I want to be able to add a line of (grey) text that when you type in the line edit, goes away. So the text is not actually there, it is just displayed.
For example:
Before I type something, the grey background text is there:
https://i.sstatic.net/2U955.png
After I type something, the text goes away, replaced by the text entered:
https://i.sstatic.net/JXVAQ.png
void QLineEdit::setPlaceholderText( const QString & )
is what you are looking for:
Ex:
ui->lineEdit->setPlaceholderText(QString("Type here"));