Extra command in subfig package

Question

I would like an extra command to be run in the captions using the \subfloat command from the subfig package. With normal captions, I just redefine \captionfont, but this doesn't work for captions of subfloats.

I have tried (the command I want to run is \sansmath:

\documentclass{article}
\usepackage{caption}
\renewcommand{\captionfont}{\footnotesize\sffamily}
\usepackage{sansmath}
\usepackage[textfont=sf]{subfig}
\let\bensubfloat\subfloat
\renewcommand{\subfloat}[2][]{\bensubfloat[\sansmath #1]{#2}}

\begin{document}
\begin{figure}
    \subfloat[Subcaption $1+1$]{\framebox[3cm]{First}} \hfill
    \subfloat[Subcaption $1+a$]{\framebox[3cm]{Second}}
    \caption{Main caption} 
\end{figure}
\end{document}

This gives the desired output, but also many errors.

Answer 1

It seems to me like you want to have this:

\documentclass{article}
\usepackage{sansmath,subfig}
\DeclareCaptionFont{sansmath}{\sansmath}
\captionsetup{font={footnotesize,sf,sansmath}}
\captionsetup[subfloat]{font={footnotesize,sf,sansmath}}

\begin{document}

\begin{figure}
    \subfloat[Subcaption $1+a$]{\framebox[3cm]{First}} \hfill
    \subfloat[Subcaption $1+b$]{\framebox[3cm]{Second}}
    \caption{Main caption $1+x$} 
\end{figure}

\end{document}

This code uses \DeclareCaptionFont which is offered by the caption package for exactly this purpose - using own commands (like \sansmath) for caption or sub-caption font settings.

Answer 2

Protect fragile commands within moving arguments:

\renewcommand{\subfloat}[2][]{\bensubfloat[\protect\sansmath #1]{#2}}

Answer 3

\documentclass{article}
\usepackage[textfont=sf]{subfig}
\usepackage{caption}
\renewcommand{\captionfont}{\footnotesize\sffamily}
\usepackage{sansmath}
\makeatletter
\long\def\sf@@subfloat[#1]{%
    \@ifnextchar [%  %] match left bracket
      {\sf@@@subfloat{sub\@captype}[{\sansmath #1}]}%
      {\sf@@@subfloat{sub\@captype}[\@empty{#1}][{\sansmath #1}]}}
\makeatother

\begin{document}

\begin{figure}
    \subfloat[Subcaption $1+1$]{\framebox[3cm]{First}} \hfill
    \subfloat[Subcaption $1+a$]{\framebox[3cm]{Second}}
    \caption{Main caption} 
\end{figure}

\end{document}