QScrollArea Memory Leak

[rossimo]

Hey, I'm currently using Qt Embedded 4.5.1. I'm using a QScrollArea to contain a QLabel. Each time I render the QScrollArea, the process eats up more memory. Is there a way I can prevent this? Is there something I'm doing incorrect?

MyWidget::MyWidget(QWidget *parent) : QWidget(parent)
{
    buildWidget();
    retranslateWidget();
}
 
void MyWidget::buildWidget()
{
    layout = new QVBoxLayout;
    scroll = new QScrollArea;
 
    scroll->setWidgetResizable(true);
    scroll->setSizePolicy(QSizePolicy::Minimum, QSizePolicy::Maximum);
    scroll->setFrameShape(QFrame::NoFrame);
    scroll->setHorizontalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
 
    text = new QLabel;
    text->setWordWrap(true);
 
    scroll->setWidget(text);
 
    layout->addWidget(scroll);
 
    setLayout(layout);
}
 
void MyWidget::retranslateWidget()
{
    aboutText->setText(tr("Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum."));
}

MyWidget::MyWidget(QWidget *parent) : QWidget(parent)
{
    buildWidget();
    retranslateWidget();
}

void MyWidget::buildWidget()
{
    layout = new QVBoxLayout;
    scroll = new QScrollArea;

    scroll->setWidgetResizable(true);
    scroll->setSizePolicy(QSizePolicy::Minimum, QSizePolicy::Maximum);
    scroll->setFrameShape(QFrame::NoFrame);
    scroll->setHorizontalScrollBarPolicy(Qt::ScrollBarAlwaysOff);

    text = new QLabel;
    text->setWordWrap(true);

    scroll->setWidget(text);

    layout->addWidget(scroll);

    setLayout(layout);
}

void MyWidget::retranslateWidget()
{
    aboutText->setText(tr("Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum."));
}

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[high_flyer]

Well, if this is a wanted behavior to create a new label each time in buildWidget(), then its not memory leak.
However some remarks:
1. you should do 'text = new QLable(this);' so that the pointer gets deleted when the parent gets deleted, OR, explicitly delete 'text' when its not needed any more.
If you didn't do that, then THAT is a memory leak.
2. If you don't really want to create a new label each time, than make it a member and either create in once in the contructor, or check to see if it is not yet created and only then allocate it like so:

MyWidget::MyWidget(QWidget *parent) : QWidget(parent)
,text(NULL)
{
...
}
 
void MyWidget::buildWidget()
{
...
if(!text)
  text = new QLabel(this);
 
{

MyWidget::MyWidget(QWidget *parent) : QWidget(parent)
,text(NULL)
{
...
}

void MyWidget::buildWidget()
{
...
if(!text)
  text = new QLabel(this);

{

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[wysota]

The label will get reparented to the scroll area after the setWidget() call so no need to delete it explicitely. I would be interested in knowing how the OP knows the application eats up memory after each redraw of the widget.

[rossimo]

I can watch the memory consumption of the process by calling "watch /proc/(pid)/status". I found a few more interesting things. This problem does not arise when placing a QPixmap in the QLabel instead of text. Also, if I hide() the QScrollArea, the problem goes away as well.

Here's the full code for the app:

mywidget.h

#ifndef MYWIDGET_H
#define MYWIDGET_H
 
#include <QWidget>
 
class QVBoxLayout;
class QScrollArea;
class QLabel;
class QPushButton;
 
class MyWidget : public QWidget
{
    Q_OBJECT
 
public:
    MyWidget(QWidget *parent = 0);
 
private:
    void buildWidget();
    void retranslateWidget();
 
    QVBoxLayout *layout;
    QScrollArea *scroll;
    QLabel *text;
    QPushButton *button;
 
private slots:
    void rebuildScrollArea();
};
 
#endif // MYWIDGET_H

#ifndef MYWIDGET_H
#define MYWIDGET_H

#include <QWidget>

class QVBoxLayout;
class QScrollArea;
class QLabel;
class QPushButton;

class MyWidget : public QWidget
{
    Q_OBJECT

public:
    MyWidget(QWidget *parent = 0);

private:
    void buildWidget();
    void retranslateWidget();

