How to patch a floating environment so that the patched command floats

Question

I'm using etoolbox to patch some environments. I want to use \AtBeginEnvironment{table}{\marginpar{foo}} to put something in the margin by the float. But the marginpar is typeset where the environment is written, not where the float is output, if you see what I mean. How can I get something to float with the table?

Answer 1

Based on Martin Scharrer's redefinition of the table environment and egreg's statement that insertions are forbidden inside insertions: This is a case where \marginpar doesn't work, but \marginnote (from the marginnote package) does.

\documentclass{article}

\usepackage[english]{babel}
\usepackage{blindtext}

\usepackage{marginnote}

\makeatletter
\renewenvironment{table}[1][]{\@float{table}[#1]\marginnote{foo}}{\end@float}
\makeatother

\begin{document}

\blindtext

\begin{table}
\centering
(Table content)
\caption{A table}
\end{table}

\end{document}

Answer 2

You can't. Both tables and marginpars are insertions and it's forbidden to have an insertion inside an insertion.

If you try to put a \marginpar in a table environment you get

! LaTeX Error: Float(s) lost.

Martin's redefinition doesn't work either:

! LaTeX Error: Not in outer par mode.

It wouldn't work anyway.

Answer 3

The AtBeginEnvironment code is executed just before the internal macro of the environment (\table) here:

$ latexdef -p etoolbox begin

\begin:
macro:#1->\csuse {@beforebegin@#1@hook}%
  \@ifundefined {#1}%
    {\def \reserved@a {\@latex@error {Environment #1 undefined}\@eha }}%
    {\def \reserved@a {\def \@currenvir {#1}\edef \@currenvline {\on@line }
     \csuse {@begin@#1@hook}\csname #1\endcsname }}%
  \@ignorefalse \begingroup \@endpefalse \reserved@a 

The important part here is \csuse {@begin@#1@hook}\csname #1\endcsname. So you can't use these hooks to do this.

Note that table is simply:

\table:
\long macro:->\@float {table}

\endtable:
\long macro:->\end@float 

So I would write:

\renewenvironment{table}[1][]{\@float{table}[#1]\marginpar{foo}}{\end@float}

and insert the code manually. Of course you need to be careful if you load any packages which change the definition of table.