QList question

[MarkoSan]

Hi to all!

I've read the docs about QList but I did not find any answers regarding my question. So, after QList method http://doc.trolltech.com/4.3/qlist.html#append, the record is not added to QList. What are possible reasons?

[roxton]

Can you show a code example?

[estanisgeyer]

Post part of its code, but you can also use the following:

QList<QString> list;
list << "One" << "Two" << "Three" << "Four";

QList<QString> list;
list << "One" << "Two" << "Three" << "Four";

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Marcelo E. Geyer

[MarkoSan]

This is my code:

m_pShoppingCartView->order()->orders().append(tmpOrder);    // adds order to internal qlist
    int iOrderSize=m_pShoppingCartView->order()->orders().size();    // calcs size
    Q_ASSERT_X(iOrderSize>0, "COperationWindow.cpp", "m_pShoppingCartView->order()->orders().size()<0");    // checks size

m_pShoppingCartView->order()->orders().append(tmpOrder);    // adds order to internal qlist
    int iOrderSize=m_pShoppingCartView->order()->orders().size();    // calcs size
    Q_ASSERT_X(iOrderSize>0, "COperationWindow.cpp", "m_pShoppingCartView->order()->orders().size()<0");    // checks size

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The application aborts on Q_ASSERT_X.

[wysota]

Doesn't by any chance orders() return a copy of the list and not a reference?

[MarkoSan]

Well, inline function order returns pointer:

inline CMerchandizeOrder* order()
        { return m_pOrder; };

inline CMerchandizeOrder* order()
        { return m_pOrder; };

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m_pOrder is declared as:

CMerchandizeOrder* m_pOrder;

CMerchandizeOrder* m_pOrder;

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and then inline member function orders() returns:

inline     QList<structOrder> orders()
        { return m_Orders; };

inline     QList<structOrder> orders()
        { return m_Orders; };

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and m_Orders are defined as:

private:
    QList<structOrder> m_Orders;

private:
    QList<structOrder> m_Orders;

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[ChristianEhrlicher]

inline     QList<structOrder> orders()
        { return m_Orders; };

inline     QList<structOrder> orders()
        { return m_Orders; };

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Here you return a copy of the List.

[MarkoSan]

Well, how do I recode this code so it will work? I do not know how to make a pointer to QList

[stevey]

Instead of

private:
  QList<structOrder> m_Orders;

private:
  QList<structOrder> m_Orders;

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use

private:
  QList<structOrder>* m_Orders;

private:
  QList<structOrder>* m_Orders;

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The are certain key things you need to be aware of with C++.
One very important thing is that when you call return at the end of the function, a COPY of the type you're returning is created to hold the data. This is because any variable declared inside a function is a STACK variable for the scope of that function. So if you declare a non pointer variable then return it, in your case in the call to orders(), you've essentially chained your call to .append(tmpOrder) on a copy of the variable that is lost as soon as the next line is called.

Now by declaring a QList<structOrder>*, at the end of your function you'll be returning a COPY still, but it will be a copy of a POINTER. The actual instantiated QList is on the heap so isn't cleaned up at the end of the function, therefore copied QList*'s data you'll be appending to is valid and won't get lost in transit.

In terms of architecture, you might want to re-consider this kind of thing anyway and use a wrapper function. A sideline of hiding the QList also is that you could schtick with your non pointer version:

void ShoppingCartView::AddOrder(structOrder order)
{
  m_Orders.append( order );
}

void ShoppingCartView::AddOrder(structOrder order)
{
  m_Orders.append( order );
}

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[wysota]

Actually he doesn't need a pointer to the list. He needs to return a reference (or pointer) to the list:

inline     QList<structOrder> &orders() const { return m_Orders; };

inline     QList<structOrder> &orders() const { return m_Orders; };

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or

inline     QList<structOrder> *orders() const { return &m_Orders; };

inline     QList<structOrder> *orders() const { return &m_Orders; };

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Of course hiding the implementation detail and exposing a method for adding items to the list as suggested is a much better idea.

[MarkoSan]

But how do I create a pointer to QList???

The code

m_pOrders=new QList();    // creates new qlist
    Q_CHECK_PTR(m_pOrders);    // checks creation

m_pOrders=new QList();    // creates new qlist
    Q_CHECK_PTR(m_pOrders);    // checks creation

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returns an compile error.

And I need this info urgently please, since I have a demo of application and the app won't compile neither run because i cannot add items to a non created list!!!!