Trouble using convertchar and thestring inside a LaTeX command or table

Question

I'm trying to generate page references inside a command that defines a row of a table, with the name of the label being referenced generated as a result of some string maniupulation using stringstrings. Here's a much simplified version of what I'm trying to do, generating a reference to the OPSFOO label based on a command parameter of FO_O, with the string processing removing underscores and prepending OPS:

\documentclass{article}
\usepackage{stringstrings}
\newcommand{\opind}[2]{
  \convertchar[q]{OPS#2}{\_}{}
  #1 & #2 & \pageref{\thestring} \\
}
\begin{document}
\begin{table}[h]
  \begin{tabular}{l l l}
    \opind{1}{FO\_O} \\
  \end{tabular}
\end{table}
\clearpage
\label{OPSFOO} FOO \\
\end{document}

That fails with the error "! Undefined control sequence. \thestring"

I thought perhaps there was something wrong with using \thestring inside a command, so I tried putting it directly into the table entry like this:

\begin{table}[h]
  \begin{tabular}{l l l}
     \convertchar[q]{OPSFO\_O}{\_}{}
     1 & FO\_O & \pageref{\thestring} \\
  \end{tabular}
\end{table}

But that gets essentially the same error. So I started wondering if there's just a problem with using \thestring as an argument to \pageref. Apparently not; if I do the string manipulation outside the table, everything works fine:

\convertchar[q]{OPSFO\_O}{\_}{}
\begin{table}[h]
 \begin{tabular}{l l l}    
   1 & FO\_O & \pageref{\thestring} \\
 \end{tabular}
\end{table}

Is there anything I can do to get the string manipulation and the use of \pageref{\thestring} to work in the command that's called inside the table, as in the first code listing above?

Answer 1

According to the stringstrings documentation, \thestring is only a \edefined macro, valid only in the local group (a table cell here). To transport the content to the later table cells, it's necessary to make a global \edef, i.e. using an \xdef with some helper macro, say \theglobalstring

\documentclass{article}
\usepackage{stringstrings}

\newcommand{\opind}[2]{
  \convertchar[q]{OPS#2}{\_}{}
  \xdef\theglobalstring{\thestring}
  #1 & #2 & \pageref{\theglobalstring} \\
}
\begin{document}
\begin{table}[h]
  \begin{tabular}{l l l}
    \opind{1}{FO\_O} \\
  \end{tabular}
\end{table}
\label{OPSFOO} FOO \\

\clearpage
\end{document}

Answer 2

Inspired by Christian Hupfer's comment to the problem, but avoiding using a global variable as in his answer:

\newcommand{\opind}[2]{
  #1 & #2 & \convertchar[q]{OPS#2}{\_}{} \pageref{\thestring} \\
}