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.