[Development] QVector reserve counterproductive?

[Christian Ehrlicher]

Hi,

while digging through the bugreports I found 
https://bugreports.qt.io/browse/QTBUG-66677 which seems to show a 
downside of QVector::reserve().
QPainterPath::addPolygon() is calling reserve() to make sure to have 
enough space for the new positions. This is exactly what I would have 
done and what e.g. CLazy suggests. But it looks like reserve() allocates 
*exactly* the amount of elements given. Therefore calling addPolygon() a 
second time will do a new reallocation and so on.
The documentation for reserve() only states that memory for 'at least 
size elements' is allocated so my naive assumption would be that calling 
reserve a second time with a slightly bigger value than before does 
nothing. But this doesn't seem to be the case here.

For the bug: removing the reserve() inside addPolygon() fixes the speed 
issues mentioned in the bug report. Don't know if the bug report is a 
valid use case though.

Thx,
Christian