如何销毁QPainter对象/消除drawText(）内存泄漏

How do I destroy the QPainter object / Get rid of drawText() memory leak?

本文关键字：内存 泄漏 drawText 消除 何销毁 QPainter 对象      更新时间：2023-10-16

我正试图减少大型应用程序的内存泄漏。使用valgrind，在调用QPainter类的drawText()函数时，我还看到了许多内存泄漏的实例。根据我读到的一些消息来源，这可能是一个Qt错误，但我在想，也许我可以像文档中所说的那样，通过销毁QPainter对象来消除它->"记住在绘图后销毁QPainter对象。"

  • 做这件事的正确方法是什么

以下是valgrind日志中drawText()导致泄漏的一个实例：

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)

如果在堆栈上创建QPainter对象（即不使用new），如文档中所述：

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

然后，当QPainter对象超出范围时（在上面的示例中：在函数的末尾），它将被销毁。文档中提到需要小心删除QPainter对象的原因是，Qt本身不会这样做，这与许多Qt小部件管理有点不同。

所以，假设您动态地分配了这个对象：

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告诉您，您负责清理QPainter对象，在我们的示例中，这是使用delete命令完成的。然而，如果您使用这种方法，您可能会考虑将其放在智能指针中，即：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");
}

现在，至于正确的方法。这取决于，如果你在本地创建QPainter对象，你需要多久创建一次，就像例子中一样？如果这会导致开销，可以考虑将QPainter创建为成员变量，并重用它。

如果您使用new动态分配内存，请记住，您还负责删除它。如果你只是像最上面的例子一样在堆栈上创建它，你不必担心，因为每当它退出创建它的范围时，它就会被销毁。

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