Why does the app crashes on delete QSqlQuery*

deleted385

Here's what I've now:

void ObjectsView::openFileDialog(){
    QFileDialog dialog;
    if(!dialog.exec()) return;
    if(db.isOpen()) {
        db.close();
        qDebug() << "deleting";
        delete query;
        qDebug() << "deleted";
    }
    auto widget = qobject_cast<QueryWidget*>(parent()->parent());
    fileName = dialog.selectedFiles().first();
    db.setDatabaseName(fileName);
    db.open();
    query = new QSqlQuery("SELECT * FROM sqlite_master", db);
    query->setForwardOnly(true);
    if(!query->exec()){
        emit widget->logMessage("e,Could not connect to " + fileName); //e, for error
        return;
    }
    makeTree();
    emit widget->logMessage("s,Connected to " + fileName); // s, for success
    isConnected = true;
}

With these it doesn't crash always. When it does, I see it printing deleting, so it crashes for this delete query line. How to fix?

eyllanesc

@Emon-Haque Why is a pointer necessary? Change QSqlQuery *query to QSqlQuery query in .h, and then make query = QSqlQuery("SELECT * FROM sqlite_master", db);, thus avoiding these unnecessary problems.

Pl45m4

@Emon-Haque said in Why does the app crashes on delete QSqlQuery*:

When it does, I see it printing deleting, so it crashes for this delete query line. How to fix?

Looks like a segfault. You're probably accessing query (i.e. trying to delete it) when there wasn't created a query object before, so it's still null.

Check if query is valid before trying to delete it.

JonB

@Emon-Haque
As @Pl45m4 has commented. Your query is a member variable, so what do you initialise it to before you call this method for the first time?!

Also, after you go delete query; please set it to nullptr, to avoid any future re-deletes....

deleted385

@Pl45m4, in the constructor, I don't do anything with the query, for the db, I have only one line:

db = QSqlDatabase::addDatabase("QSQLITE");

I changed that qDebug() << "deleting"; to qDebug() << "deleting" << query;. Here's, without a crash, what I did:

and here's the qDebug output:

deleting 0x0
deleted
deleting 0x41a5c00
deleted
deleting 0x41a59e0
deleted

first, I attached quran.db and it deleted a nullptr? I didn't call db.Open() anywhere, this is the only function where I call db.Open(). Looks like db.isOpen() returned true although I didn't call Open(). For the rest two database I attached, it got something to delete.

deleted385

@JonB, do I've to initialize it in constructor? I check whether the db.isOpen(), if it is opened, it should get into that block BUT, to me, it looks like it's getting into that block when the db is not open.

eyllanesc

@Emon-Haque Why is a pointer necessary? Change QSqlQuery *query to QSqlQuery query in .h, and then make query = QSqlQuery("SELECT * FROM sqlite_master", db);, thus avoiding these unnecessary problems.

SGaist

Hi,

@Emon-Haque said in Why does the app crashes on delete QSqlQuery*:

How to fix?

In the case of QSqlQuery: do not create it on the heap. There's no reason for that.
If you look at all the code related to QSqlQuery, you can see that none of them allocate it on the heap.

deleted385

@eyllanesc, can you change the database of such query variable later?

EDIT
I did try that approach and at that time I got some error. Now, I don't have any, I'll go with that, Thanks.

eyllanesc

@Emon-Haque Your question is unclear. If you don't pass a QSqlDatabase to QSqlQuery then it will use the default database.

deleted385

@eyllanesc, it can't be changed later. I've to reassign query = QSqlQuery("SELECT * FROM sqlite_master", db); everytime. Not a problem, I think.

JonB

@Emon-Haque
In your code, if when you enter ObjectsView::openFileDialog() db is already open but query is a member variable and uninitialised it will crash. Initialise member variables before first use.

I also said that for safety you should query = nullptr immediately after delete query, to prevent accidental re-deletion.

If you delete it anywhere else it will also crash here.

Finally, if nothing works. I don't know about this, but when you create query you associate it with db at that time. I do not know if you close or change db whether that could cause a problem when you later delete query associated with a now-closed-or-different db.

Frankly the whole business of deleting query only next time you open/create db is "messy". And it doesn't solve what happens to allocated query if you never come to openFileDialog() again. Do you need to do things this way?

JonB

@Emon-Haque said in Why does the app crashes on delete QSqlQuery*:

@eyllanesc, it can't be changed later. I've to reassign query = QSqlQuery("SELECT * FROM sqlite_master", db); everytime. Not a problem, I think.

Just what is waiting to trip someone up when the code gets changed at a later date. Things like "set to nullptr after you delete a member variable" is just good practice.

deleted385

@JonB, later after a break, I'll try with a global QSqlQuery* again and will try to use that in other view, where I've used QSqlQueryModel testModel (in previous post), to execute multiple statements AND instead of db.isOpen(), I'll use my global isConnected to see whether it crashes with that or not.