在QWidget子類中調用width（），height（），size（）或rect（）以segfault結尾

Question

我對QWidget的width() ， height() ， size()或rect()函數有問題； 調用它時會出現段錯誤。 這是Qt 4.7。

這是有問題的類的標題：

class PlotCanvas : public QWidget
{ 
    void paintEvent(QPaintEvent * e);
    uint64_t smallestDiv();
    uint64_t longestLength();
    void drawGrid(QPainter * painter);
    QVector<Plot*> plots;
    int calculateHeight() const;
    int calculateWidth() const;
    uint32_t cursorPosition;
public:
    PlotCanvas(QWidget * parent = 0) : QWidget(parent) 
    {
        setStyleSheet("background-color: black");
        setAutoFillBackground(true);
        setSizePolicy(QSizePolicy::MinimumExpanding, QSizePolicy::MinimumExpanding);
        setFocusPolicy(Qt::StrongFocus);
        update();
    }
    void addPlot(Plot * plot);
    int getDivCount();
};

和實際的段錯誤代碼：

int PlotCanvas::getDivCount()
{
   QRect bounds = rect(); //segfaults
   qDebug() << bounds.size().width();
   qDebug() << bounds.size().height();
   return 10;
}

追溯到Qt的qrect.h ：

inline int QRect::width() const
{ return  x2 - x1 + 1; } //this line finally kills it

看起來QWidget內部似乎沒有正確初始化一些東西，但是我不知道是什么，因為如您所見，我並沒有真正改變QWidget初始化。

PlotCanvas小部件位於QAbstractScrollArea子類中（尚未實現，因此只是普通的QAbstractScrollArea ：

class PlotCanvasScrollArea : public QAbstractScrollArea
{
public:
    PlotCanvasScrollArea(QWidget * parent = 0) : QAbstractScrollArea(parent) 
    {

    }
};

我只需要確定QWidget繪畫區域的大小，因為我正在繪制過渡圖，並且我需要知道多少時分適合當前的窗口小部件大小，因為顯然應用程序窗口是可調整大小的。

編輯：

構建PlotCanvas代碼：

Board(QString & name) : name(name)
{
    hbox = new QHBoxLayout();
    commonKnobGroup = new QGroupBox("Common settings:");
    commonKnobGroup->setMinimumWidth(190);
    commonKnobGroup->setSizePolicy(QSizePolicy(QSizePolicy::Minimum, QSizePolicy::Minimum));
    commonKnobs = new CommonKnobs(commonKnobGroup);
    hsplitter = new QSplitter();
    plotGroupBox = new QGroupBox("Measurement plots:", hsplitter);
    settingsGroupBox = new QGroupBox("Plot settings:", hsplitter);
    plotSplitter = new QSplitter();
    plotHbox = new QHBoxLayout();
    plotHbox->addWidget(plotSplitter);
    plotGroupBox->setLayout(plotHbox);
    settingsVLayout = new QVBoxLayout();
    plotNamesColumn = new QListWidget(plotSplitter);
    scrollArea = new PlotCanvasScrollArea(plotSplitter);
    //scrollArea->setWidgetResizable(true);
    plotsCanvas = new PlotCanvas();
    scrollArea->setViewport(plotsCanvas);
    scrollArea->setBackgroundRole(QPalette::Dark);
    specializedKnobGroup = new QGroupBox("Probe settings:");
    plotSettings = new QStackedWidget(specializedKnobGroup);
    settingsVLayout->addWidget(commonKnobGroup);
    settingsVLayout->addWidget(specializedKnobGroup);
    settingsGroupBox->setLayout(settingsVLayout);
    hbox->addWidget(hsplitter);
    setLayout(hbox);
    connect(plotNamesColumn, SIGNAL(currentItemChanged(QListWidgetItem *, QListWidgetItem *)), this,
            SLOT(changePageNames(QListWidgetItem*, QListWidgetItem*)));
    connect(commonKnobs, SIGNAL(scaleUpdated()), plotsCanvas, SLOT(update()));

};

