Caption package destroys center/varwidth

Question

I have received a wonderful comment how to get listings centered:

\documentclass[a4paper]{book}
%\usepackage[hang,small,it,hypcap=true]{caption}  % screw hypcap
\usepackage[T1]{fontenc}
\usepackage{fancyvrb}
\usepackage{listings}
\lstset{
  fancyvrb=true,%
}
\usepackage{varwidth}

\lstset{
    basicstyle=\linespread{0.94}\ttfamily,%
    fancyvrb=true,%
    captionpos=b
}

\begin{document}

\begin{center}\begin{varwidth}{\linewidth}%
\begin{lstlisting}[caption={I want to be centered},label={lst:label2}]
trait Sys[S <: Sys[S]] {
  type Tx     <: stm.Txn[S]
  type Var[A] <: stm.Var[S#Tx, A]
  type ID     <: stm.Identifier[S#Tx]
  type Acc
  ...
}
\end{lstlisting}\end{varwidth}
\end{center}

\end{document}

So far so good:

If I add the caption package (uncomment in the example above) which I must do because a lot of stuff relies on it, the centering is destroyed:

I also found that adding the showframe package has the same problem, so perhaps the culprit is center or varwidth and not caption. How to fix this?

Answer 1

varwidth is not able to cope with arbitrary contents and you get the warning

Package varwidth Warning: Failed to reprocess entire contents

In these cases the environment becomes simply a minipage with the stated width. You can do with the caption outside the lstlisting environment:

\documentclass[a4paper]{book}
\usepackage[T1]{fontenc}

\usepackage[
  hang,
  small,
  it,
  hypcap=true
]{caption}
\usepackage{fancyvrb}
\usepackage{listings}
\lstset{
  fancyvrb=true,
}
\usepackage{varwidth}

\lstset{
    basicstyle=\linespread{0.94}\ttfamily,
    fancyvrb=true,
    captionpos=b
}

\begin{document}

\begin{center}
\begin{varwidth}{\linewidth}
\begin{lstlisting}
trait Sys[S <: Sys[S]] {
  type Tx     <: stm.Txn[S]
  type Var[A] <: stm.Var[S#Tx, A]
  type ID     <: stm.Identifier[S#Tx]
  type Acc
  ...
}
\end{lstlisting}\end{varwidth}
\captionof{lstlisting}{I want to be centered}\label{lst:label2}
\end{center}

\end{document}

Watch out about possible page breaks between the listing and the caption. A float would be better; figure (with \captionof{lstlisting}) would be good.

Answer 2

Here is another solution:

\documentclass[a4paper]{book}
\usepackage[T1]{fontenc}

\usepackage{caption}
\usepackage{fancyvrb}
\usepackage{listings}

\begin{document}

\begin{figure}
\centering
\lstset{xleftmargin=0.18\textwidth,xrightmargin=0.18\textwidth}
\begin{lstlisting}
trait Sys[S <: Sys[S]] {
  type Tx     <: stm.Txn[S]
  type Var[A] <: stm.Var[S#Tx, A]
  type ID     <: stm.Identifier[S#Tx]
  type Acc
  ...
}
\end{lstlisting}
\captionof{lstlisting}{Type members of the system abstraction}\label{lst:scala_system_decl}
\end{figure}

\end{document}