Qt5's documentation doesn't mention that QPlainTextEdit has setText(QString) like QTextEdit does. But, I don't think it's impossible. The only way I found is to use QTextDocument which can has setPlainText(const QString& text). So I have to do this:
plain_text_edit->setDocument(text_document);
The problem is text_document should be a pointer. Not like QTextEdit's setText which can take a local variable as it's parameter. So, is there anyway to do setText like to QPlainTextEdit?