    QVBoxLayout *layout;
    QScrollArea *scroll;
    QLabel *text;
    QPushButton *button;

private slots:
    void rebuildScrollArea();
};

#endif // MYWIDGET_H

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mywidget.cpp

#include "mywidget.h"
 
#include <QScrollArea>
#include <QVBoxLayout>
#include <QLabel>
#include <QPushButton>
#include <QApplication>
#include <QImage>
 
MyWidget::MyWidget(QWidget *parent) : QWidget(parent)
{
    scroll = NULL;
 
    setFixedHeight(240);
    setFixedWidth(320);
 
    buildWidget();
 
    connect(button, SIGNAL(clicked()), this, SLOT(rebuildScrollArea()));
}
 
void MyWidget::buildWidget()
{
    layout = new QVBoxLayout;
    setLayout(layout);
 
    button = new QPushButton;
    layout->addWidget(button);
 
    rebuildScrollArea();
}
 
void MyWidget::rebuildScrollArea()
{
    if (scroll) scroll->deleteLater();
 
    scroll = new QScrollArea;
    scroll->setWidgetResizable(true);
    //scroll->hide();
 
    text = new QLabel;
    text->setWordWrap(true);
    //text->setPixmap(QPixmap(":/stewart.png"));
    //text->adjustSize();
 
    scroll->setWidget(text);
 
    layout->addWidget(scroll);
 
    retranslateWidget();
}
 
void MyWidget::retranslateWidget()
{
    button->setText(tr("Click to Rebuild"));
    text->setText(tr("Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum."
                     "Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum."
                     "Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum."));
 
}

#include "mywidget.h"

#include <QScrollArea>
#include <QVBoxLayout>
#include <QLabel>
#include <QPushButton>
#include <QApplication>
#include <QImage>

MyWidget::MyWidget(QWidget *parent) : QWidget(parent)
{
    scroll = NULL;

    setFixedHeight(240);
    setFixedWidth(320);

    buildWidget();

    connect(button, SIGNAL(clicked()), this, SLOT(rebuildScrollArea()));
}

void MyWidget::buildWidget()
{
    layout = new QVBoxLayout;
    setLayout(layout);

    button = new QPushButton;
    layout->addWidget(button);

    rebuildScrollArea();
}

void MyWidget::rebuildScrollArea()
{
    if (scroll) scroll->deleteLater();

    scroll = new QScrollArea;
    scroll->setWidgetResizable(true);
    //scroll->hide();

    text = new QLabel;
    text->setWordWrap(true);
    //text->setPixmap(QPixmap(":/stewart.png"));
    //text->adjustSize();

    scroll->setWidget(text);

    layout->addWidget(scroll);

    retranslateWidget();
}

void MyWidget::retranslateWidget()
{
    button->setText(tr("Click to Rebuild"));
    text->setText(tr("Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum."
                     "Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum."
                     "Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum."));

}

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main.ccp

#include <QtGui/QApplication>
#include "mywidget.h"
#include "zdefaultstyle.h"
 
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MyWidget w;
 
#if defined(Q_WS_S60)
    w.showMaximized();
#else
    w.show();
#endif
 
    return a.exec();
}

#include <QtGui/QApplication>
#include "mywidget.h"
#include "zdefaultstyle.h"

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

#if defined(Q_WS_S60)
    w.showMaximized();
#else
    w.show();
#endif

    return a.exec();
}

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[high_flyer]

This problem does not arise when placing a QPixmap in the QLabel instead of text.

That is probably because the QLabel is allocated on the heap, and the QPixmap on the stack, and the stack size is fixed, and 'watch' probably is looking at the dynamic memory allocation.

I still fail to understand the problem.
Allocating new object consumes more memory, that is the nature of things.

Each time I render the QScrollArea, the process eats up more memory.

As long as 'MyWidget' object is alive, it only allocates new QLabels.
If each time you render the QScrollArea its done with the same 'MyWidget' object (with rebuildScrollArea()), then you allocate new memory.
You also should see a new QLabel in your scroll area each time you call rebuildScrollArea().

Also, if I hide() the QScrollArea, the problem goes away as well.

Maybe its because you call rebuildScrollArea() as part of the paint sequence, so it is not called when the widget is not visible.