Argument of \caption@ydblarg has an extra }

Question

A misleading error message from the following code:

\documentclass{article}
\usepackage{graphicx}
\usepackage{subfig}

\begin{document}

\begin{figure}[!ht]
\centering%
\subfloat[]{
    \label{subfig1}
    \includegraphics[width=.4\linewidth]{fig1.pdf}%
}%
\qquad%
\subfloat[]{
    \label{subfig2}
    \includegraphics[width=.4\linewidth]{fig2.pdf}
}%
\label{myfig}
\caption{A caption for figures \subref{subfig1} and \subref{subfig2}.}
\end{figure}

\end{document}

gives

! Argument of \caption@ydblarg has an extra }.
<inserted text> 
                \par 
l.19 ...es \subref{subfig1} and \subref{subfig2}.}

?

What have I done wrong? I can't see any extra } anywhere.

Answer 1

The easiest solution in these cases is to add \protect in front of the usual suspects; \subref is one of them because it has a *-variant.

\documentclass{article}
\usepackage{graphicx}
\usepackage{subfig}

\begin{document}

\begin{figure}[!ht]
\centering
\subfloat[]{%
    \label{subfig1}%
    \includegraphics[width=.4\linewidth]{fig1.pdf}%
}%
\qquad
\subfloat[]{%
    \label{subfig2}%
    \includegraphics[width=.4\linewidth]{fig2.pdf}%
}

\caption{A caption for figures \protect\subref{subfig1} and \protect\subref{subfig2}.}
\label{myfig}

\end{figure}

\end{document}

Note where I added/removed % at end of lines and the position of \label{myfig} after \caption.

This might be annoying if you have several appearances of \subref in moving arguments. In this case you can “robustify” the command:

\documentclass{article}
\usepackage{graphicx}
\usepackage{subfig}
\usepackage{etoolbox} % for \robustify

\robustify{\subref}

\begin{document}

\begin{figure}[!ht]
\centering
\subfloat[]{%
    \label{subfig1}%
    \includegraphics[width=.4\linewidth]{fig1.pdf}%
}%
\qquad
\subfloat[]{%
    \label{subfig2}%
    \includegraphics[width=.4\linewidth]{fig2.pdf}%
}

\caption{A caption for figures \subref{subfig1} and \subref{subfig2}.}
\label{myfig}

\end{figure}

\end{document}

Another solution is to avoid subfig for subcaption, which is the path I'd recommend:

\documentclass{article}
\usepackage{graphicx}
\usepackage{subcaption}

\begin{document}

\begin{figure}[!ht]
\centering
\subcaptionbox{\label{subfig1}}{%
    \includegraphics[width=.4\linewidth]{fig1.pdf}%
}%
\qquad
\subcaptionbox{\label{subfig2}}{%
    \includegraphics[width=.4\linewidth]{fig2.pdf}%
}

\caption{A caption for figures \subref{subfig1} and \subref{subfig2}.}
\label{myfig}

\end{figure}

\end{document}