How do I use an auxilliary file for my own commands?

Question

I would like to extend the \label and \ref commands to provide me not only with the number of the section but also some sort of heading.

What I have in mind is something like this:

\documentclass{article}
\usepackage{xparse}
\makeatother
\let\oldlabel\label
\def\Ref#1{see~\ref{#1}~(\csname l@#1\endcsname)}
\DeclareDocumentCommand{\label}{m o}%
{
\oldlabel{#1}
\IfValueTF{#2}%
    {\expandafter\gdef\csname l@#1\endcsname{#2}}
    {}
}
\makeatother
\begin{document}
    \section{Intro}\label{intro}[Introduction]
    Some text referencing \Ref{intro} or \Ref{outro}
    \section{Outro}\label{outro}[Outro]
    blah...
\end{document}

However, if I use \Ref with a forward reference I get an error because \l@outro is not yet defined. I would like to fix that by writing these \gdefs into an aux file and including at the begin of the document at the second latex run.

Therefore, my questions are: 1) How can I properly write \gdefs into an aux file, say \jobname.ref ? 2) How can I safely read the aux file? (It might not yet exist...)

The problem with undefined commands isn't a problem since \@ifundefined can be used. But let's assume all labels specify the optional argument so far.

Thanks for your help!

Answer 1

I figured out how to use the newfile package for my purposes.

    \documentclass{article}
    \usepackage{xparse,ifthen}
    \usepackage{newfile,hyperref}
    \makeatother
    \AtBeginDocument{%
        \newoutputstream{refs}
        \let\oldlabel\label
        \let\oldsect\section
        \DeclareDocumentCommand{\label}{m o}%
        {
        \oldlabel{#1}
        \IfValueTF{#2}%
                {%
                \addtostream{refs}{\protect\DeclareDocumentCommand{\string\l@#1}{}{#2}}
                }
                {}
        }
        \def\Ref#1{see~\ref{#1}~(\expandafter\ifx\csname l@#1\endcsname\undefined{??}\else{\csname l@#1\endcsname}\fi)}
        \def\section#1\label#2{\oldsect{#1}\label{#2}[#1]}}
    \makeatother
    \begin{document}
            \IfFileExists{\jobname.ref}{\input{\jobname.ref}}{}
            \openoutputfile{\jobname.ref}{refs}
            \addtostream{refs}{\protect\makeatletter}

            \section{Intro}\label{intro}
            Some text referencing \Ref{intro} or \Ref{outro}
            \section{Outro}\label{outro}
            blah... see \pageref{intro} or \autoref{intro}

            \addtostream{refs}{\protect\makeatother}
            \closeoutputstream{refs}
    \end{document}

In the end, I redefined the commands \section#1\label#2 to store the section title #1 into \l@#2 for later use by \Ref. Of course, in the end I need to redefine \chapter, \subsection etc. too.

If you have any further improvements or found some errors please let me know.