How to correctly re-assign a pointer?

**pkohut** · 4th January 2011, 15:18

Originally Posted by homerun4711

so the crash should not happen at all?!?

It happens because DataManager still thinks m_customerTableView is valid. Post full versions of AddressBook and DataManager (both header and cpp). It's diffecult to get the big picture at this point with just code fragments.

Added after 19 minutes:

I've got an idea, will take a few to compose my thoughts. Hold off on posting the code if you like.

**homerun4711** · 4th January 2011, 15:28

One minute to late

Here is the complete code:

AddressBook.h

Qt Code:

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class AddressBook : public QDialog, public Ui::AddressBook
{
    Q_OBJECT
 
public:
    AddressBook(DataManager* dmgr, QWidget *parent = 0);
    ~AddressBook();
 
private:
   DataManager* data;
 
};
 
#endif

class AddressBook : public QDialog, public Ui::AddressBook
{
    Q_OBJECT

public:
    AddressBook(DataManager* dmgr, QWidget *parent = 0);
    ~AddressBook();

private:
   DataManager* data;

};

#endif

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AddressBook.cpp

Qt Code:

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AddressBook::AddressBook(DataManager* dmgr, QWidget *parent): QDialog(parent)
{
 
    Q_ASSERT(dmgr);
    data = dmgr;
    Q_ASSERT(data);
 
    setupUi(this);
    connect(buttonBox, SIGNAL(rejected()), this, SLOT(reject()));
 
    tableView = dmgr->customerTableView(this);
    Q_ASSERT(tableView);
 
}
 
AddressBook::~AddressBook()
{
data->DeleteCustomerTableView();
}

AddressBook::AddressBook(DataManager* dmgr, QWidget *parent): QDialog(parent)
{

    Q_ASSERT(dmgr);
    data = dmgr;
    Q_ASSERT(data);

    setupUi(this);
    connect(buttonBox, SIGNAL(rejected()), this, SLOT(reject()));
    
    tableView = dmgr->customerTableView(this);
    Q_ASSERT(tableView);

}

AddressBook::~AddressBook()
{
data->DeleteCustomerTableView();
}

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DataManager.h

Qt Code:

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class DataManager
{
public:
    DataManager();
 
    QStandardItemModel *customerModel();
    QStandardItemModel *articlesModel();
    QStandardItemModel *invoicesModel();
    QStandardItemModel *invoiceEntriesModel();
 
    QTableView *customerTableView(QWidget *parent = 0);
    QTableView *articlesTableView(QWidget *parent = 0);
    QTableView *invoicesTableView(QWidget* parent = 0);
    QTableView *invoiceEntriesTableView(QWidget* parent = 0);
 
    QTableView *DeleteCustomerTableView();
    QTableView *DeleteArticlesTableView();
    QTableView *DeleteInvoicesTableView();
    QTableView *DeleteInvoiceEntriesTableView();
 
private:
 
    QStandardItemModel *model_customer;
    QStandardItemModel *model_articles;
    QStandardItemModel *model_invoices;
    QStandardItemModel *model_invoiceEntries;
 
    QTableView *m_customerTableView;
    QTableView *m_articlesTableView;
    QTableView *m_invoicesTableView;
    QTableView *m_invoiceEntriesTableView;
 
    QItemSelectionModel *selectionModelCustomer;
    QItemSelectionModel *selectionModelArticles;
    QItemSelectionModel *selectionModelInvoices;
    QItemSelectionModel *selectionModelInvoiceEntries;
 
    QHeaderView *headerViewCustomer;
    QHeaderView *headerViewArticles;
    QHeaderView *headerViewInvoices;
    QHeaderView *headerViewInvoiceEntries;
 
    void setupModels();
    void setupTableViews();
 
};

class DataManager
{
public:
    DataManager();
     
    QStandardItemModel *customerModel();
    QStandardItemModel *articlesModel();
    QStandardItemModel *invoicesModel();
    QStandardItemModel *invoiceEntriesModel();
    
    QTableView *customerTableView(QWidget *parent = 0);
    QTableView *articlesTableView(QWidget *parent = 0);
    QTableView *invoicesTableView(QWidget* parent = 0);
    QTableView *invoiceEntriesTableView(QWidget* parent = 0);
    
    QTableView *DeleteCustomerTableView();
    QTableView *DeleteArticlesTableView();
    QTableView *DeleteInvoicesTableView();
    QTableView *DeleteInvoiceEntriesTableView();
    
private:
    
