QList<T> runtime error

Question

I have a QList for store some item on QgraphicsScene like:

QList<QGraphicsItem*> lineList;

when I want to use it like:

lineList[itemIndex++]=scene->createItemGroup(groupItems);

i got a runtime error. I'm curios why?

by the way I know about linelist.append()

thanks.

Answer 1

Assuming that you want to create a new QList, you should use QList::append() or << operator. From Qt Docs:

T & QList::operator[](int i) Returns the item at index position i as a modifiable reference. i must be a valid index position in the list (i.e., 0 <= i < size()). This function is very fast (constant time). See also at() and value().

So QList::operator[] can't be used to populate list like that.

Answer 2

Assuming that itemIndex++ is equals or below to lineList.size(), you can use QList::insert

lineList.insert(itemIndex++, scene->createItemGroup(groupItems));

Anyway, it is preferable to use append for extending your list by one unit.