[Solved]QList adds garbage values even though object to be added has been initialised correctly

goetz

You have multiple errors in your class:

QList, like all other container classes, requires the stored objects to be "assignable":http://doc.qt.nokia.com/4.7/containers.html#assignable-data-type.

Your class misses an assignment operator.
Your default constructor does not initialize the members to some sane default values.
Your copy constructor does nothing (i.e. it does not copy the values!)

rishabhc

Thanks a lot Volker! Works like a charm. Just to be sure about what I've written, I'm attaching the revised code below. Please correct me if things are wrong:
@
#include<datelist.h>

DateList::DateList()
{
this->count=0;
this->index=0;
QDate d(1900,1,1);
this->date=d;
}

DateList::DateList(const DateList &other)
{
this->count=other.count;
this->index=other.index;
this->date=other.date;
}

DateList::~DateList()
{

}

DateList& DateList::operator =(DateList rhs)
{
return rhs;
}

DateList::DateList(QDate inputdate, int inputindex,int inputcount)
{
this->date=inputdate;
this->index=inputindex;
this->count=inputcount;
}

QDate DateList::getDate()
{
return this->date;
}

int DateList::getIndex()
{
return this->index;
}

int DateList::getCount()
{
return this->count;
}

void DateList::setDate(QDate inputdate)
{
this->date=inputdate;
}

void DateList::setIndex(int inputindex)
{
this->index=inputindex;
}

void DateList::setCount(int inputcount)
{
this->count=inputcount;
}
@

goetz

Your assignment operator operator= needs a proper implementation (it does not change your data right now and you do not check for self-assignmet!) and it should take a const rhs, so that you can assign a const value to a non-const variable. Have a look at the "C++ FAQ lite/Assignment operators":http://www.parashift.com/c++-faq-lite/assignment-operators.html for some further details.

Franzk

In this specific case, the assignment operator and copy constructor can be left out, because no specific actions are required. This is something the compiler can handle. It is however never a bad thing to have them around (if your class is copyable). Just remember to update them if you add new data fields :P.

shivangim

I am also having a similar problem, but I have no clue how to proceed. Kindly Help.

savings class
@#ifndef SAVEITEM_H
#define SAVEITEM_H
#include <QtGui>
#include <QPoint>

class Savings
{
public:

Savings();
Savings(int item, QPoint pt);
Savings& Savings::operator= (Savings const& s);
Savings(const Savings &other);

QPoint position;
int itemtype;
};

#endif@

here are the copy constructor and assignment operator i wrote
@Savings::Savings(const Savings &other)
{
this->position= other.position;
this->itemtype= other.itemtype;
}

Savings& Savings::operator= (Savings const& s)
{
if (this != &s)
{
//this->position = QPoint(s.position);
this->position = s.position;
this->itemtype = s.itemtype;
}
return *this;
} @

and hence defined a QList
@QList<Savings> saved;
Savings anitem = Savings(intToSave, ptToSave);
saved.append(anitem);@

Franzk

What exactly is the problem you have? and what do your other constructors look like?

In the Savings class declaration, remove the extra qualifier:
@Savings& Savings::operator= (Savings const& s);@
should be
@Savings& operator= (Savings const& s);@
gcc would throw an error on that one.

shivangim

I am using visual studio. It is giving no error in compilation.
Savings& operator = (Savings const &s) throws compilation error.

During runtime-
@QList<Savings> saved;
Savings anitem = Savings(intToSave, ptToSave);
saved.append(anitem);@
'anitem' is correctly created, but as soon as line 3 is executed, the object appended in the list 'saved' is garbage.

other constructors of Savings class-
@#include "saveitem.h"

Savings::Savings()
{
itemtype = 0;
position = QPoint(0,0);
}

Savings::Savings(int item, QPoint pt)
{
itemtype= item;
position = pt;
}

void Savings::setItemType(int item)
{
itemtype = item;
}

void Savings::setPosition(QPoint pt)
{
position = pt;
}
int Savings::getItemType() const
{
return itemtype;
}

QPoint Savings::getPosition() const
{
return position ;
}

Savings::Savings(const Savings &other)
{
this->position= other.position;
this->itemtype= other.itemtype;
}

Savings& Savings::operator= (Savings const& s)
{
if (this != &s)
{
//this->position = QPoint(s.position);
this->position = s.position;
this->itemtype = s.itemtype;
}
return *this;
} @

Franzk

I copied your exact code. Used the following header:
@#ifndef SAVINGS_H
#define SAVINGS_H

#include <QPoint>

class Savings
{
public:
Savings();
Savings(int item, QPoint pt);
Savings(const Savings &other);
Savings& operator= (Savings const& s);

void setItemType(int item);
void setPosition(QPoint pt);
int getItemType() const;
QPoint getPosition() const;

private:
QPoint position;
int itemtype;
};

#endif // SAVINGS_H
@

and main.cpp:
@#include <QtCore/QCoreApplication>
#include <QList>
#include <QDebug>
#include "savings.h"
#include <QTimer>

QDebug &operator << (QDebug &dbg, const Savings &in)
{
dbg.nospace() << "<" << in.getItemType() << ", " << in.getPosition() << ">";
return dbg.space();
}

int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
QList<Savings> s;
s << Savings(1, QPoint(2,3));
s << Savings(2, QPoint(1, 0));
qDebug() << s;

QTimer::singleShot(5000, &a, SLOT(quit()));
return a.exec();
}@

Nothing wrong with your code, I would say (tested on mingw, but no reason why this shouldn't work on msvc).

shivangim

Thank you Franzk :)
I'l try it out once more!
Though i just gave it a shot and changed QList to QVector instead, and it gives the correct output, no clue how!

Franzk

It might just be the fact that you rebuilt a lot of the code after the change. Technically QVector and QList require the same behavior from the stored types.