    QStandardItemModel *model_customer;
    QStandardItemModel *model_articles;
    QStandardItemModel *model_invoices;
    QStandardItemModel *model_invoiceEntries;

    QTableView *m_customerTableView;
    QTableView *m_articlesTableView;
    QTableView *m_invoicesTableView;
    QTableView *m_invoiceEntriesTableView;

    QItemSelectionModel *selectionModelCustomer;
    QItemSelectionModel *selectionModelArticles;
    QItemSelectionModel *selectionModelInvoices;
    QItemSelectionModel *selectionModelInvoiceEntries;

    QHeaderView *headerViewCustomer;
    QHeaderView *headerViewArticles;
    QHeaderView *headerViewInvoices;
    QHeaderView *headerViewInvoiceEntries;

    void setupModels();
    void setupTableViews();
   
};

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DataManager.cpp

Qt Code:

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DataManager::DataManager()
{
    QTextStream out(stdout);
    out << "DataManager()\n";
 
    setupModels();
    setupTableViews(); //set QTableViews to NULL
 
}
 
void DataManager::setupModels()
{
 
    QTextStream out(stdout);
    out << "setupModels()\n";
 
    model_customer = new QStandardItemModel;
    model_customer->setObjectName("Customers");
 
    model_articles  = new QStandardItemModel;
    model_articles->setObjectName("Articles");
 
    model_invoices  = new QStandardItemModel;
    model_invoices->setObjectName("Invoices");
 
    model_invoiceEntries  = new QStandardItemModel;
    model_invoiceEntries->setObjectName("InvoiceEntries");
 
}
 
 
 
void DataManager::setupTableViews()
{
    QTextStream out(stdout);
    out << "setupTableViews()\n";
 
    //set to NULL, else the delete later will crash the app
    m_customerTableView = NULL;
    m_articlesTableView = NULL;
    m_invoicesTableView = NULL;
    m_invoiceEntriesTableView = NULL;
 
}
 
QStandardItemModel *DataManager::customerModel()
{
   Q_ASSERT(model_customer);
   return model_customer;
}
 
QStandardItemModel *DataManager::articlesModel()
{
    Q_ASSERT(model_articles);
    return model_articles;
}
 
QStandardItemModel *DataManager::invoicesModel()
{
    Q_ASSERT(model_invoices);
    return model_invoices;
}
 
QStandardItemModel *DataManager::invoiceEntriesModel()
{    
    Q_ASSERT(model_invoiceEntries);
    return model_invoiceEntries;
}
 
QTableView *DataManager::customerTableView(QWidget* parent)
    {
 
    DeleteCustomerTableView();
 
    m_customerTableView = new QTableView(parent);
    m_customerTableView->setModel(model_customer);
    m_customerTableView->setGeometry(QRect(10, 10, 700, 400));
    m_customerTableView->setObjectName("CustomerTableView");;
    m_customerTableView->setSortingEnabled(true);
    m_customerTableView->sortByColumn(0, Qt::DescendingOrder);
    m_customerTableView->setSelectionMode(QAbstractItemView::SingleSelection);
    m_customerTableView->setSelectionBehavior(QAbstractItemView::SelectRows);
    selectionModelCustomer = new QItemSelectionModel(model_customer);
    m_customerTableView->setSelectionModel(selectionModelCustomer);
    headerViewCustomer = m_customerTableView->horizontalHeader();
    return m_customerTableView;
    }
 
 
QTableView *DataManager::articlesTableView(QWidget* parent)
{
 
    DeleteArticlesTableView();
 
    m_articlesTableView = new QTableView(parent);
    m_articlesTableView->setModel(model_articles);
    m_articlesTableView->setGeometry(QRect(10, 10, 700, 400));
    m_articlesTableView->setObjectName("ArticlesTableView");;
    m_articlesTableView->setSortingEnabled(true);
    m_articlesTableView->sortByColumn(0, Qt::DescendingOrder);
    m_articlesTableView->setSelectionMode(QAbstractItemView::SingleSelection);
    m_articlesTableView->setSelectionBehavior(QAbstractItemView::SelectRows);
    selectionModelArticles = new QItemSelectionModel(model_articles);
    m_articlesTableView->setSelectionModel(selectionModelArticles);
    headerViewArticles = m_articlesTableView->horizontalHeader();
    return m_articlesTableView;
}
 
