object to another one, because it will get deleted and the parent will have a NULL pointer.Consequently it's safer to use pointers because they get deleted by their parents.
Ya. My thought on it is that if it is going to be a member object, then the parent should be something withing that container that ultimately has the class as the parent. Unless it is a special case where it is holding a pointer to some other object, in which case it should be clear about not taking ownership (setting as parent)

benjarobin · Accepted Answer · 2013-01-02 18:58:09Z

0

Maybe to simply using a different constructor than the default one ?

answered Jan 2, 2013 at 18:58

benjarobin

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1 Comment

Youssef Bouhjira Over a year ago

thanks for the answer, but how can using member variables instead of pointers prevent restrict us to the default constructor

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