How to get the line height of a QTextBlock?

Therefore

Windows 7 SP1
MSVS 2010
Qt 4.8.4

Given this code:

@#include <QTGui>

int main(int argc, char argv[])
{
QTextDocument text_document = new QTextDocument("testing");
QTextBlock text_block = text_document->begin();
qDebug() << text_block.text() << text_block.blockFormat().lineHeight()
<< text_block.blockFormat().lineHeightType();
}@

The console displays:

@"testing" 0 0@

Question: Why doesn't lineHeight return "the LineHeight property for the paragraph"? The lineHeightType is set for single spacing.

I am clearly not understanding this. When I try to set the line height before outputting, nothing happens (lineHeight() is still zero):

@text_block.blockFormat().setLineHeight(30,QTextBlockFormat::SingleHeight);@

To be clear, in my application nothing happens when outputting to a GUI window.

Even trying:

@qDebug() << text_block.text() << text_block.layout()->boundingRect().height();@

yields 0.