編輯2：
完整的堆棧跟蹤：

#0  0x000000010000b9ac in QWidget::rect() const at /Volumes/Data/shinji/QtSDK/Desktop/Qt/474/gcc/include/QtGui/qwidget.h:1007
#1  0x000000010000b454 in PlotCanvas::getDivCount() ()
#2  0x000000010000ab39 in Board::getDivCount() ()
#3  0x0000000100009fbc in CommonKnobs::updateScaleByBeginEnd() ()
#4  0x00000001000015b4 in CommonKnobs::qt_metacall(QMetaObject::Call, int, void**) ()
#5  0x0000000100d09b66 in QMetaObject::activate(QObject*, QMetaObject const*, int, void**) ()
#6  0x000000010070df9d in QSpinBox::valueChanged(int) ()
#7  0x00000001004d9684 in QSpinBoxPrivate::emitSignals(EmitPolicy, QVariant const&) ()
#8  0x000000010043b321 in QAbstractSpinBoxPrivate::setValue(QVariant const&, EmitPolicy, bool) ()
#9  0x000000010043c11a in QAbstractSpinBoxPrivate::_q_editorTextChanged(QString const&) ()
#10 0x000000010043cca7 in QAbstractSpinBox::qt_metacall(QMetaObject::Call, int, void**) ()
#11 0x000000010070e435 in QSpinBox::qt_metacall(QMetaObject::Call, int, void**) ()
#12 0x0000000100d09b66 in QMetaObject::activate(QObject*, QMetaObject const*, int, void**) ()
#13 0x000000010048a1e6 in QLineEdit::textChanged(QString const&) ()
#14 0x000000010048d23a in QLineEdit::qt_metacall(QMetaObject::Call, int, void**) ()
#15 0x0000000100d09b66 in QMetaObject::activate(QObject*, QMetaObject const*, int, void**) ()
#16 0x000000010070c9f9 in QLineControl::textChanged(QString const&) ()
#17 0x0000000100494337 in QLineControl::finishChange(int, bool, bool) ()
#18 0x0000000100496664 in QLineControl::processKeyEvent(QKeyEvent*) ()
#19 0x000000010048b8eb in QLineEdit::keyPressEvent(QKeyEvent*) ()
#20 0x00000001001056bd in QWidget::event(QEvent*) ()
#21 0x000000010048c18b in QLineEdit::event(QEvent*) ()
#22 0x000000010043d9f0 in QAbstractSpinBox::keyPressEvent(QKeyEvent*) ()
#23 0x00000001001056bd in QWidget::event(QEvent*) ()
#24 0x000000010043bb1b in QAbstractSpinBox::event(QEvent*) ()
#25 0x00000001004d7be5 in QSpinBox::event(QEvent*) ()
#26 0x00000001000a9e8d in QApplicationPrivate::notify_helper(QObject*, QEvent*) ()
#27 0x00000001000b198b in QApplication::notify(QObject*, QEvent*) ()
#28 0x0000000100d0321c in QCoreApplication::notifyInternal(QObject*, QEvent*) ()
#29 0x00000001000a9f2c in qt_sendSpontaneousEvent(QObject*, QEvent*) ()
#30 0x0000000100125cc1 in QKeyMapper::sendKeyEvent(QWidget*, bool, QEvent::Type, int, QFlags<Qt::KeyboardModifier>, QString const&, bool, int, unsigned int, unsigned int, unsigned int, bool*) ()
#31 0x0000000100126b43 in QKeyMapperPrivate::translateKeyEvent(QWidget*, OpaqueEventHandlerCallRef*, OpaqueEventRef*, void*, bool) ()
#32 0x000000010006072e in qt_dispatchKeyEvent(void*, QWidget*) ()
#33 0x000000010005478b in -[QCocoaView keyDown:] ()
#34 0x00007fff815a90c7 in -[NSWindow sendEvent:] ()
#35 0x0000000100059891 in -[QCocoaWindow sendEvent:] ()
#36 0x00007fff814ddafa in -[NSApplication sendEvent:] ()
#37 0x000000010005cf0a in -[QNSApplication sendEvent:] ()
#38 0x00007fff814746de in -[NSApplication run] ()
#39 0x0000000100066c04 in QEventDispatcherMac::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) ()
#40 0x0000000100def774 in QEventLoop::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) ()
#41 0x0000000100defa94 in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) ()
#42 0x0000000100df10bc in QCoreApplication::exec() ()
#43 0x0000000100001cf6 in main ()

