\protect with \cite inside \caption

Question

I've read several posts and other webpages now on using \protect in front of \cite inside \caption. They all advise using \protect in this situation, and many go as far as to say that you must use it.

Now, I've been able to use \cite inside \caption without \protect and still get no errors. For example, the following works fine:

\documentclass{article}

\begin{document}

\begin{figure}
Hello
\caption{I cite this \cite[p. 41]{lang:cyc}.}
\end{figure}

\begin{thebibliography}{1}

\bibitem{lang:cyc}
S.~Lang.
\newblock {\em Cyclotomic fields {I} and {II}}, volume 121 of {\em Graduate Texts in Mathematics}.
\newblock Springer-Verlag, New York, second edition, 1990.
\newblock With an appendix by Karl Rubin.

\end{thebibliography}

\end{document}

I'm interested to know, therefore, whether \protect is strictly necessary in this context. Could it be that it's sometimes not necessary (as in the above), but that it's best to use it anyway because you never know when some intricate circumstance might make it necessary?

Thanks for your help.

Answer 1

It shouldn't be necessary, because the kernel \cite command is robust. Also the main packages for managing bibliographies, such as natbib and biblatex have care to redefine \cite in a robust fashion.

Some packages, unfortunately don't, for example apacite (but natbib emulates it pretty well). In such a case, \protect would become necessary.

Adding \protect when not strictly necessary shouldn't be a problem, though.