Widgets handling without new (and without double-delete!)

[nicesw123]

Hi,

the main Qt idiom seems to be to use widgets on the heap via new.
But this is not strictly needed.

Here's a nice example of handling things without new:

dialog.h

#ifndef DIALOG_H
#define DIALOG_H

#include <QDialog>
#include <QPushButton>
#include <QVBoxLayout>
#include <QTextStream>

QTextStream qout{stdout, QIODevice::WriteOnly};

class MyPushButton : public QPushButton {
 public:
 MyPushButton(                                          QWidget * parent = 0) : QPushButton{            parent} { ctorMsg(); }
 MyPushButton(const QString & text,                     QWidget * parent = 0) : QPushButton{text,       parent} { ctorMsg(); }
 MyPushButton(const QIcon & icon, const QString & text, QWidget * parent = 0) : QPushButton(icon, text, parent) { ctorMsg(); }
  ~MyPushButton()  { dtorMsg(); }

 private:
    void ctorMsg() { qout << "ctor MyPushButton (" << text() << ")" << endl; }
    void dtorMsg() { qout << "dtor MyPushButton (" << text() << ")" << endl; }
};

class MyLayout : public QVBoxLayout {
 public:
 MyLayout(QWidget * parent = 0) : QVBoxLayout{ parent } { ctorMsg(); }
  ~MyLayout() { dtorMsg(); }

 private:
    void ctorMsg() { qout << "ctor MyLayout" << endl; }
    void dtorMsg() { qout << "dtor MyLayout" << endl; }
};


class Dialog : public QDialog {
 public:
 Dialog(QWidget *parent = nullptr) : QDialog{ parent }, layout{ this }, oneButton{"One"}, twoButton{"Two"}
    {
      ctorMsg();

      layout.addWidget(&oneButton);
      layout.addWidget(&twoButton);
    }
  
  ~Dialog()
    {
      dtorMsg();
    }


 private:

  void ctorMsg() { qout << "ctor Dialog" << endl; }
  void dtorMsg() { qout << "dtor Dialog" << endl; }

  MyLayout     layout;
  MyPushButton oneButton;
  MyPushButton twoButton;

};

#endif

main.cpp

#include <QApplication>
#include "dialog.h"

int main(int argc, char *argv[])
{
  QApplication app{argc, argv};
  Dialog dialog;

  dialog.show();
  return app.exec();
}

project.pro

TEMPLATE = app
TARGET = go
CONFIG += c++11
QT += widgets

INCLUDEPATH += .
HEADERS += dialog.h
SOURCES += main.cpp

The output is:

ctor MyLayout
ctor MyPushButton (One)
ctor MyPushButton (Two)
ctor Dialog
dtor Dialog
dtor MyPushButton (Two)
dtor MyPushButton (One)
dtor MyLayout

You might ask:
Is this completely safe? Are there no double deletes?

Answer: yes in this manner it is completely safe.
Even if I change the constructor-order, by changing (in dialog.h) to

  MyPushButton oneButton;
  MyLayout     layout;                /* now layout is inbetween ->what happens on delete? */
  MyPushButton twoButton;

Then the output would be

ctor MyPushButton (One)
ctor MyLayout
ctor MyPushButton (Two)
ctor Dialog
dtor Dialog
dtor MyPushButton (Two)
dtor MyLayout
dtor MyPushButton (One)

You may wonder: Does not MyPushButton (One) get double-deleted?
No. MyLayout is not a parent of oneButton. Instead the parent is Dialog - or rather Dialog's baseclass QWidget (because, we set layout{ this }, which is equivalent to calling setLayout(layout)).

Here is the appropriate reference: http://doc.qt.io/qt-5/qwidget.html#setLayout
"The QWidget will take ownership of layout." and of it's added Widgets.

But then if we look above, we see that dtor Dialog is called before other dtors!
Does dtor Dialog not delete the children?? (lurking double delete?)
No, not directly: Dialog is derived from the baseclass QWidget.
And if we were to list the ctor and dtor of the baseclass as well, we get (via normal C++ rules):

ctor QWidget
ctor MyPushButton (One)
ctor MyLayout
ctor MyPushButton (Two)
ctor Dialog
dtor Dialog
dtor MyPushButton (Two)
dtor MyLayout
dtor MyPushButton (One)
dtor QWidget

So for children: Only dtor QWidget would delete children in the layout, if there are any left. But since these children's destructor was already called (normal stack... destructor), the children have already been removed from the parent QWidget.

So all is good.
No double deletes.

Right?

[nicesw123]

For double delete examples:

main.cpp

#include <QObject>
#include <QTextStream>

QTextStream qout{stdout, QIODevice::WriteOnly};

class MyClass : public QObject {
 public:
  MyClass(const QString& name, QObject * parent = nullptr) : QObject{ parent }, m_name{ name } { ctorMsg(); }
  ~MyClass()  { dtorMsg(); }

 private:
  void ctorMsg() { qout << "ctor MyClass (" << m_name << ')' << endl; }
  void dtorMsg() { qout << "dtor MyClass (" << m_name << ')' << endl; }

  const QString m_name;
};

int main()
{
  MyClass child{"child"};
  MyClass parent{"parent"};
  child.setParent(&parent);
  
  return 0;
}

outputs:

ctor MyClass (child)
ctor MyClass (parent)
dtor MyClass (parent)
dtor MyClass (child)
*** Error in `./go': free(): invalid size: 0x00007ffdae7af870 ***
Aborted

To fix it:
change main.cpp to

//...

int main()
{

  MyClass parent{ "parent" };
  MyClass child{ "child", &parent };

  return 0;
}

Now the output is correct:

ctor MyClass (parent)
ctor MyClass (child)
dtor MyClass (child)
dtor MyClass (parent)

PS: you need the following project file:
project.pro

TEMPLATE = app
TARGET = go
CONFIG += c++11
INCLUDEPATH += .
HEADERS +=
SOURCES += main.cpp