\protect
All LaTeX commands are either fragile or robust. A
fragile command can break when it is used in the argument to certain
other commands. Commands that contain data that LaTeX writes to an
auxiliary file and re-reads later are fragile. This includes material
that goes into a table of contents, list of figures, list of tables,
etc. Fragile commands also include line breaks, any command that has an
optional argument, and many more. To prevent such commands from
breaking, one solution is to preceded them with the command
\protect
.
For example, when LaTeX runs the \section{section
name}
command it writes the section name text to the
.aux auxiliary file, moving it there for use elsewhere in the
document such as in the table of contents. Any argument that is
internally expanded by LaTeX without typesetting it directly is
referred to as a
moving argument. A command is fragile if it can
expand during this process into invalid TeX code. Some examples of
moving arguments are those that appear in the \caption{...}
command (see figure
), in the \thanks{...}
command
(see \maketitle
), and in @-expressions in the tabular
and
array
environments (see tabular
).
If you get strange errors from commands used in moving arguments, try
preceding it with \protect
. Every fragile commands must be
protected with their own \protect
.
Although usually a \protect
command doesn’t hurt, length
commands are robust and should not be preceded by a \protect
command. Nor can a \protect
command be used in the argument to
\addtocounter
or \setcounter
command.
In this example the \caption
command gives a mysterious error
about an extra curly brace. Fix the problem by preceding each
\raisebox
command with \protect
.
\begin{figure} ... \caption{Company headquarters of A\raisebox{1pt}{B}\raisebox{-1pt}{C}} \end{figure}
In the next example the \tableofcontents
command gives an error
because the \(..\)
in the section title expands to illegal TeX
in the .toc file. You can solve this by changing \(..\)
to \protect\(..\protect\)
.
\begin{document} \tableofcontents ... \section{Einstein's \( e=mc^2 \)} ...