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!!!!

[MarkoSan]

Ok, I've managed to do it. I forgot to pass the type of qlist. Now let me check some things I will report the progress. Thanks for all!

[wysota]

You don't need to create the list on the heap. Simply return a pointer to the list by using the ampersand.

[MarkoSan]

Hello, let me continue this thread of mine after 3 years!

I am trying to implement wrapper around QList and when trying to compile following code:

#ifndef BSPITEM_H
#define BSPITEM_H
 
#include <QObject>
#include <QtCore/QList>
 
#include "BSPItemProperty.h"
 
class BSPItem : public QObject
{
    Q_OBJECT
 
private:
    QList<BSPItemProperty> m_lsProperties;
 
public:
    explicit BSPItem(QObject *parent = 0);
 
    inline QList<BSPItemProperty>& properties()
        { return this->m_lsProperties; }
 
    void addProperty(const QString& name,
                     const QVariant& value);
    void deleteProperty(const QString& name);
};
 
#endif // BSPITEM_H

#ifndef BSPITEM_H
#define BSPITEM_H

#include <QObject>
#include <QtCore/QList>

#include "BSPItemProperty.h"

class BSPItem : public QObject
{
    Q_OBJECT

private:
    QList<BSPItemProperty> m_lsProperties;

public:
    explicit BSPItem(QObject *parent = 0);

    inline QList<BSPItemProperty>& properties()
        { return this->m_lsProperties; }

    void addProperty(const QString& name,
                     const QVariant& value);
    void deleteProperty(const QString& name);
};

#endif // BSPITEM_H

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and its implementation - .cpp file:

#include "bspitem.h"
 
BSPItem::BSPItem(QObject *parent) :
    QObject(parent)
{
    this->properties().clear();
}
 
void BSPItem::addProperty(const QString& name,
                          const QVariant& value)
{
    BSPItemProperty tempProperty(this,
                                 name,
                                 value);
 
 
    this->properties().append(tempProperty);
}
 
void BSPItem::deleteProperty(const QString& name)
{
    for(quint64 iIndex=0; iIndex<(quint64)this->properties().size(); iIndex++)
        if(this->properties().at(iIndex).propertyName()==name)
            this->deleteProperty(name);
}

#include "bspitem.h"

BSPItem::BSPItem(QObject *parent) :
    QObject(parent)
{
    this->properties().clear();
}

void BSPItem::addProperty(const QString& name,
                          const QVariant& value)
{
    BSPItemProperty tempProperty(this,
                                 name,
                                 value);


    this->properties().append(tempProperty);
}

void BSPItem::deleteProperty(const QString& name)
{
    for(quint64 iIndex=0; iIndex<(quint64)this->properties().size(); iIndex++)
        if(this->properties().at(iIndex).propertyName()==name)
            this->deleteProperty(name);
}

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I get following errors:

Running build steps for project QtBSPPlayer...
Configuration unchanged, skipping qmake step.
Starting: "C:\QtSDK\mingw\bin\mingw32-make.exe" -w
mingw32-make: Entering directory `C:/Users/HP2/Desktop/QtBSPPlayer-build-desktop'
C:/QtSDK/mingw/bin/mingw32-make -f Makefile.Debug
mingw32-make[1]: Entering directory `C:/Users/HP2/Desktop/QtBSPPlayer-build-desktop'
g++ -c -g -frtti -fexceptions -mthreads -Wall -DUNICODE -DQT_LARGEFILE_SUPPORT -DQT_DLL -DQT_GUI_LIB -DQT_CORE_LIB -DQT_HAVE_MMX -DQT_HAVE_3DNOW -DQT_HAVE_SSE -DQT_HAVE_MMXEXT -DQT_HAVE_SSE2 -DQT_THREAD_SUPPORT -DQT_NEEDS_QMAIN -I"..\..\..\..\QtSDK\Desktop\Qt\4.7.1\mingw\include \QtCore" -I"..\..\..\..\QtSDK\Desktop\Qt\4.7.1\mingw\include \QtGui" -I"..\..\..\..\QtSDK\Desktop\Qt\4.7.1\mingw\include " -I"..\..\..\..\QtSDK\Desktop\Qt\4.7.1\mingw\include \ActiveQt" -I"debug" -I"..\QtBSPPlayer" -I"." -I"..\..\..\..\QtSDK\Desktop\Qt\4.7.1\mingw\mkspecs \win32-g++" -o debug\bspitem.o ..\QtBSPPlayer\bspitem.cpp
mingw32-make[1]: Leaving directory `C:/Users/HP2/Desktop/QtBSPPlayer-build-desktop'
mingw32-make: Leaving directory `C:/Users/HP2/Desktop/QtBSPPlayer-build-desktop'
In file included from ..\QtBSPPlayer\/bspitem.h:7,
from ..\QtBSPPlayer\bspitem.cpp:1:
..\..\..\..\QtSDK\Desktop\Qt\4.7.1\mingw\include\Q tCore/qobject.h: In copy constructor 'BSPItemProperty::BSPItemProperty(const BSPItemProperty&)':
..\QtBSPPlayer\/BSPItemProperty.h:9: instantiated from 'void QList<T>::node_construct(QList<T>::Node*, const T&) [with T = BSPItemProperty]'
..\..\..\..\QtSDK\Desktop\Qt\4.7.1\mingw\include/QtCore/qlist.h:499: instantiated from 'void QList<T>::append(const T&) [with T = BSPItemProperty]'
..\QtBSPPlayer\bspitem.cpp:17: instantiated from here
..\..\..\..\QtSDK\Desktop\Qt\4.7.1\mingw\include\Q tCore/qobject.h:309: error: 'QObject::QObject(const QObject&)' is private
..\QtBSPPlayer\/BSPItemProperty.h:9: error: within this context
In file included from ..\..\..\..\QtSDK\Desktop\Qt\4.7.1\mingw\include\Q tCore/qobject.h:50,
from ..\..\..\..\QtSDK\Desktop\Qt\4.7.1\mingw\include\Q tCore/QObject:1,
from ..\QtBSPPlayer\/bspitem.h:4,
from ..\QtBSPPlayer\bspitem.cpp:1:
..\..\..\..\QtSDK\Desktop\Qt\4.7.1\mingw\include/QtCore/qlist.h: In member function 'void QList<T>::node_construct(QList<T>::Node*, const T&) [with T = BSPItemProperty]':
..\..\..\..\QtSDK\Desktop\Qt\4.7.1\mingw\include/QtCore/qlist.h:359: note: synthesized method 'BSPItemProperty::BSPItemProperty(const BSPItemProperty&)' first required here
In file included from ..\QtBSPPlayer\/bspitem.h:7,
from ..\QtBSPPlayer\bspitem.cpp:1:
..\..\..\..\QtSDK\Desktop\Qt\4.7.1\mingw\include\Q tCore/qobject.h: In member function 'BSPItemProperty& BSPItemProperty::operator=(const BSPItemProperty&)':
..\..\..\..\QtSDK\Desktop\Qt\4.7.1\mingw\include\Q tCore/qobject.h:309: error: 'QObject& QObject::operator=(const QObject&)' is private
..\QtBSPPlayer\/BSPItemProperty.h:9: error: within this context
In file included from ..\..\..\..\QtSDK\Desktop\Qt\4.7.1\mingw\include\Q tCore/qobject.h:50,
from ..\..\..\..\QtSDK\Desktop\Qt\4.7.1\mingw\include\Q tCore/QObject:1,
from ..\QtBSPPlayer\/bspitem.h:4,
from ..\QtBSPPlayer\bspitem.cpp:1:
..\..\..\..\QtSDK\Desktop\Qt\4.7.1\mingw\include/QtCore/qlist.h: In member function 'void QList<T>::node_construct(QList<T>::Node*, const T&) [with T = BSPItemProperty]':
..\..\..\..\QtSDK\Desktop\Qt\4.7.1\mingw\include/QtCore/qlist.h:365: note: synthesized method 'BSPItemProperty& BSPItemProperty::operator=(const BSPItemProperty&)' first required here
mingw32-make[1]: *** [debug/bspitem.o] Error 1
mingw32-make: *** [debug] Error 2
The process "C:\QtSDK\mingw\bin\mingw32-make.exe" exited with code 2.
Error while building project QtBSPPlayer (target: Desktop)
When executing build step 'Make'

What is wrong??

[nightghost]

..\..\..\..\QtSDK\Desktop\Qt\4.7.1\mingw\include\Q tCore/qobject.h:309: error: 'QObject::QObject(const QObject&)' is private
..\QtBSPPlayer\/BSPItemProperty.h:9: error: within this context

Are you trying to make a copy of a qobject?

[MarkoSan]

Are you trying to make a copy of a qobject?

I do not think so, but if I do, then I do not know where. If I comment the core of methods addProperty() - append method generates errors and in constructor, clear method generates errors and I simply do not know why. Does these two methods use copy constructor internally??

[nightghost]

I assume that append creates a copy of the appened object, as well as functions like value

[MarkoSan]

I assume that append creates a copy of the appened object, as well as functions like value

Well, so what do I do now to get rid of these errors

[nish]

define a copy constructor in you BP item class