带有父应用程序崩溃的QSqlQueryModel。

I am rather new to Qt, maybe that's why I cant fully understand the child-parent concept. I need to perform some sql query. I set the QSqlQuery, perform the "prepare and bind" operation and exec it. Next I pass it to the model and display the data. The problem occurs when closing the window - I get a memory violation error. The error occurs only, when I create the model with a parent. Here's the code:

我对Qt很陌生，也许这就是为什么我不能完全理解孩子父母的概念。我需要执行一些sql查询。我设置了QSqlQuery，执行“准备和绑定”操作并执行它。接下来，我将它传递给模型并显示数据。当关闭窗口时出现问题—我有内存违规错误。只有当我与父类创建模型时，才会出现错误。这是代码:

QSqlQuery query;
query.prepare(QString("SELECT \
        %1 as nazwa \
        , kontrahentid \
        FROM kontrahent WHERE %2 ilike ?"
    ).arg(showWhat, searchBy) //handled above, no need to escape
);
query.addBindValue(searchString); //user input data - so bind it

if (!query.exec()) {
    qDebug() << query.lastError();
    QApplication::restoreOverrideCursor();
    return;
}

if (model == NULL)
//  model = new QSqlQueryModel; // app closes the window correctly
    model = new QSqlQueryModel(this); // app crashes when closing the window

model->setQuery(query);
if (model->lastError().isValid()) {
    qDebug() << model->lastError();
    QApplication::restoreOverrideCursor();
    return;
}

model->setHeaderData(0, Qt::Horizontal, "ID");
ui.kontrahenciList->setModel(model);
//ui.kontrahenciList->setModelColumn(1);
ui.kontrahenciList->show();

Here's the error I'm getting:

这是我的错误:

Unhandled exception at 0x0fe29f9a (qsqlpsqld.dll) in HurBudClientGUI.exe: 0xC0000005: Access violation reading location 0x00000004.

and the call stack:

和调用堆栈:

qsqlpsqld.dll!QScopedPointer<QObjectData,QScopedPointerDeleter<QObjectData> >::data()  Line 143 + 0x3 bytes C++
qsqlpsqld.dll!qGetPtrHelper<QScopedPointer<QObjectData,QScopedPointerDeleter<QObjectData> > >(const QScopedPointer<QObjectData,QScopedPointerDeleter<QObjectData> > & p)  Line 919 + 0xb bytes  C++
qsqlpsqld.dll!QPSQLDriver::d_func()  Line 106 + 0x13 bytes  C++
qsqlpsqld.dll!QPSQLResultPrivate::privDriver()  Line 212    C++
qsqlpsqld.dll!QPSQLResultPrivate::deallocatePreparedStmt()  Line 306 + 0xc bytes    C++
qsqlpsqld.dll!QPSQLResult::~QPSQLResult()  Line 328 C++
qsqlpsqld.dll!QPSQLResult::`scalar deleting destructor'()  + 0xf bytes  C++
Qt5Sqld.dll!QSqlQueryPrivate::~QSqlQueryPrivate()  Line 94 + 0x23 bytes C++
Qt5Sqld.dll!QSqlQueryPrivate::`scalar deleting destructor'()  + 0xf bytes   C++
Qt5Sqld.dll!QSqlQuery::~QSqlQuery()  Line 245 + 0x1e bytes  C++
Qt5Sqld.dll!QSqlQueryModelPrivate::~QSqlQueryModelPrivate()  Line 90 + 0x3d bytes   C++
Qt5Sqld.dll!QSqlQueryModelPrivate::`scalar deleting destructor'()  + 0xf bytes  C++
Qt5Cored.dll!672cbf06()     
[Frames below may be incorrect and/or missing, no symbols loaded for Qt5Cored.dll]  
Qt5Cored.dll!672cb92a()     
Qt5Cored.dll!672c03f4()     
Qt5Cored.dll!67200dc4()     
Qt5Cored.dll!67203608()     
Qt5Sqld.dll!QSqlQueryModel::~QSqlQueryModel()  Line 175 + 0x9 bytes C++

