Weird behavior of QList<T>

[advtaco]

Hello there!
I have a slight problem with a QList.
In the given code, rooms is a QList. Using the "at(i)" method i am trying to get data out of that list.
But when i am using variables instead of a number as the "i" attribute i am getting an assert error.
Thanks in advance, Jan

    //doesn't work
    int n = 1;
    printf(building->rooms.at(n).name.toLatin1().data());
    
    //works
    printf(building->rooms.at(1).name.toLatin1().data());

[jsulm]

@advtaco What is the error you get?

[advtaco]

I am getting a segmentation fault

[VRonin]

@advtaco said in Weird behavior of QList<T>:

am getting a segmentation fault

Could you post stack trace?

[jsulm]

@advtaco Can you show the real code (I guess you posted just parts or an example)?

[advtaco]

@jsulm
This is the function in which it happens, roomCount is 2 in this example, the for-loop works for i = 0 but fails when it increments to i = 1

void CmdHandler::listRooms()
{
   int roomCount = building->rooms.count();
   printf("Room listing: \n\r");
   printf(QString(QString::number(building->rooms.count()) + "\r\n").toLatin1().data());

   printf("+---------------------------+\n\r");
   for (int i = 0; i < roomCount; i++)
   {
       printf(QString::number(i).toLatin1().data());
       printf(QString("Room #" + QString::number(i) + ", " + building->rooms.at(i).name + "\n\r").toLatin1().data());
       printf(QString("Switches: " + QString::number(building->rooms.at(i).switches.count()) + "\n\r").toLatin1().data());
       printf(QString("Blinds: " + QString::number(building->rooms.at(i).blinds.count()) + "\n\r").toLatin1().data());
       printf("+---------------------------+\n\r");
   }
}

@VRonin
This is what i get
https://pydio.advtaco.de/public/33a27b

[VRonin]

@advtaco said in Weird behavior of QList<T>:

This is what i get

that's the assertion, not the stack trace. start your debugger and see the "stack window"

[Taz742]

@advtaco

@advtaco said in Weird behavior of QList<T>:

This is the function in which it happens, roomCount is 2 in this example, the for-loop works for i = 0 but fails when it increments to i = 1
void CmdHandler::listRooms()
{
int roomCount = building->rooms.count();
printf("Room listing: \n\r");
printf(QString(QString::number(building->rooms.count()) + "\r\n").toLatin1().data());

printf("+---------------------------+\n\r");
for (int i = 0; i < roomCount; i++)
{
printf(QString::number(i).toLatin1().data());
printf(QString("Room #" + QString::number(i) + ", " + building->rooms.at(i).name + "\n\r").toLatin1().data());
printf(QString("Switches: " + QString::number(building->rooms.at(i).switches.count()) + "\n\r").toLatin1().data());
printf(QString("Blinds: " + QString::number(building->rooms.at(i).blinds.count()) + "\n\r").toLatin1().data());
printf("+---------------------------+\n\r");
}
}

can your share full code in building and room class ?

[Taz742]

@advtaco said in Weird behavior of QList<T>:

//doesn't work
int n = 1;
printf(building->rooms.at(n).name.toLatin1().data());

//works
printf(building->rooms.at(1).name.toLatin1().data());

int n = 0;
printf(building->rooms.at(n).name.toLatin1().data());
its works
?

[advtaco]

@Taz742

kbuilding.cpp :

#include "kbuilding.h"

kBuilding::kBuilding(QString name)
{
    this->name = name;
}

void kBuilding::addRoom(kRoom newRoom)
{
    rooms.append(newRoom);
}

void kBuilding::removeRoom(int remRoom)
{
    rooms.removeAt(remRoom);
}

kbuilding.h :

#ifndef KBUILDING_H
#define KBUILDING_H

#include <QList>
#include <QString>
#include "kroom.h"

class kBuilding
{
public:
    kBuilding(QString name);
    QString name;
    QList<kRoom> rooms;
    void addRoom(kRoom newRoom);
    void removeRoom(int remRoom);
};

#endif // KBUILDING_H

kRoom.cpp :

#include "kroom.h"

kRoom::kRoom(QString name)
{
    this->name = name;
}

void kRoom::addSwitch(kSwitch newSwitch)
{
    switches.append(newSwitch);
}

void kRoom::removeSwitch(int remSwitch)
{
    switches.removeAt(remSwitch);
}

void kRoom::addBlind(kBlind newBlind)
{
    blinds.append(newBlind);
}

void kRoom::removeBlind(int remBlind)
{
    blinds.removeAt(remBlind);
}

kroom.h :

#ifndef KROOM_H
#define KROOM_H

#include <QList>
#include <QString>
#include <kswitch.h>
#include <kblind.h>

class kRoom
{
public:
    kRoom(QString name);
    QList<kSwitch> switches;
    QList<kBlind> blinds;
    QString name;
    void addSwitch(kSwitch newSwitch);
    void removeSwitch(int remSwitch);
    void addBlind(kBlind newBlind);
    void removeBlind(int remBlind);
};

