[Development] QVector: clear while keeping capacity

[Milian Wolff]

Hello all,

could it be that QVector changed its behavior in Qt5? I just noticed that 
resize(0) does deallocate memory, which is highly unexpected from my side. See 
e.g. this old thread where it was said to use resize(0) to clear a vector 
while keeping its capacity:

http://comments.gmane.org/gmane.comp.lib.qt.general/42277,using

But now in Qt5, I see in QVector::reallocData

    if (aalloc != 0) {
       ...
    } else {
        x = Data::sharedNull();
    }

which triggers the deallocation of the memory. This can be seen by this 
example program:

#include <QVector>
#include <QDebug>

int main()
{
  QVector<int> v;
  v.fill(1, 100);
  qDebug() << v.size() << v.capacity();
  v.resize(0);
  qDebug() << v.size() << v.capacity();
  v.fill(1, 100);
  qDebug() << v.size() << v.capacity();
  v.clear();
  qDebug() << v.size() << v.capacity();
  return 0;
}

The output is for me: 
100 122
0 0
100 122
0 0

Is this truly desired behavior? It can trigger serious performance issues...
-- 
Milian Wolff | milian.wolff at kdab.com | Software Engineer
KDAB (Deutschland) GmbH&Co KG, a KDAB Group company
Tel. Germany +49-30-521325470, Sweden (HQ) +46-563-540090
KDAB - Qt Experts - Platform-independent software solutions