Does this code provide memory leaks?

Question

Finally i have installed Ubuntu and set up Qt+Valgrind to prevent memory leaks which i could not do in Windows. So i can not understand does this code provide a memory leak? In fact that Valgrind says i have only 500+ issues but nothing about the leak. I

#include <QWidget>
#include <QFrame>
#include <QVBoxLayout>
#include <QApplication>

int main(int argc, char *argv[])

{
    QApplication a(argc, argv);

    QWidget * wdgt = new QWidget;  //this line should be the cause of leakage 
                                   //if it exist (as far as i know)
    QVBoxLayout *layout = new QVBoxLayout;
    QFrame * frame = new QFrame;

    frame->setFrameStyle(QFrame::Panel | QFrame::Plain);
    frame->setLineWidth(5);
    layout->addWidget(frame);

    wdgt->setLayout(layout);
    wdgt->setFixedSize(800,600);
    wdgt->show();

    return a.exec();
}

Answer 1

See this post: Creating and deallocating a Qt widget object

It explaines that if one Qt object has a parent, it will be automatically deleted when the parent is destroyed.

In your code:

• wdgt is the parent of layout because you did wdgt->setLayout(layout).
• wdgt is the parent of frame because you did layout->addWidget(frame) and layout's parent is wdgt. As commented by thuga, layout pass the ownership to their own parent.

In your code, only wdgt is orphan (no Qt parent to delete it automatically).

To fix this, you can either give him a parent:

QWidget * wdgt = new QWidget(&app);

So that wdgt is a child of app and will then be deleted automatically when app is destroyed.

or delete it yourself:

int main(int argc, char *argv[])
{
    ...
    int res = a.exec();
    delete wdgt; // this will delete wdgt, but also frame and layout
    return res;
}

or, fianlly, create it as an object, so that it is deleted when going out of scope:

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

    QWidget wdgt;

    QVBoxLayout *layout = new QVBoxLayout;
    QFrame * frame = new QFrame;

    frame->setFrameStyle(QFrame::Panel | QFrame::Plain);
    frame->setLineWidth(5);
    layout->addWidget(frame);

    wdgt.setLayout(layout);
    wdgt.setFixedSize(800,600);
    wdgt.show();

    return a.exec();
}

By the way, note that if you do QVBoxLayout *layout = new QVBoxLayout(wdgt), there is no need to do wdgt->setLayout(layout). So those two piece of codes are equivalent:

QVBoxLayout *layout = new QVBoxLayout(wdgt); // parenting upon construction

is the same as:

QVBoxLayout *layout = new QVBoxLayout; // no parent
wdgt->setLayout( layout ); // reparenting