如何销毁QPainter对象/摆脱drawText（）内存泄漏？

Question

我正在尝试减少大型应用程序的内存泄漏。 使用valgrind，我在调用QPainter类的drawText()函数时也看到了许多内存泄漏实例。 根据我读过的一些消息来源，它可能是一个Qt bug，但我想也许我可以通过破坏QPainter对象来解决它，正如文档所说 - >“记住在绘制后销毁QPainter对象。 “

• 这样做的正确方法是什么？

这是valgrind日志的一个实例，其中drawText()导致泄漏：

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

如果在堆栈上创建QPainter对象（即不使用new ），则如文档中所述：

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

然后QPainter对象在超出范围时将被销毁（在上面的例子中：在函数的末尾）。 文档提到您需要小心删除QPainter对象的原因是因为Qt本身不会这样做，这与许多Qt小部件管理有点不同。

所以，假设你动态分配了这个对象：

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，使用delete命令完成。 但是，如果你使用这种方法，你可以考虑把它放在智能指针中，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，至于正确的方式。 这取决于，如果您在本地创建QPainter对象，您需要多久创建一次QPainter对象，如示例中所示？ 如果这会导致开销，请考虑将QPainter创建为成员变量，然后重复使用它。 （但是，如果你担心速度：“早期优化永远不是一个好主意”是应该致力于内心的事情）。

如果使用new动态分配内存，请记住您也负责删除它。 如果您只是在堆栈中创建它，就像在最顶层的示例中一样，您不必担心，因为它会在退出创建它的范围时被销毁。