QTableView *DataManager::invoicesTableView(QWidget* parent)
{
 
    DeleteInvoicesTableView();
 
    m_invoicesTableView = new QTableView(parent);
    m_invoicesTableView->setModel(model_invoices);
    m_invoicesTableView->setGeometry(QRect(10, 10, 700, 400));
    m_invoicesTableView->setObjectName("ArticlesTableView");;
    m_invoicesTableView->setSortingEnabled(true);
    m_invoicesTableView->sortByColumn(0, Qt::DescendingOrder);
    m_invoicesTableView->setSelectionMode(QAbstractItemView::SingleSelection);
    m_invoicesTableView->setSelectionBehavior(QAbstractItemView::SelectRows);
    selectionModelInvoices = new QItemSelectionModel(model_invoices);
    m_invoicesTableView->setSelectionModel(selectionModelInvoices);
    headerViewInvoices = m_invoicesTableView->horizontalHeader();
    return m_invoicesTableView;
}
 
QTableView *DataManager::invoiceEntriesTableView(QWidget* parent)
{
    DeleteInvoiceEntriesTableView();
 
    m_invoiceEntriesTableView = new QTableView(parent);
    m_invoiceEntriesTableView->setModel(model_invoiceEntries);
    m_invoiceEntriesTableView->setGeometry(QRect(10, 10, 700, 400));
    m_invoiceEntriesTableView->setObjectName("ArticlesTableView");;
    m_invoiceEntriesTableView->setSortingEnabled(true);
    m_invoiceEntriesTableView->sortByColumn(0, Qt::DescendingOrder);
    m_invoiceEntriesTableView->setSelectionMode(QAbstractItemView::SingleSelection);
    m_invoiceEntriesTableView->setSelectionBehavior(QAbstractItemView::SelectRows);
    selectionModelInvoiceEntries = new QItemSelectionModel(model_invoiceEntries);
    m_invoiceEntriesTableView->setSelectionModel(selectionModelInvoiceEntries);
    headerViewInvoices = m_invoiceEntriesTableView->horizontalHeader();
    return m_invoiceEntriesTableView;
}
 
 
 
//suggestion from forum, to avoid leaks
QTableView *DataManager::DeleteCustomerTableView()
{
    delete m_customerTableView;
    m_customerTableView = NULL;
    return m_customerTableView;
}
 
QTableView *DataManager::DeleteArticlesTableView()
{
    delete m_articlesTableView;
    m_articlesTableView = NULL;
    return m_articlesTableView;
}
 
QTableView *DataManager::DeleteInvoicesTableView()
{
    delete m_invoicesTableView;
    m_invoicesTableView = NULL;
    return m_invoicesTableView;
}
 
QTableView *DataManager::DeleteInvoiceEntriesTableView()
{
    delete m_invoiceEntriesTableView;
    m_invoiceEntriesTableView = NULL;
    return m_invoiceEntriesTableView;
}

DataManager::DataManager()
{
    QTextStream out(stdout);
    out << "DataManager()\n";
    
    setupModels();
    setupTableViews(); //set QTableViews to NULL

}

void DataManager::setupModels()
{

    QTextStream out(stdout);
    out << "setupModels()\n";

    model_customer = new QStandardItemModel;
    model_customer->setObjectName("Customers");

    model_articles  = new QStandardItemModel;
    model_articles->setObjectName("Articles");

    model_invoices  = new QStandardItemModel;
    model_invoices->setObjectName("Invoices");

    model_invoiceEntries  = new QStandardItemModel;
    model_invoiceEntries->setObjectName("InvoiceEntries");

}



void DataManager::setupTableViews()
{
    QTextStream out(stdout);
    out << "setupTableViews()\n";

    //set to NULL, else the delete later will crash the app
    m_customerTableView = NULL;
    m_articlesTableView = NULL;
    m_invoicesTableView = NULL;
    m_invoiceEntriesTableView = NULL;

}

QStandardItemModel *DataManager::customerModel()
{
   Q_ASSERT(model_customer);
   return model_customer;
}

QStandardItemModel *DataManager::articlesModel()
{
    Q_ASSERT(model_articles);
    return model_articles;
}

QStandardItemModel *DataManager::invoicesModel()
{
    Q_ASSERT(model_invoices);
    return model_invoices;
}

QStandardItemModel *DataManager::invoiceEntriesModel()
{    
    Q_ASSERT(model_invoiceEntries);
    return model_invoiceEntries;
}

