babel, frenchb and em dash

Question

If I'm not wrong, in French the em-dashes, typeset with --- in LaTeX, are usually treated like the guillemets as far as non-breaking spaces go. An "opening" em-dash must be followed by a non-breaking space, while a "closing" em-dash must be preceded by a non-breaking space.

[french]{babel} doesn't apply this rule, so I ended up making my own macros:

\newcommand{\ocadr}{\mbox{---~}}
\newcommand{\fcadr}{\mbox{~---}}

Is there a better way to ensure these non-breaking spaces?

Edit:

Here is a minimal example to justify the use of \mbox:

\documentclass{minimal}

\begin{document}

\parbox{9cm}{\textsc{
Dieu se sert des epreuves pour nous montrer nos c\oe{}urs,
 afin de pouvoir nous purifier et nous faire ensuite entrer
 dans le pays de la benediction ---~physiquement et spirituellement.
}}

\end{document}

which produces:

Whereas with \mbox around the ---~, I get:

Nota:

I kept the \textsc because it's a case that I know produces this effect with the given size of the \parbox.

Answer 1

I see that the \mbox is needed as the ligature inserts a \discretionary. However, I'd keep the space out of the box if I were you. So

\newcommand*{\ocadr}{\mbox{---}~}
\newcommand*{\fcadr}{\unskip~---}

using the primitives \unskip to avoid any additional space (the opening macro should skip spaces in any case) and the \mbox to prevent a break after the opening material would seem best.