QGraphicsView Won't Expand - c++

Question

I'm having extreme trouble getting QWidgets to expand as needed. This isn't the first time I've run into this problem, and last time I solved it by hacking in a large sizeHint(). This is the WRONG approach and I would really like to learn the CORRECT approach.
If anyone could help me out it would be greatly appreciated. Here's what it looks like and the layouts I have written in code. If necessary I can supply the code. Please help me learn Layouts.
Edit: The first layout mock up actually works correctly. I am attaching another layout mockup which causes a problem.
Code. Three classes.
MainWindow Class:
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent)
{
QWidget * w = new MainCentralWidget(this);
setCentralWidget(w);
}
MainCentralWidget Class:
MainCentralWidget::MainCentralWidget(QWidget *parent) :
QWidget(parent)
{
SetupLayout();
}
void MainCentralWidget::SetupLayout()
{
QVBoxLayout * main_layout;
QFormLayout * plugin_layout;
//Start
main_layout = new QVBoxLayout();
//Setup the plugin chooser
plugin_layout = new QFormLayout();
QComboBox * plugins_box = new QComboBox();
plugin_layout->addRow("Choose Plugin: ", plugins_box);
QFrame* line = new QFrame();
line->setFrameShape(QFrame::HLine);
line->setFrameShadow(QFrame::Sunken);
plugin_layout->addRow(line);
main_layout->insertLayout(0, plugin_layout);
main_layout->insertWidget(1, new SubWidget());
//Finish
setLayout(main_layout);
setSizePolicy(QSizePolicy::Expanding, QSizePolicy::Expanding);
}
SubWidget Class:
SubWidget::SubWidget(QWidget *parent) :
QWidget(parent)
{
setStyleSheet("QWidget { background: yellow }");
setMaximumSize(10000,10000);
setMinimumSize(100,100);
setSizePolicy(QSizePolicy::Expanding, QSizePolicy::Expanding);
SetupLayout();
//setMaximumSize(10000,10000);
}
void SubWidget::SetupLayout()
{
QHBoxLayout main_layout;
main_layout.setAlignment(Qt::AlignTop | Qt::AlignLeft);
main_layout.addWidget(&m_graphics_view);
m_graphics_view.setMaximumSize(10000,100000);
setLayout(&main_layout);
}

Answer 1

void SubWidget::SetupLayout()
{
QHBoxLayout main_layout;
main_layout.setAlignment(Qt::AlignTop | Qt::AlignLeft);
main_layout.addWidget(&m_graphics_view);
m_graphics_view.setMaximumSize(10000,100000);
setLayout(&main_layout);
}
Your main_layout gets destroyed immediately after SetupLayout finished, as it goes out of scope. So, actually, your SubWidget has no any layout. That is why widgets are shown incorrectly.