QTableView *DataManager::customerTableView(QWidget* parent)
    {

    DeleteCustomerTableView();

    m_customerTableView = new QTableView(parent);
    m_customerTableView->setModel(model_customer);
    m_customerTableView->setGeometry(QRect(10, 10, 700, 400));
    m_customerTableView->setObjectName("CustomerTableView");;
    m_customerTableView->setSortingEnabled(true);
    m_customerTableView->sortByColumn(0, Qt::DescendingOrder);
    m_customerTableView->setSelectionMode(QAbstractItemView::SingleSelection);
    m_customerTableView->setSelectionBehavior(QAbstractItemView::SelectRows);
    selectionModelCustomer = new QItemSelectionModel(model_customer);
    m_customerTableView->setSelectionModel(selectionModelCustomer);
    headerViewCustomer = m_customerTableView->horizontalHeader();
    return m_customerTableView;
    }


QTableView *DataManager::articlesTableView(QWidget* parent)
{

    DeleteArticlesTableView();

    m_articlesTableView = new QTableView(parent);
    m_articlesTableView->setModel(model_articles);
    m_articlesTableView->setGeometry(QRect(10, 10, 700, 400));
    m_articlesTableView->setObjectName("ArticlesTableView");;
    m_articlesTableView->setSortingEnabled(true);
    m_articlesTableView->sortByColumn(0, Qt::DescendingOrder);
    m_articlesTableView->setSelectionMode(QAbstractItemView::SingleSelection);
    m_articlesTableView->setSelectionBehavior(QAbstractItemView::SelectRows);
    selectionModelArticles = new QItemSelectionModel(model_articles);
    m_articlesTableView->setSelectionModel(selectionModelArticles);
    headerViewArticles = m_articlesTableView->horizontalHeader();
    return m_articlesTableView;
}

QTableView *DataManager::invoicesTableView(QWidget* parent)
{

    DeleteInvoicesTableView();

    m_invoicesTableView = new QTableView(parent);
    m_invoicesTableView->setModel(model_invoices);
    m_invoicesTableView->setGeometry(QRect(10, 10, 700, 400));
    m_invoicesTableView->setObjectName("ArticlesTableView");;
    m_invoicesTableView->setSortingEnabled(true);
    m_invoicesTableView->sortByColumn(0, Qt::DescendingOrder);
    m_invoicesTableView->setSelectionMode(QAbstractItemView::SingleSelection);
    m_invoicesTableView->setSelectionBehavior(QAbstractItemView::SelectRows);
    selectionModelInvoices = new QItemSelectionModel(model_invoices);
    m_invoicesTableView->setSelectionModel(selectionModelInvoices);
    headerViewInvoices = m_invoicesTableView->horizontalHeader();
    return m_invoicesTableView;
}

QTableView *DataManager::invoiceEntriesTableView(QWidget* parent)
{
    DeleteInvoiceEntriesTableView();

    m_invoiceEntriesTableView = new QTableView(parent);
    m_invoiceEntriesTableView->setModel(model_invoiceEntries);
    m_invoiceEntriesTableView->setGeometry(QRect(10, 10, 700, 400));
    m_invoiceEntriesTableView->setObjectName("ArticlesTableView");;
    m_invoiceEntriesTableView->setSortingEnabled(true);
    m_invoiceEntriesTableView->sortByColumn(0, Qt::DescendingOrder);
    m_invoiceEntriesTableView->setSelectionMode(QAbstractItemView::SingleSelection);
    m_invoiceEntriesTableView->setSelectionBehavior(QAbstractItemView::SelectRows);
    selectionModelInvoiceEntries = new QItemSelectionModel(model_invoiceEntries);
    m_invoiceEntriesTableView->setSelectionModel(selectionModelInvoiceEntries);
    headerViewInvoices = m_invoiceEntriesTableView->horizontalHeader();
    return m_invoiceEntriesTableView;
}



//suggestion from forum, to avoid leaks
QTableView *DataManager::DeleteCustomerTableView()
{
    delete m_customerTableView;
    m_customerTableView = NULL;
    return m_customerTableView;
}

QTableView *DataManager::DeleteArticlesTableView()
{
    delete m_articlesTableView;
    m_articlesTableView = NULL;
    return m_articlesTableView;
}

QTableView *DataManager::DeleteInvoicesTableView()
{
    delete m_invoicesTableView;
    m_invoicesTableView = NULL;
    return m_invoicesTableView;
}

QTableView *DataManager::DeleteInvoiceEntriesTableView()
{
    delete m_invoiceEntriesTableView;
    m_invoiceEntriesTableView = NULL;
    return m_invoiceEntriesTableView;
}

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**pkohut** · 4th January 2011, 15:36

AddressBook "has-a" QTableView, it should own it. DataManager should only reference it. So remove m_customerTableView from DataManager and move it to AddressBook.
Create a new member function in DataManager called CustomerModel.
If DataManager needs to operate on a QTableView then pass a pointer to the function.

