Re: Underline the current line (function inside)

[Eli Zaretskii <eliz@gnu.org>]

> Date: Fri, 11 Jun 2021 17:32:26 +0300
> From: Jean Louis <bugs@gnu.support>
> 
> This function is handy to underline the current line:
> 
> (defun underline-line ()
>   "Underline the current line."
>   (interactive)
>   (let* ((start (line-beginning-position))
> 	 (end (line-end-position))
> 	 (length (- end start)))
>     (end-of-line)
>     (newline)
>     (insert (make-string length ?=))
>     (newline)))

This assumes that each character takes 1 column.  That is false in
general, because some characters have zero width on display, and some
others take closer to two columns.

You need to use string-width instead, or count columns instead of
buffer positions.

And, of course, all this assumes fixed-pitch font.