There are functions like textEdit->textCursor()->selectionStart() and textEdit->textCursor()->selectionEnd(), but there are no functions setSelectionStart, setSelectionEnd.
Is there any way to select some part of text manually?
QTextCursor c = textEdit->textCursor();
c.setPosition(startPos);
c.setPosition(endPos, QTextCursor::KeepAnchor);
textEdit->setTextCursor(c);
This piece of code moves the cursor to the start position of the selection using setPosition, then moves it to the end of the selection, but leaves the selection anchor at the old position by specifying a MoveMode as the second parameter.
The last line sets the selection to be visible inside the edit control, so you should skip it if you just want to do some manipulations with the selected text.
Also, if you don't have the exact positions, movePosition is helpful: you can move the cursor in various ways, such as one word to the right or down one line.