Qt Code:

Switch view

AddressBook::AddressBook(DataManager* dmgr, QWidget *parent): QDialog(parent)
{
    m_customerTableView = new QTableView(this);
    m_customerTableView->setModel(dmgr->customerModel());
}
 
DataManager::DataManager()
{
   ...   
   ...
}
 
QAbstractItemMode * DataManager::customerModel(void)
{
    return model_customer;
}
 
void DataManager::someFunctionThatNeedsToWorkonQTableView(QTableView * tableView)
{
    tableView->doWhatEverNeedsToBeDone();
}

AddressBook::AddressBook(DataManager* dmgr, QWidget *parent): QDialog(parent)
{
    m_customerTableView = new QTableView(this);
    m_customerTableView->setModel(dmgr->customerModel());
}

DataManager::DataManager()
{
   ...   
   ...
}

QAbstractItemMode * DataManager::customerModel(void)
{
    return model_customer;
}

void DataManager::someFunctionThatNeedsToWorkonQTableView(QTableView * tableView)
{
    tableView->doWhatEverNeedsToBeDone();
}

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Of course follow proper Qt naming conventions and code defensively.

**homerun4711** · 4th January 2011, 15:48

Yes, I understand what you mean, but I like to keep the QTableView central in DataManager, because it is used by more components, not only by AddressBook, to provide a uniform interface where ever in the GUI a customer list is shown.
I would prefer to fix the errors and completly understand why they happen to prevent them in the future.

**pkohut** · 4th January 2011, 16:03

Originally Posted by homerun4711

One minute to late

Here is the complete code:

Move all the table view code to the AddressBook. The views are owned by the dialog and will be automatically deleted when the dialog is dismissed. Headache gone, code simplified.

Originally Posted by homerun4711

Yes, I understand what you mean, but I like to keep the QTableView central in DataManager, because it is used by more components, not only by AddressBook, to provide a uniform interface where ever in the GUI a customer list is shown.
I would prefer to fix the errors and completly understand why they happen to prevent them in the future.

Ask yourself these questions -
Does DataManager have a tableView?
Does AddressBook have a tableView?
Who should be the owner?

If DataManager does most of the work, then pass pointers of the tableView to the worker functions. In the furture, if you derive a class from QTableView, then overload the DataManager worker function with the derived classes.

Qt Code:

Switch view

class MyTableView1 : public QTableView;
class MyTableView2 : public QTableView;
AddressBook::DoSomeWork()
{
    dmgr->DoSomeWork(pMyTableView1);
    dmgr->DoSomeWork(pMyTableView2);
}
 
void DataManager::DoSomeWork(MyTableView1 * pTableView);
void DataManager::DoSomeWork(MyTableView2 * pTableView);

class MyTableView1 : public QTableView;
class MyTableView2 : public QTableView;
AddressBook::DoSomeWork()
{
    dmgr->DoSomeWork(pMyTableView1);
    dmgr->DoSomeWork(pMyTableView2);
}
 
void DataManager::DoSomeWork(MyTableView1 * pTableView);
void DataManager::DoSomeWork(MyTableView2 * pTableView);

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ps, and I see that address book is just one of many Dialog classes planned. Each of those classes will own their own table view and pass that to the data manager.

**homerun4711** · 4th January 2011, 16:14

Does DataManager have a tableView?
Does AddressBook have a tableView?
Who should be the owner?

If DataManager does most of the work, then pass pointers of the tableView to the worker functions. In the furture, if you derive a class from QTableView, then overload the DataManager worker function with the derived classes.

Yes, you are right. I will do that in the future, well, maybe I will rewrite the application I am working on right now to work this way.

Thanks a lot for your help and your suggestions!

**pkohut** · 4th January 2011, 16:39

Originally Posted by homerun4711

Yes, you are right. I will do that in the future, well, maybe I will rewrite the application I am working on right now to work this way.

With any code, but Qt for sure, if you feel like you're fighting the code or framework you probably are. Always be open minded about refactoring a chunk of code, even if it's a 1000 lines and took a week to write. Especially when first learning a language or framework, because most of that learning is code thrashing and the what's written is probably fragile.

Originally Posted by homerun4711

Thanks a lot for your help and your suggestions!

Thank you as well, this provided me with an opportunity to explore the API differently than I might have, definitely sooner.

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