c++ - 当 C++ 尝试访问 vector 的元素(在范围内)时，Qt Quick Windows 应用程序崩溃

Question

我在 Windows 上的 Qt Quick 应用程序遇到了非常奇怪的崩溃。

我有一个名为 QObjectVector 的 QObject 派生 c++ 类，它允许 QML 端访问容器中的 QObject 指针。在 QML 方面，我有一堆按钮显示该容器中对象的名称。当我滚动鼠标滚轮时，我手动更新每个按钮的 currentItem 属性。

奇怪的是，只有有时，在不可预测 次数的快速滚动之后，程序崩溃(停止工作)。调试后发现，当at函数访问m_data[i]时会发生崩溃。我已经确保 i 在有效范围 [0, m_data.size()) 内，因此它不是索引超出范围错误。我的猜测是，不知何故，QML 引擎在尝试管理内存时删除了我在堆上的对象。

QObjectVector.h

#pragma once

#include <QObject>
#include <QVector>

#include "Types.h"

namespace LPP
{
    class QObjectVector : public QObject
    {
        Q_OBJECT

        Q_PROPERTY(Int size READ size NOTIFY sizeChanged)

    public:
        explicit QObjectVector();
        virtual ~QObjectVector();

        Int size();
        Q_INVOKABLE QObject* at(Int);
        void push(QObject*);
        void remove(Int);
        void remove(QObject*);

        QVector<QObject*> &getData();

        bool deleteChildrenOnDestroy;

    private:
        QVector<QObject*> m_data;

    signals:
        void sizeChanged();

    public slots:
    };
}

QObjectVector.cpp

...
    QObject* QObjectVector::at(Int i)
    {
        qDebug() << "size: " << this->m_data.size();
        for (int i = 0; i < this->m_data.size(); i++){
            qDebug() << "i: " << i << " item: ";
            //when I scroll rapidly, crash happens here when I trace objects in vector
            qDebug() << this->m_data[i];
        }
        qDebug() << "returning.";
        return (i >= 0 && i < this->m_data.size()) ? this->m_data[i] : nullptr;
    }
....

按钮容器.qml

....
    //this function is called when I scroll mouse wheel.
    //itemList contains list of Buttons

    function refreshDisplay() {
        var i, j = 0;

        for (i = 0; i < itemList.length; i++){
            itemList[i].visible = itemList[i].enabled = false;
        }

        var start = Utils.clamp(Math.floor(scrollView.flickableItem.contentY / (itemHeight + itemVSpacing)), 0, currentFolder.size);
        var end = Utils.clamp(Math.ceil((scrollView.flickableItem.contentY + scrollView.flickableItem.height) / (itemHeight + itemVSpacing)), 0, currentFolder.size);

        var item;

        for (i = start; i < end; i++){
            if (j >= itemList.length){
                itemList.push(tableItem_comp.createObject(tableArea));
            }

            itemList[j].visible = itemList[j].enabled = true;

            item = currentFolder.at(i);
            itemList[j].currentItem = item;
            j++;
        }
    }
....

按钮.qml

....
    property var currentItem: null;

    anchors.left: parent.left
    anchors.right: parent.right

    SimpleButton {
        id: button
        width: 100
        height: 50
        text: currentItem.name;
        onClicked: {
            viewer.select(currentItem);
        }
    }
....

这个问题非常奇怪和不可预测，解决它对我的应用程序的开发非常关键，所以请拜托，任何帮助将不胜感激。

谢谢!

汤米

Answer 1

这是因为您要返回的对象归 QML 引擎所有，因此可以随时进行垃圾回收。

试着打电话

QQmlEngine::setObjectOwnership(object, QQmlEngine::CppOwnership)

在每个对象返回到 QML 之前(例如，当它被构造时)。

参见 this section文档:

When data is transferred from C++ to QML, the ownership of the data always remains with C++. The exception to this rule is when a QObject is returned from an explicit C++ method call: in this case, the QML engine assumes ownership of the object, unless the ownership of the object has explicitly been set to remain with C++ by invoking QQmlEngine::setObjectOwnership() with QQmlEngine::CppOwnership specified.

Additionally, the QML engine respects the normal QObject parent ownership semantics of Qt C++ objects, and will not ever take ownership of a QObject instance which already has a parent.