Fragile error on robust command using relsize

Question

I created a supposedly robust command through \DeclareRobustCommand{\plusplus}{\relsize{-3}{++}} that modifies some ++ sign, and I use it in the document. However, when I use the \relsize and the hyperref package with the tocdepth counter set, I obtain a cryptic error (at least for me)

Runaway argument? {\@tempcnta >\z@ \relax \advance \@tempcnta \m@ne \@tempdimb ! TeX capacity exceeded, sorry [main memory size=10000000]. ...\@tempcnta \m@ne \@tempdimb \@tempa \@tempdimb \relax l.15 \subsubsection{test\plusplus}

I even try to increase the memory size, as you can see, up to 10000000. And, in my full document the error changes to

Runaway definition? ->\protect 5.3.4 test\protect \<>-\kernel@ifnextchar [4]\<>-\@ifnextchar \ETC . ! TeX capacity exceeded, sorry [main memory size=10000000]. \new@ifnex tchar l.108 \subsection{test\plusplus}

I don't understand why the error changes. However, the code below is the shortest code I get before removing the error. However, I need to use the hyperref package and the table of contents. How can I solve this problem?

\documentclass{book}

\usepackage{relsize}
\DeclareRobustCommand{\plusplus}{\relsize{-3}{++}}

\usepackage{hyperref}%remove hyperref and it compiles

\begin{document}

\setcounter{tocdepth}{3}%or remove teh tocdepth and it compiles
%but use both and it doesn't

\tableofcontents

\subsubsection{test\plusplus}

\end{document}

Answer 1

The problem is with hyperref that tries to expand \plusplus and finds itself in an infinite loop.

Solution: provide a special definition of \plusplus for the bookmarks.

\documentclass{book}

\usepackage{relsize}
\usepackage{hyperref}

\DeclareRobustCommand{\plusplus}{\relsize{-3}{++}}
\pdfstringdefDisableCommands{%
  \def\plusplus{++}%
}

\begin{document}

\tableofcontents

%%% the following would work also without \pdfstringdefDisableCommands
\section{test\texorpdfstring{\plusplus}{++}}

\section{test\plusplus}

\end{document}

For a "one shot" occurrence, using \texorpdfstring{\plusplus}{++} is simpler. The \pdfstringDisableCommands way is better as it doesn't require thinking whether a command is safe for the bookmarks or not.

---

You can define \plusplus differently, without relsize:

\documentclass{book}

\usepackage{hyperref}

\makeatletter
\DeclareRobustCommand{\plusplus}{%
  {\check@mathfonts\fontsize{\ssf@size}{\f@baselineskip}\selectfont++}%
}
\pdfstringdefDisableCommands{%
  \def\plusplus{++}%
}
\makeatother

\begin{document}

\tableofcontents

%%% the following would work also without \pdfstringdefDisableCommands
\section{test\texorpdfstring{\plusplus}{++}}

\section{test\plusplus}

\end{document}

If you want to raise the two + symbols, you can change the definition into

\makeatletter
\DeclareRobustCommand{\plusplus}{%
  \raisebox{.25ex}{\check@mathfonts\fontsize{\ssf@size}{\f@baselineskip}\selectfont++}%
}
\pdfstringdefDisableCommands{%
  \def\plusplus{++}%
}
\makeatother