As I mentioned above: the error doesn't happen when (one of below):

正如我上面提到的:错误不会发生在(以下一个):

I create QSqlQueryModel without the parent (model = new QSqlQueryModel;)
我创建的QSqlQueryModel没有父模型(model = new QSqlQueryModel;)
I pass "static" query to QSqlQueryModel (regardless of having the parent).
我将“静态”查询传递给QSqlQueryModel(不考虑是否有父类)。

eg:

例如:

model->setQuery(
    QSqlQuery(
        QString("SELECT \
            %1 as nazwa \
            , kontrahentid \
            FROM kontrahent"
        ).arg(showWhat)
    )
);

What am I doing wrong? And the real question is: what is the purpose for QSqlQueryModel having a parent? If I delete it manually in the window's destructor - is there any diffrence?

我做错了什么?真正的问题是:QSqlQueryModel有父类的目的是什么?如果我在窗口的析构函数中手动删除它，有什么不同吗?

I guess this is a bug - I reported it on qt bugtracker: https://bugreports.qt.io/browse/QTBUG-43889

我想这是一个bug——我在qt bugtracker上报告了它:https://bugreports.qt.io/browse/QTBUG-43889。

3 个解决方案

#1

This is the key part:

这是关键部分:

if (model == NULL)
    //  model = new QSqlQueryModel; // app closes the window correctly
    model = new QSqlQueryModel(this); // app crashes when closing the window

The parent-child concept in Qt provides many features, with automatic child destruction being one of them.

Qt中的父-子概念提供了许多特性，其中之一是自动的子破坏。

If any QObject has other QObject set as a parent, then when parent QObject is deleted, the child object will also be deleted.

如果任何QObject有其他QObject集作为父对象，那么当父QObject被删除时，子对象也将被删除。

Now, you mentioned you do explicitly delete the model in destructor of the window. You should not. If you define this as a parent, then the model will be automatically deleted together with this, but it was already deleted by you in the destructor, so model is deleted twice, thus your error appears.

现在，您提到了在窗口的析构函数中显式地删除模型。你不应该。如果您将其定义为父类，那么模型将会被自动删除，但是在析构函数中已经被删除了，所以模型被删除了两次，因此出现了错误。

#2

So I removed the "delete operation" code in the destructor. Error still occurs, the only difference is diffrent call stack. error message:

因此，我删除了析构函数中的“删除操作”代码。错误仍然发生，唯一的区别是不同的调用堆栈。错误信息:

Unhandled exception at 0x0f249f9a (qsqlpsqld.dll) in HurBudClientGUI.exe: 0xC0000005: Access violation reading location 0x00000004.

call stack:

调用堆栈:

