我試圖通過傳遞一個lambda函數作爲第三個參數來調用QObject::connect
。作爲QObject的第三個參數傳遞一個lambda函數時出錯:: connect
不過,我視覺的Visual Studio給了我以下錯誤:
Unhandled exception at 0x0000000066B48265 (Qt5Cored.dll) in QCustomPlotInVS_FirstTry.exe: 0xC0000005: Access violation reading location 0x0000000000000008.
這裏是關於調用堆棧的頂部3線時出現錯誤:
Qt5Cored.dll!QListData::size() Line 91
QCustomPlotInVS_FirstTry.exe!QList::size() Line 132 QCustomPlotInVS_FirstTry.exe!QCustomPlot::graph(int index) Line 9657
91號線在QList作。 h是: inline int size() const { return d->end - d->begin; }
我想我得到這個錯誤,因爲我不正確地嘗試使用一個指針(即QCustomPlot* plot
)無線一個lambda函數。
我的lambda函數的語法是否正確? 如果不是,我做錯了什麼?
這裏是我稱之爲QObject::connect
與lambda函數功能:
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
}
你的回答是正確的。我將'[&](){// ...}'改爲'[=](){// ...}'並且它工作正常。但是,我不清楚爲什麼。我想如果我通過價值捕獲它會創建一個變量的副本(在我的例子中是'QCustomPlot * plot'),那麼對副本的任何更改都不適用於原始變量。我想我只是意識到我的擔憂是錯誤的,因爲複製一個指針會給出一個新的變量,它仍然指向我想修改的對象。我的解釋看起來是否正確? – user3731622
沒錯。您可以更容易地將引用解析爲指針:通過引用捕獲本地指針變量就像將指針存儲在雙指針內部一樣。當以後你嘗試使用它時,它指向一個不再存在的局部變量。相反,通過值捕獲它就像創建一個指向原始對象的新QCustomPlot *(指針的值是指向對象的地址)。 –