Missing increment of index in foreach with QMap

[EMStegehuis]

Hello,

Since we updated from VS 16.3.10 to 16.5.4 we had some failures in our unittest. We were able to track them down to an old usage of QMap in a foreach.

In the example code below the rowNr variable is not incremented at all in release mode. In debug it is. Optimisation level is set to /O2.

When yo uncomment the last logging line the counter is incremented.

Has anybody a clue whether this is a bug in Qt?

The funny thing is ist does not happen with the stringlist so it has something to do with the foreach + QMap.

And yes: There are better ways to iterate over a QMap.

    QMap <QString, QString> mapContainer;
    mapContainer.insert (QString ("One"), QString ("1"));
    mapContainer.insert (QString ("Two"), QString ("2"));
    mapContainer.insert (QString ("Three"), QString ("3"));
    mapContainer.insert (QString ("Four"), QString ("4"));
    mapContainer.insert (QString ("Five"), QString ("5"));

    //QStringList mapContainer = { QString ("One"), QString ("Two"), QString ("Three"), QString ("Four"), QString ("Five") };

    unsigned int rowNr = 0;

    foreach (QString contour, mapContainer)
    //for (unsigned int secondLoopIndex = 0; secondLoopIndex < 5; ++secondLoopIndex)
    {

      LOGINFO (QString ("<-----------------------Index row in loop before incr.: (%1)").arg (rowNr));
      ++rowNr;
      //LOGINFO (QString ("<-----------------------Index row in loop after incr.: (%1)").arg (rowNr));
    }

Output

<-----------------------Index row in loop before incr.: (0)
<-----------------------Index row in loop before incr.: (0)
<-----------------------Index row in loop before incr.: (0)
<-----------------------Index row in loop before incr.: (0)
<-----------------------Index row in loop before incr.: (0)

[EMStegehuis]

And it helps to not put the increment (++rowNr) at the end of the for loop (without direct usage) but at the beginning prior to its usage.

So it seems related to some optimisation.

[Christian Ehrlicher]

Please provide a minimal compilable example so we can test you code. This code is working fine:

int main(int argc, char **argv)
{
  QApplication app(argc, argv);

  QMap <QString, QString> mapContainer;
  mapContainer.insert (QString ("One"), QString ("1"));
  mapContainer.insert (QString ("Two"), QString ("2"));
  mapContainer.insert (QString ("Three"), QString ("3"));
  mapContainer.insert (QString ("Four"), QString ("4"));
  mapContainer.insert (QString ("Five"), QString ("5"));

  unsigned int rowNr = 0;
  foreach (QString contour, mapContainer)
    //for (unsigned int secondLoopIndex = 0; secondLoopIndex < 5; ++secondLoopIndex)
  {
    qDebug() << QString ("<-----------------------Index row in loop before incr.: (%1)").arg (rowNr);
    ++rowNr;
    qDebug() << QString ("<-----------------------Index row in loop after incr.: (%1)").arg (rowNr);
  }

  return app.exec();
}

[kshegunov]

@EMStegehuis said in Missing increment of index in foreach with QMap:

Has anybody a clue whether this is a bug in Qt?

Probably not. Compile in release with debug info and inspect the assembly, probably an optimizer problem. Whatever the reason, however, you should move to a range-based for, foreach's deprecated.