qsqlpsqld.dll!QScopedPointer<QObjectData,QScopedPointerDeleter<QObjectData> >::data()  Line 143 + 0x3 bytes C++
qsqlpsqld.dll!qGetPtrHelper<QScopedPointer<QObjectData,QScopedPointerDeleter<QObjectData> > >(const QScopedPointer<QObjectData,QScopedPointerDeleter<QObjectData> > & p)  Line 919 + 0xb bytes  C++
qsqlpsqld.dll!QPSQLDriver::d_func()  Line 106 + 0x13 bytes  C++
qsqlpsqld.dll!QPSQLResultPrivate::privDriver()  Line 212    C++
qsqlpsqld.dll!QPSQLResultPrivate::deallocatePreparedStmt()  Line 306 + 0xc bytes    C++
qsqlpsqld.dll!QPSQLResult::~QPSQLResult()  Line 328 C++
qsqlpsqld.dll!QPSQLResult::`scalar deleting destructor'()  + 0xf bytes  C++
Qt5Sqld.dll!QSqlQueryPrivate::~QSqlQueryPrivate()  Line 94 + 0x23 bytes C++
Qt5Sqld.dll!QSqlQueryPrivate::`scalar deleting destructor'()  + 0xf bytes   C++
Qt5Sqld.dll!QSqlQuery::~QSqlQuery()  Line 245 + 0x1e bytes  C++
Qt5Sqld.dll!QSqlQueryModelPrivate::~QSqlQueryModelPrivate()  Line 90 + 0x3d bytes   C++
Qt5Sqld.dll!QSqlQueryModelPrivate::`scalar deleting destructor'()  + 0xf bytes  C++
Qt5Cored.dll!QScopedPointerDeleter<QObjectData>::cleanup(QObjectData * pointer)  Line 62 + 0x20 bytes   C++
Qt5Cored.dll!QScopedPointer<QObjectData,QScopedPointerDeleter<QObjectData> >::~QScopedPointer<QObjectData,QScopedPointerDeleter<QObjectData> >()  Line 109 + 0x9 bytes  C++
Qt5Cored.dll!QObject::~QObject()  Line 940 + 0x15 bytes C++
Qt5Cored.dll!QAbstractItemModel::~QAbstractItemModel()  Line 1454 + 0xf bytes   C++
Qt5Cored.dll!QAbstractTableModel::~QAbstractTableModel()  Line 3299 + 0x8 bytes C++
Qt5Sqld.dll!QSqlQueryModel::~QSqlQueryModel()  Line 175 + 0x9 bytes C++
HurBudClientGUI.exe!QSqlQueryModel::`scalar deleting destructor'()  + 0x10 bytes    C++
Qt5Cored.dll!QObjectPrivate::deleteChildren()  Line 1841 + 0x24 bytes   C++
Qt5Widgetsd.dll!QWidget::~QWidget()  Line 1488  C++
Qt5Widgetsd.dll!QDockWidget::~QDockWidget()  Line 1172 + 0x22 bytes C++
HurBudClientGUI.exe!searchDock::~searchDock()  Line 28 + 0x1c bytes C++

searchDock is the class I'm working with.

searchDock是我正在学习的课程。

#3

I guess I solved it. When the model is initialized as view's child:

我想我解决了。当模型初始化为视图的子节点时:

model = new QSqlQueryModel(this);

The problem is order of operations. When I close the connection and remove the database in the destructor of my class - the database gets disconnected and no further operations are possible. But after my class' destructor, destructors of its base classes get into action - one by another, as ordered by inheritance. When QObject::~QObject() is executed, it gets to the

问题是操作的顺序。当我关闭连接并在类的析构函数中删除数据库时，数据库就会被断开，没有进一步的操作。但是，在我的类“析构函数”之后，它的基类的析构函数会按照继承的顺序依次执行。当QObject::~QObject()被执行时，它会到达。

QObjectPrivate::deleteChildren()

method. And then, it finally gets to delete the model. The model wants to free the resources - which means QSqlResult (QPSQLResult to be specific in that case). This it how it looks:

方法。然后，它最终会删除模型。模型希望释放资源——这意味着QSqlResult(在这种情况下，QPSQLResult是特定的)。这就是它的样子:

QPSQLResult::~QPSQLResult()
{
    Q_D(QPSQLResult);
    cleanup();
    if (d->preparedQueriesEnabled && !d->preparedStmtId.isNull())
        d->deallocatePreparedStmt();
};

So here Qt tries to deallocate preparedStatement - regardless of the fact, that the connection no longer exists:

所以这里Qt尝试释放preparedStatement——不管事实是什么，连接不再存在:

void QPSQLResultPrivate::deallocatePreparedStmt()
{
    const QString stmt = QLatin1String("DEALLOCATE ") + preparedStmtId;
    PGresult *result = privDriver()->exec(stmt);
    if (PQresultStatus(result) != PGRES_COMMAND_OK)
        qWarning("Unable to free statement: %s", PQerrorMessage(privDriver()->connection));
    PQclear(result);
    preparedStmtId.clear();
};

So - to make it work properly, I wolud have to call

所以，为了让它正常工作，我必须打电话。

QSqlQueryModel::~QSqlQueryModel()

或

QSqlQueryModel::clear()

BEFORE closing the connection with the DB. I still think it's a bug.

在关闭与DB的连接之前。我仍然认为这是一个错误。

#1