Call by reference with QVector

Question

I have in an Object an QVector of Coordinates (my type) that I want to transfer to an other Vector ( I validate and than want to use ist ).

Header

bool getVector(QVector<Coordinates> &getCoordinates );

C File

static QVector<Coordinates> current;


int getVector( QVector<Coordinates> &getCoordinates)
{

.... stuff ...

 getCoordinates = current;

.... stuff ....
return 0;
}

And I use it like

....

QVector<Coordinates> currentCoordinates;
getVector(currentCoordinates);

currentCoordinates.X // CRASH

The debugger goes to this line where an Live Crash happens

  inline QVector(const QVector<T> &v) : d(v.d) { d->ref.ref(); if (!d->sharable) detach_helper(); }

So my how can I fix this? As I can use this to get all the other Variables with this methode.

score 0 · Answer 1 · edited May 23 '17 at 12:13

0

A likely cause of your problem is that current has not been constructed before getVector is called. Initialization of static objects in C++ is a thorny area, and a frequent source of bugs - for more information, see this question, and the static initialization order fiasco FAQ entry.

A simple solution to this problem is to provide access to current via a function, i.e. replace

static QVector<Coordinates> current;

with

static QVector<Coordinates>& getCurrent()
{
    static QVector<Coordinates> current;
    return current;
}

Note, however, that the function as written above is not thread-safe. If multiple threads may call getCurrent, then it should be protected with a QMutex.

edited May 23 '17 at 12:13

Community

1
1

answered May 03 '10 at 16:11

Gareth Stockwell

3,112
18
23

I did what you said. It worked just fine. But than i was told, that if i just change QVector to an own Type like typdef QVector VesselCoordinates, that i should work. And it work just fine as well. Even as i was deleting your Function it still worked just fine. thank you – Thomas May 04 '10 at 10:07
@Thomas, just using a typedef should not change the behaviour of the program at all - it is just 'syntatic sugar' for the compiler. Are you sure that just adding a typedef to your original program fixes the crash? – Gareth Stockwell May 04 '10 at 12:09
yes just the typedef fixed it. As i was told it makes that the compiler resolve it just in time and than the Vector is been constructed. We trick the compiler. – Thomas May 04 '10 at 19:28

score 0 · Answer 2 · answered May 04 '10 at 19:36

For gareth and the Forum :

the header:

typedef  QVector<Coordinates> VesselCoordinates;

  bool (*getVessel)(Charakter forCharakter, Vessel& getVessel,VesselCoordinates &getCoordinates );

later i bind tis function pointer to an static function ( cause this part of my Program will be one day convertet to c)

cpp file lower layer:

     static struct {
            Charakter currentPlayerVessel;
            VesselCoordinates possibility;
        }data;
         static bool getVessel(Charakter forCharakter, Vessel& getVessel,VesselCoordinates &getCoordinates );

// funktion to bind the funktion pointer to this static funktion so it can be called outside the File

        static bool serverNamespace::getVessel(Charakter forCharakter, Vessel& getVessel,VesselCoordinates &getCoordinates )
        {
            bool retValue= false;

            if ( forCharakter == data.currentPlayerVessel){
                // TODO abfragen ob die Adresse regestriert ist!
                if ((true == minSize()) and  ((true == shipsInRow())or (true == shipsInLine())))
            {
                retValue = true;
                Vessel test = (Vessel)data.possibility.size();
                getVessel =  test;
                getCoordinates = data.possibility;
            }
        }
        return retValue;
    }

And then i can use this in the upper layer cpp file to get the information i need:

// in an Funktion : 

 VesselCoordinates currentCoordinates;
    currentCoordinates.clear();

    Vessel currentVessel;

if (true == basicFleet->getVessel(currentCharakter,currentVessel, currentCoordinates ))

// doing stuff to it

so its worik fine but your idea worked just as fine. Maybe you can see why my idea is also working.

Thank you

elektor

Call by reference with QVector

2 Answers2