将lambda函数作为第三个参数传递给QObject::connect时出错

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。

但是，给出了以下错误：

未处理的异常在0x0000000000B48265 (Qt5Cored.dll)中QCustomPlotInVS_FirstTry.exe: 0xC0000005:访问冲突读取位置0x0000000000000008。

下面是调用堆栈中发生错误时的前3行：

Qt5Cored.dll!QListData::size()第91行 QList：：size()第132行QCustomPlotInVS_FirstTry.exe!QCustomPlot::graph(int索引)第9657行

qList.h中的第91行是：inline int size() const { return d->end - d->begin; }

我认为我得到了这个错误，因为我不正确地尝试使用一个指针(即QCustomPlot* plot)和lambda函数。

我的lambda函数语法正确吗？如果没有，我做错了什么？

下面是使用lambda函数调用QObject::connect的函数：

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

您将通过引用捕获堆栈分配的变量(初犯：plot)，该闭包将在稍后调用；一旦调用闭包，所有这些内容都将成为垃圾。取而代之的是按价值捕获。