#endif // KROOM_H

kswitch.cpp :

#include "kswitch.h"

kSwitch::kSwitch(bool initialState, QString knx_address)
{
    this->currentState = initialState;
    this->knx_address = knx_address;
}

bool kSwitch::changeState()
{
    bool newState;
    if( currentState == true )
    {
        newState = false;
    }
    else
    {
        newState = true;
    }

    // change state via knx

    if( 1 /* if changing state succeeds */)
    {
        this->currentState = newState;
        return true;
    }
    return false;
}

kswitch.h :

#ifndef KSWITCH_H
#define KSWITCH_H

#include <QString>

class kSwitch
{
public:
    kSwitch(bool initialState, QString knx_address);
    QString knx_address;
    bool currentState;
    bool changeState();
};

#endif // KSWITCH_H

kblind.cpp :

#include "kblind.h"

kBlind::kBlind(float initialHeight, QString knx_address)
{
    if(!(0 < initialHeight < 1))
    {
        // error;
    }
    else
    {
        this->currentHeight = initialHeight;
    }
    this->knx_address = knx_address;
}

bool kBlind::changeHeight(float newHeight)
{
    if(!(0 < newHeight < 1))
    {
        return false;
    }
    else
    {
        // change state via knx

        if(1 /* if changing state succeeds */)
        {
            this->currentHeight = newHeight;
            return true;
        }
        return false;
    }
}

kblind.h

#ifndef KBLIND_H
#define KBLIND_H

#include <QString>

class kBlind
{
public:
    kBlind(float initialHeight, QString knx_address);
    QString knx_address;
    float currentHeight;
    bool changeHeight(float newHeight);
};

#endif // KBLIND_H

@VRonin

https://pydio.advtaco.de/public/217d9a
That's the stack trace

[VRonin]

@advtaco said in Weird behavior of QList<T>:

That's the stack trace

Which one is line 55 of cmdhandler.cpp?

I know this is something unbelievably obvious that I'm missing but I just can't see the bug yet

[jsulm]

@advtaco Are you sure you really have roomCount entries in the list? You should check that first.

[advtaco]

@jsulm Yep, i checked that in the debugger

@VRonin That's this line:

 printf(QString("Room #" + QString::number(i) + ", " + building->rooms.at(i).name + "\n\r").toLatin1().data());

[Taz742]

@VRonin @jsulm

@advtaco said in Weird behavior of QList<T>:

#ifndef KROOM_H
#define KROOM_H

#include <QList>
#include <QString>
#include <kswitch.h>
#include <kblind.h>

class kRoom
{
public:
kRoom(QString name);
QList<kSwitch> switches;
QList<kBlind> blinds;
QString name;
void addSwitch(kSwitch newSwitch);
void removeSwitch(int remSwitch);
void addBlind(kBlind newBlind);
void removeBlind(int remBlind);
};

When I used class in other classes I had a problem with the following:
like this

> #include <QList>
> #include <QString>
> #include <kswitch.h>
> #include <kblind.h>

class kSwtich;
class kBlind;

Could this be a problem anyway?

[jsulm]

@Taz742 Sory, I don't follow: what problem do you mean?

[jsulm]

@advtaco Not related to your question: may I ask you why you use printf C function? It makes your code more complex as you need to get char* out of QString. Why not use qDebug for debugging or std::cout for regular std out - you're using C++ not C :-)

[Taz742]

@jsulm undefined refernce (class name).
@advtaco you are using Qt 5.9 yeap?
please check other version if you can.

[VRonin]

I'm speechless... I can only thing of something weird going on in building

ok, let's try with an alternative.
replace

for (int i = 0; i < roomCount; i++)
   {
       printf(QString::number(i).toLatin1().data());
       printf(QString("Room #" + QString::number(i) + ", " + building->rooms.at(i).name + "\n\r").toLatin1().data());
       printf(QString("Switches: " + QString::number(building->rooms.at(i).switches.count()) + "\n\r").toLatin1().data());
       printf(QString("Blinds: " + QString::number(building->rooms.at(i).blinds.count()) + "\n\r").toLatin1().data());
       printf("+---------------------------+\n\r");
   }

with

foreach(const kRoom& singleRoom, building->rooms){
qDebug() << singleRoom.name;
}

[jsulm]

@Taz742 Well, you need to have the class definition somewhere, a forward declaration isn't a definition.

[VRonin]

@jsulm said in Weird behavior of QList<T>:

@advtaco Not related to your question: may I ask you why you use printf C function? It makes your code more complex as you need to get char* out of QString. Why not use qDebug for debugging or std::cout for regular std out - you're using C++ not C :-)

For more infos on this front, see https://stackoverflow.com/questions/3886105/how-to-print-to-console-when-using-qt