How to correctly re-assign a pointer?

[homerun4711]

Hello!

I started another topic yesterday, but I guess it went far into detail.
My questions comes down to this:

How can I correctly re-assign a pointer?

In my case, I have a QtDesigner-Layout, which creates a QTableView:

public:
QTableView *tableView;
...
void setupUi(QDialog *AddressBook)
    {
       tableView = new QTableView(AddressBook);
    ...
    }

public:
QTableView *tableView;
...
void setupUi(QDialog *AddressBook)
    {
       tableView = new QTableView(AddressBook);
    ...
    }

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In the class which uses this layout I want to pass a pointer to a pre-defined QTableView to the one created by the layout.

AddressBook-class:

AddressBook::AddressBook(DataManager* dmgr, QWidget *parent): QDialog(parent)
{
    ...
   tableView = dmgr->customerTableView();
    ...
}

AddressBook::AddressBook(DataManager* dmgr, QWidget *parent): QDialog(parent)
{
    ...
   tableView = dmgr->customerTableView();
    ...
}

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DataManager-class

...
QTableView *m_customerTableView;
QTableView *customerTableView();
...
m_customerTableView = new QTableView;
...
QTableView *DataManager::customerTableView()
{
   return m_customerTableView;
}
...

...
QTableView *m_customerTableView;
QTableView *customerTableView();
...
m_customerTableView = new QTableView;
...
QTableView *DataManager::customerTableView()
{
   return m_customerTableView;
}
...

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I guess to avoid memory leaks I have to delete the original tableView created by QtDesigner.

I tried the following but could get it running, since I am not so familiar with pointers yet. (coming from Java) So there could be a misunderstanding on how pointers work.

In AddressBook-class:

delete tableView;
QTableView *tableView = dmgr->customerTableView();

delete tableView;
QTableView *tableView = dmgr->customerTableView();

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I hope you can help me.

Kind regards,
HomeR

[high_flyer]

You are shadowing your member pointer.
There is no need to redeclare it:

delete tableView;
/*QTableView * */ tableView = dmgr->customerTableView();

delete tableView;
/*QTableView * */ tableView = dmgr->customerTableView();

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[homerun4711]

Thanks for you anwser.
I tried what you said, but it's still not working.

Does shadowing mean, that a global variable is hidden because a local variable is used instead?

What exactly happens here?

delete tableView;
/*QTableView * */ tableView = dmgr->customerTableView();

delete tableView;
/*QTableView * */ tableView = dmgr->customerTableView();

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As far as I unterstand, delete tableView does remove the QTableView object created in the QtDesigner-file ui_addressbook.h :

class Ui_AddressBook
{
public:
    QTableView *tableView;
..
}

class Ui_AddressBook
{
public:
    QTableView *tableView;
..
}

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Then my prepared QTableView returned by dmgr->customerTableView(); is assigned - but assigned to what? There is no other variable named tableView existing, I guess. Do I have to define an additional one with the same name in
AddressBook.h ?

Right now my code is

delete tableView;
Q_CHECK_PTR(tableView);
Q_CHECK_PTR(dmgr->customerTableView());
tableView = dmgr->customerTableView();

delete tableView;
Q_CHECK_PTR(tableView);
Q_CHECK_PTR(dmgr->customerTableView());
tableView = dmgr->customerTableView();

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Why does die application not quit on "Q_CHECK_PTR(tableView)" after "delete tableView" ?

[high_flyer]

I tried what you said, but it's still not working.

please define what 'not working' mean here.

Does shadowing mean, that a global variable is hidden because a local variable is used instead?

Yes, but instead of 'global' here its in the class scope. (so its only visible in the scope of the class).

What exactly happens here?

Well, unless there is more code that I havn't seen then:
In the very first code segment you posted in the firt post, you declare

QTableView *tableView

QTableView *tableView

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As a class member.
So if this code:

delete tableView;
/*QTableView * */ tableView = dmgr->customerTableView();

delete tableView;
/*QTableView * */ tableView = dmgr->customerTableView();

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Is in the same clase, then tableViewe is already defined, and defining it again in the method, would shadow the one defined in the class header, and the address will be assigned to the local shadow, which gets destroyed when the method exits.
Note - the POINTER gets destroyed, but the address allocated still lives - and this called a dangling pointer - which is a memory leak - since you can't get it and delete it any more. (gets deleted when the memory of the application is returned)

Do I have to define an additional one with the same name in
AddressBook.h

No, and if you would, you would get a complication error.

As far as I unterstand, delete tableView does remove the QTableView object created in the QtDesigner-file ui_addressbook.h :

Yes and no.
'delete' deletes the object pointed to by the poiter (calling its destructor etc).
In your case, its ture, that the first allocation is done in the ui file.

Then my prepared QTableView returned by dmgr->customerTableView(); is assigned - but assigned to what? There is no other variable named tableView existing, I guess.

Don't guess - be sure!

Why does die application not quit on "Q_CHECK_PTR(tableView)" after "delete tableView" ?

because delete does not set the deleted pointer to NULL.
So you should do:

delete tableView;
Q_CHECK_PTR(tableView); //will go through, since tableViewer is not NULL (but filled with garbage)
tableView = NULL;
Q_CHECK_PTR(tableView); // this will assert!
Q_CHECK_PTR(dmgr->customerTableView());
tableView = dmgr->customerTableView();