QT question

This is a discussion on QT question within the C++ Programming forums, part of the General Programming Boards category; In the examples of QT, address book, I see this code in the AddressBook class: Code: //in class AddressBook* ui; ...

  1. #1
    Registered User C_ntua's Avatar
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    QT question

    In the examples of QT, address book, I see this code in the AddressBook class:
    Code:
        
    //in class
    AddressBook* ui;
    QLineEdit* nameLine;
    
    //in constructor
    nameLine = new QLineEdit;
    nameLine = ui->nameLine;
    So my question is since nameLine points to a new object QLineEdit and then points to ui->nameLine, isn't a memory leak created?

  2. #2
    Captain Crash brewbuck's Avatar
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    Quote Originally Posted by C_ntua View Post
    In the examples of QT, address book, I see this code in the AddressBook class:
    Code:
        
    //in class
    AddressBook* ui;
    QLineEdit* nameLine;
    
    //in constructor
    nameLine = new QLineEdit;
    nameLine = ui->nameLine;
    So my question is since nameLine points to a new object QLineEdit and then points to ui->nameLine, isn't a memory leak created?
    Is ui pointing to this? If so, it's just a pointless circuitous reference, not a memory leak. If not, it's definitely a memory leak.
    Code:
    //try
    //{
    	if (a) do { f( b); } while(1);
    	else   do { f(!b); } while(1);
    //}

  3. #3
    Registered User C_ntua's Avatar
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    The constructor starts
    Code:
    AddressBook::AddressBook(QWidget *parent)
        : QWidget(parent), ui(new Ui::AddressBook)
    {
         ui->setupUi(this);
    Anyway, I think it is kind of meaningless to see what QTCreator actually does.
    The point is that there seems to be ne meaning for all of this, except if the example was taken from an older versiont of QT, which I believe is the case.
    The whole point was that a basic object is created that is pointed by ui and we then can only play around with this. So we just link the pointer of this to the objects of the basic object pointed by ui.

    If I don't allocate memory it seems to work fine.

  4. #4
    Captain Crash brewbuck's Avatar
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    If ui->nameLine is not the same object as nameLine (and it seems to be the case) then it's definitely a leak, yeah.
    Code:
    //try
    //{
    	if (a) do { f( b); } while(1);
    	else   do { f(!b); } while(1);
    //}

  5. #5
    Registered User C_ntua's Avatar
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    I am quite sure it is a leak. And there is nothing that deletes the object in the destructor. But the thing is why would you ever do
    Code:
    pointer = new Obj;
    pointer = something;
    I mean, no matter what the case is, how could the above code possibly not to something weird?

    They actually have two examples in there. The example I am using is the correct one, the other is how to create widgets without not graphically, but purely code-wise.

    I really don't know why do they have this line of code...

  6. #6
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    qt's memory management is utter fail. it at least encourages, if not outright requires, bad practices.

    that first (leaking) line would normally create a qwidget in it's own window. if you were to close the window, it would clean up after itself.

    i don't understand the purpose of phrasing the code in that manner in the slightest either.

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