Adding a circle to a section title

Question

I want a circle around a roman numeral inside a section title.

A command \circled is made from:

\documentclass[11pt,a4paper]{article}
\usepackage{tikz}   
...

%Circle around characters
\newcommand*\circled[1]{%
    \tikz[baseline=(C.base)]\node[draw,circle,inner sep=0.5pt](C) {#1};\!
}

a solution I have from the latex community forum. It works very well surrounding a character with a circle. When I try to put this inside a \subsection command to add it to a section title, it actually works, but I get many errors.

Result:

Warnings:

What is wrong here? It works apparently, but can I fix the errors?

Answer 1

\circled is a fragile command this way (due to the \tikz etc content).

It must be \protected then. Or define it with \DeclareRobustCommand, as for example \robustcircled, which can be used without \protect then.

As a general rule: Use robust commands as arguments to the \section etc. commands.

\documentclass[11pt,a4paper]{article}
\usepackage{tikz}   

%Circle around characters
\newcommand*\circled[1]{%
    \tikz[baseline=(C.base)]\node[draw,circle,inner sep=0.5pt](C) {#1};
}


\DeclareRobustCommand*\robustcircled[1]{%
    \tikz[baseline=(C.base)]\node[color=blue,draw,circle,inner sep=0.5pt](C) {#1};
}


\usepackage{pgffor}

\begin{document}

\foreach \x in {1,...,5}{%
\section{\protect\circled{\x} The super theory on brontosaurs}
\foreach \y in {1,...,6}{%
\subsection{\robustcircled{\y} The theory of brontosaurs -- version \y}
}}
\end{document}