作为QObject的第三个参数传递一个lambda函数时出错:: connect

Question

我试图通过传递一个lambda函数作为第三个参数来调用QObject::connect。

不过，我视觉的Visual Studio给了我以下错误：

Unhandled exception at 0x0000000066B48265 (Qt5Cored.dll) in QCustomPlotInVS_FirstTry.exe: 0xC0000005: Access violation reading location 0x0000000000000008.

这里是关于调用堆栈的顶部3线时出现错误：

Qt5Cored.dll!QListData::size() Line 91
QCustomPlotInVS_FirstTry.exe!QList::size() Line 132 QCustomPlotInVS_FirstTry.exe!QCustomPlot::graph(int index) Line 9657

91号线在QList作。 h是： inline int size() const { return d->end - d->begin; }

我想我得到这个错误，因为我不正确地尝试使用一个指针（即QCustomPlot* plot）无线一个lambda函数。

我的lambda函数的语法是否正确？ 如果不是，我做错了什么？

这里是我称之为QObject::connect与lambda函数功能：

void setupRealTimePlot(QCustomPlot* plot, QTimer* dataTimer) 
{ 
    plot->addGraph(); // blue line 
    plot->graph(0)->setPen(QPen(Qt::blue)); 
    plot->graph(0)->setBrush(QBrush(QColor(240, 255, 200))); 
    plot->graph(0)->setAntialiasedFill(false); 
    plot->addGraph(); // red line 
    plot->graph(1)->setPen(QPen(Qt::red)); 
    plot->graph(0)->setChannelFillGraph(plot->graph(1)); 

    plot->addGraph(); // blue dot 
    plot->graph(2)->setPen(QPen(Qt::blue)); 
    plot->graph(2)->setLineStyle(QCPGraph::lsNone); 
    plot->graph(2)->setScatterStyle(QCPScatterStyle::ssDisc); 
    plot->addGraph(); // red dot 
    plot->graph(3)->setPen(QPen(Qt::red)); 
    plot->graph(3)->setLineStyle(QCPGraph::lsNone); 
    plot->graph(3)->setScatterStyle(QCPScatterStyle::ssDisc); 

    plot->xAxis->setTickLabelType(QCPAxis::ltDateTime); 
    plot->xAxis->setDateTimeFormat("hh:mm:ss"); 
    plot->xAxis->setAutoTickStep(false); 
    plot->xAxis->setTickStep(2); 
    plot->axisRect()->setupFullAxesBox(); 

    // make left and bottom axes transfer their ranges to right and top axes: 
    QObject::connect(plot->xAxis, SIGNAL(rangeChanged(QCPRange)), plot->xAxis2, SLOT(setRange(QCPRange))); 
    QObject::connect(plot->yAxis, SIGNAL(rangeChanged(QCPRange)), plot->yAxis2, SLOT(setRange(QCPRange))); 

    // setup a timer that repeatedly calls MainWindow::realtimeDataSlot: 
    QObject::connect(dataTimer, &QTimer::timeout, 
     [&]() 
     { 
      // calculate two new data points: 
      double key = QDateTime::currentDateTime().toMSecsSinceEpoch()/1000.0; 
      static double lastPointKey = 0; 
      if (key-lastPointKey > 0.01) // at most add point every 10 ms 
      { 
      double value0 = qSin(key); //qSin(key*1.6+qCos(key*1.7)*2)*10 + qSin(key*1.2+0.56)*20 + 26; 
      double value1 = qCos(key); //qSin(key*1.3+qCos(key*1.2)*1.2)*7 + qSin(key*0.9+0.26)*24 + 26; 
      // add data to lines: 
      plot->graph(0)->addData(key, value0); 
      plot->graph(1)->addData(key, value1); 
      // set data of dots: 
      plot->graph(2)->clearData(); 
      plot->graph(2)->addData(key, value0); 
      plot->graph(3)->clearData(); 
      plot->graph(3)->addData(key, value1); 
      // remove data of lines that's outside visible range: 
      plot->graph(0)->removeDataBefore(key-8); 
      plot->graph(1)->removeDataBefore(key-8); 
      // rescale value (vertical) axis to fit the current data: 
      plot->graph(0)->rescaleValueAxis(); 
      plot->graph(1)->rescaleValueAxis(true); 
      lastPointKey = key; 
      } 
      // make key axis range scroll with the data (at a constant range size of 8): 
      plot->xAxis->setRange(key+0.25, 8, Qt::AlignRight); 
      plot->replot(); 

      // calculate frames per second: 
      static double lastFpsKey; 
      static int frameCount; 
      ++frameCount; 
      if (key-lastFpsKey > 2) // average fps over 2 seconds 
      { 
      lastFpsKey = key; 
      frameCount = 0; 
      } 
     }); 

    dataTimer->start(0); // Interval 0 means to refresh as fast as possible 

} 

Answer 1

你通过引用捕获栈上分配的变量（初犯：plot）在将被称为closure后来;所有这些东西一旦被调用就会变成垃圾。取而代之的是价值观。