How to add children to QGraphicsItem

Question

I have the following code, where I'm trying to have two rectangles, and then inside each, display "sub" rectangles (this is the code for one parent and one child)

auto graphicsView = std::make_unique<QGraphicsView>();
auto scene = std::make_unique<QGraphicsScene>();
graphicsView->setScene(scene.get());

auto parent1 = std::make_unique<QGraphicsRectItem>(0, 0, 100, 200);
parent1->setBrush(QBrush(QColor(Qt::cyan)));

scene->addItem(parent);

auto subRect1 = std::make_unique<QGraphicsRectItem>(10, 10, 50, 50, parent1);
subRect1->setBrush(QBrush(QColor(Qt::yellow)));

And I was excepting that when I display my QGraphicsView , I would see a cyan rectangle ( parent1 ), on top of which I would also see a yellow smaller rectangle ( subRect1 ), but I'm only able to see the cyan one.

Looking at Qt's documentation, I see that they talk about the fact that QGraphicsItem can have children, but why am I not seeing the child here?

PS: I tried adding the child in 3 different ways, but no luck:

1. Passing the parent to the constructor (as I've shown above)
2. Calling setParentItem on the child and passing the parent's pointer
3. Calling parent's childItems() and then push_back or append and passing child's pointer to it

Answer 1

make_unique won't work here. When you add an object to the parent in Qt, you've passed off ownership. The trouble is that you also have an unique_ptr which owns it. As soon as it goes out of scope, it deletes youur object. Use new instead.

auto graphicsView = new QGraphicsView();
auto scene = new QGraphicsScene();
graphicsView->setScene(scene);

auto parent1 = new QGraphicsRectItem(0, 0, 100, 200);
parent1->setBrush(QBrush(QColor(Qt::cyan)));

scene->addItem(parent);

auto subRect1 = new QGraphicsRectItem(10, 10, 50, 50, parent1);
subRect1->setBrush(QBrush(QColor(Qt::yellow)));

(This is not fully exception proof, but Qt isn't designed to use exceptions. You could make scene, for example, with make_unique and then hand it off to the graphicsView with release(), but in practice nobody ever does that with Qt.)