Why won't this macro work in a \caption?

Question

I created a macro to easily shift into typewriter mode, ala markdown or fancyvrb's "short verbatim" command. It works everywhere but in \captions. In that case, TeX fails saying I've exceeded capacity. Is my approach untenable? Below is a MWE. The example outside \caption, !+foo+!, works fine. The example inside \caption fails.

\chardef\plus=`+\chardef\bang=`!
\catcode`!=\active\catcode`+=\active
\def\startTTA{\futurelet\next\startTTB} 
\long\def\startTTB#1{\ifx\next+\expandafter\startTTC\else\bang\expandafter#1\fi}
\def\startTTC{\begingroup\ttfamily\global\let+=\stopTTA\global\let!=\bang}
\def\stopTTA{\futurelet\next\stopTTB}
\long\def\stopTTB#1{\ifx\next!\expandafter\stopTTC\else\plus\expandafter#1\fi}
\def\stopTTC{\endgroup\global\let+=\plus\global\let!=\startTTA}
\let!=\startTTA\let+=\plus
\documentclass{article}
\begin{document}
!+foo+!
\begin{figure}
\tracingmacros=1\tracingcommands=2
\caption{!+foo.+!}
\tracingmacros=0\tracingcommands=0
\end{figure}
\end{document}

Suggestions for improving this macro are welcome as well!

Answer 1

The problem is in the definition of \startTTB and \stopTTB with an argument; moreover, the active ! and + should be protected against expansion in write operations:

\makeatletter
\chardef\plus=`+\chardef\bang=`!
\catcode`!=\active\catcode`+=\active
\def\startTTA{\futurelet\next\startTTB}
\def\startTTB{\ifx\next+\expandafter\startTTC\expandafter\@gobble\else\bang\fi}
\def\startTTC{\begingroup\ttfamily\protected\def+{\stopTTA}\protected\def!{\bang}}
\def\stopTTA{\futurelet\next\stopTTB}
\def\stopTTB{\ifx\next!\expandafter\stopTTC\expandafter\@gobble\else\plus\fi}
\def\stopTTC{\endgroup}
\protected\def!{\startTTA}\protected\def+{\plus}
\makeatother

It's not necessary to do global redefinitions. With the two \@gobble commands, the + or ! characters are swallowed.

A shorter set of macros will do the same:

\chardef\plus=`+\chardef\bang=`!
\catcode`!=\active\catcode`+=\active
\protected\def!{\startTTA}\protected\def+{\plus}
\def\startTTA{\futurelet\next\startTTB}
\def\startTTB{\ifx\next+\expandafter\startTTC\else\bang\fi}
\long\def\startTTC+#1+!{\begingroup\ttfamily
  \let+\plus\let!\bang#1\endgroup}

However, I don't see where this is more efficient than declaring

\def\?+#1+?{\texttt{#1}}

and writing

\?+foo+?
\caption{\?+foo+?}