如何销毁QPainter对象/摆脱drawText（）内存泄漏？

Question

I'm trying to trim down the memory leaks of a large application. Using valgrind, I also saw many instances of memory leaks when calling the drawText() function of the QPainter class. It could be a Qt bug according to some sources that I've read, but I was thinking maybe I could get rid of it by destroying the QPainter object just as the documentation has said -> "Remember to destroy the QPainter object after drawing."

• What is the right way of doing that?

Here's one instance of valgrind log where drawText() causes a leak:

127971 ==00:00:05:31.916 24132== 68,594 (768 direct, 67,826 indirect) bytes in 2 blocks     are definitely lost in loss record 4,979 of 4,982
127972 ==00:00:05:31.916 24132==    at 0x4C2683D: malloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
127973 ==00:00:05:31.916 24132==    by 0x81861FF: ft_mem_qalloc (in /usr/lib64/libfreetype.so.6.6.2)
127974 ==00:00:05:31.916 24132==    by 0x8186242: ft_mem_alloc (in /usr/lib64/libfreetype.so.6.6.2)
127975 ==00:00:05:31.916 24132==    by 0x81876BE: FT_New_Library (in  /usr/lib64/libfreetype.so.6.6.2)
127976 ==00:00:05:31.916 24132==    by 0x81819C3: FT_Init_FreeType (in /usr/lib64/libfreetype.so.6.6.2)
127977 ==00:00:05:31.916 24132==    by 0x54E4667: ??? (in /usr/lib64/libQtGui.so.4.7.1)
127978 ==00:00:05:31.916 24132==    by 0x54E4A64:  QFontEngineFT::init(QFontEngine::FaceId, bool, QFontEngineFT::GlyphFormat) (in  /usr/lib64/libQtGui.so.4.7.1)
127979 ==00:00:05:31.916 24132==    by 0x54DE0B5: QFontEngineX11FT::QFontEngineX11FT(_FcPattern*, QFontDef const&, int) (in /usr/lib64/libQtGui.so.4.7.1)
127980 ==00:00:05:31.916 24132==    by 0x542EE80: ??? (in /usr/lib64/libQtGui.so.4.7.1)
127981 ==00:00:05:31.916 24132==    by 0x543721A: QFontDatabase::load(QFontPrivate const*, int) (in /usr/lib64/libQtGui.so.4.7.1)
127982 ==00:00:05:31.916 24132==    by 0x5414D46: QFontPrivate::engineForScript(int) const (in /usr/lib64/libQtGui.so.4.7.1)
127983 ==00:00:05:31.916 24132==    by 0x5429FDD: QFontMetricsF::leading() const (in /usr/lib64/libQtGui.so.4.7.1)
127984 ==00:00:05:31.916 24132==    by 0x534ED45: ??? (in /usr/lib64/libQtGui.so.4.7.1)
127985 ==00:00:05:31.916 24132==    by 0x534FCBD: QPainter::drawText(QRect const&, int, QString const&, QRect*) (in /usr/lib64/libQtGui.so.4.7.1)
127986 ==00:00:05:31.916 24132==    by 0x5863AC: gui::base::Printer::printTitle() (in /home/bed/workspace/tasks/MemProfile/MemoryProfiling/build-pc-debug/src/application-ui)
127987 ==00:00:05:31.916 24132==    by 0x585559: gui::base::Printer::run() (in /home/bed/workspace/tasks/MemProfile/MemoryProfiling/build-pc-debug/src/application-ui)
127988 ==00:00:05:31.916 24132==    by 0x5D49A5D: ??? (in /usr/lib64/libQtCore.so.4.7.1)
127989 ==00:00:05:31.916 24132==    by 0x7620A3E: start_thread (in /lib64/libpthread- 2.11.3.so)
127990 ==00:00:05:31.916 24132==    by 0x738067C: clone (in /lib64/libc-2.11.3.so)

Answer 1

If you create the QPainter object on the stack (that is, not using new ), as is mentioned in the documentation:

void SimpleExampleWidget::paintEvent(QPaintEvent *)
{
    QPainter painter(this);
    painter.setPen(Qt::blue);
    painter.setFont(QFont("Arial", 30));
    painter.drawText(rect(), Qt::AlignCenter, "Qt");
}

Then the QPainter object will be destroyed when it falls out of scope (in the example above: at the end of the function). The reason the documentation mentions that you need to be careful to delete the QPainter object, is because Qt will not do so itself, which is a bit different from a lot of Qt widget management.

So, say you allocated this object dynamically instead:

void SimpleExampleWidget::paintEvent(QPaintEvent *)
{
    QPainter* painter = new QPainter(this);
    painter->setPen(Qt::blue);
    painter->setFont(QFont("Arial", 30));
    painter->drawText(rect(), Qt::AlignCenter, "Qt");
    // Delete object, since Qt wont do it for you:
    delete painter;
}

Qt is telling you that you are in charge of cleaning up the QPainter object, which in our example, is done using the delete command. However, if you're using this approach, you might consider putting it in a smart pointer, ie: boost::scoped_ptr<QPainter> painter = new QPainter(this);

void SimpleExampleWidget::paintEvent(QPaintEvent *)
{
    boost::scoped_ptr<QPainter> painter = new QPainter(this);
    painter->setPen(Qt::blue);
    painter->setFont(QFont("Arial", 30));
    painter->drawText(rect(), Qt::AlignCenter, "Qt");
}

Now, as for the right way to do it. It depends, how often would you have to create QPainter objects if you created them locally, like in the example? If this causes overhead, consider creating the QPainter as a member variable, and reusing it. (Though, if you're concerned about speed: "optimizing early is never a good idea" is something that should be committed to heart).

If you allocate memory dynamically using new , remember that you are in charge of deleting it as well. If you just create it on the stack like in the top most example, you shouldn't have to worry, as it will be destroyed whenever it exits the scope where it was created.