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我正在尝试减少大型应用程序的内存泄漏。drawText()使用 valgrind，我还看到了很多调用类函数时内存泄漏的实例QPainter。根据我读过的一些资料，这可能是一个Qt错误，但我在想也许我可以通过销毁对象来摆脱它，QPainter就像文档所说的那样 - >“记住QPainter在绘制后销毁对象。”

  • 这样做的正确方法是什么？

drawText()这是导致泄漏的 valgrind 日志的一个实例：

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)
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1 回答 1

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如果您在堆栈上创建 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 QPainter 对象将在超出范围时被销毁（在上面的示例中：在函数的末尾）。文档中提到需要小心删除 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，这是使用delete命令完成的。但是，如果您使用这种方法，您可能会考虑将其放入智能指针中，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，如果您在本地创建 QPainter 对象，您需要多久创建一次，就像在示例中一样？如果这会导致开销，请考虑将 QPainter 创建为成员变量并重用它。（不过，如果您担心速度：“尽早优化绝不是一个好主意”是应该牢记在心的事情）。

如果您使用 动态分配内存new，请记住您也负责删除它。如果您只是像在最上面的示例中那样在堆栈上创建它，则不必担心，因为只要它退出创建它的范围，它就会被销毁。

于 2012-03-13T08:11:41.123 回答