Answer 1

看一下崩潰堆棧跟蹤-它是否在某個地方通向構造函數？ 我的第一個想法是，我不確定您是否應該在構造函數中調用update-我可能不在基礎上，但這對我來說似乎是錯誤的。 如果update應該在QWidget構造函數中調用，則基本構造函數將調用它（使您的調用成為多余的），否則，我根本不知道會發生什么。

Answer 2

您的scrollArea是作為plotSplitter的子級創建的，但永遠不會使用addWidget（）添加到拆分器中，此外plotSplitter沒有父級，這意味着它將是自己的窗口。 我可以想象問題可能出在將scrollArea的視口設置為plotsCanvas時，而后者是另一個沒有父級的QWidget，因此也沒有它自己的窗口。

嘗試這樣的事情，然后從那里開始工作：

Board(QString & name) : name(name)
{
    hbox = new QHBoxLayout();
    commonKnobGroup = new QGroupBox("Common settings:");
    commonKnobGroup->setMinimumWidth(190);
    commonKnobGroup->setSizePolicy(QSizePolicy(QSizePolicy::Minimum, QSizePolicy::Minimum));
    commonKnobs = new CommonKnobs(commonKnobGroup);
    hsplitter = new QSplitter();
    plotGroupBox = new QGroupBox("Measurement plots:", hsplitter);
    settingsGroupBox = new QGroupBox("Plot settings:", hsplitter);
    plotSplitter = new QSplitter(this);
    plotHbox = new QHBoxLayout();
    plotHbox->addWidget(plotSplitter);
    plotGroupBox->setLayout(plotHbox);
    settingsVLayout = new QVBoxLayout();
    plotNamesColumn = new QListWidget(plotSplitter);
    scrollArea = new PlotCanvasScrollArea(plotSplitter);
    plotSplitter->addWidget(plotNamesColumn);
    plotSplitter->addWidget(scrollArea);
    //scrollArea->setWidgetResizable(true);
    plotsCanvas = new PlotCanvas(this);
    scrollArea->setViewport(plotsCanvas);
    scrollArea->setBackgroundRole(QPalette::Dark);
    specializedKnobGroup = new QGroupBox("Probe settings:");
    plotSettings = new QStackedWidget(specializedKnobGroup);
    settingsVLayout->addWidget(commonKnobGroup);
    settingsVLayout->addWidget(specializedKnobGroup);
    settingsGroupBox->setLayout(settingsVLayout);
    hbox->addWidget(hsplitter);
    setLayout(hbox);
    connect(plotNamesColumn, SIGNAL(currentItemChanged(QListWidgetItem *, QListWidgetItem *)), this,
            SLOT(changePageNames(QListWidgetItem*, QListWidgetItem*)));
    connect(commonKnobs, SIGNAL(scaleUpdated()), plotsCanvas, SLOT(update()));

};

Answer 3

嘗試使用小部件geometry而不是rect 。 另外，請注意復制構造函數。 如果使用成員變量，則需要使用setGeometry或setRect代替副本構造函數。

請參閱Qt Window Geometry文檔。

Answer 4

我已經解決了它，它使用了parent（）作為指向Board的指針（使用scatic_cast進行了廣播，我認為使用static_cast轉換為錯誤的對象會殺死它，但不會）從PlotCanvas和我正在使用的另一個類中實際上，如果是PlotCanvas，則將parent（）引用到PlotCanvasScrollArea；如果是其他類，則將其引用到QGroupBox。
調試器顯示出我實際上跳入了Board的代碼，甚至認為被調用對象的地址是完全錯誤的，這很奇怪，而更奇怪的是，它崩潰了而不是起源於PlotCanvas代碼（Board的代碼）。
但是現在可以了。 感謝大伙們 ：）