Center children widget in parent widget(parent widget was added in layout) - c++

Question

I created a layout contain a parent widget. In that parent widget i created another widget.
My code is similarly to this:
QGridLayout *layout = new QGridLayout();
QWidget *parentWidget = new QWidget();
layout->addWidget(parentWidget );
QWidget *childWidget = new QWidget(parentWidget);
How can i center the child widget in parent widget ?
The problem is we cannot get the true size of parent widget because it's in a layout.

Answer 1

Move the child inside the parent's showeEvent. You can use a bool flag to do it only when the parent is shown for the first time.
void Parent::showEvent(QShowEvent *)
{
if(_first_show)
{
_first_show = false;
_child->move(this->rect().center() - _child->rect().center());
}
}
Proof that it works (red is the parent, and blue is the child):

Answer 2

You can do this by setting fixed size of child widget and placing it inside grid layout of parent widget.
QGridLayout *layout = new QGridLayout();
QWidget *parentWidget = new QWidget();
layout->addWidget(parentWidget );
QWidget *childWidget = new QWidget(parentWidget);
QGridLayout *parentLayout = new QGridLayout();
parentWidget->setLayout(parentLayout);
parentLayout->addWidget(childWidget);
childWidget->setFixedSize(/*some fixed size for child widget*/);