Problem with subcaption package - undefined command

Question

I am currently writing my own document class (which is very inspired by the book class) and I am facing a problem when using the caption and subcaption packages.

Here's a part of my classemphasized text:

\RequirePackage{caption}
\RequirePackage{subcaption}

\newcounter{figure}

\newenvironment{figure}{%
    \@float{figure}%
}
{\end@float}

\renewcommand\thefigure{%
    \@arabic\c@figure%
}

\renewcommand\thesubfigure{%
    \arabic{subfigure}%
}

\newcommand\figurename{Fig}

\def\fps@figure{tbp}
\def\ftype@figure{1}
\def\ext@figure{lof}
\def\fnum@figure{\figurename\nobreakspace\thefigure}

\DeclareCaptionLabelFormat{format}{\textbf{\textsc{#1.~#2. -- }}}

\captionsetup[figure]{
    name={Fig},
    labelsep=none,
    labelformat=format,
    textformat=simple,
    justification=justified,
    singlelinecheck=true,
    font=footnotesize,
    textfont=it,
    position=bottom,
}

\captionsetup[subfigure]{
    name={Fig},
    labelsep=none,
    labelformat=format,
    textformat=simple,
    justification=justified,
    singlelinecheck=true,
    font=footnotesize,
    textfont=it,
    position=bottom,
}

For the simple figures, no problem, everything goes perfectly well. But when I try to insert subfigures I get the following error concerning the document class:

Command \thesubfigure undefined

Do you know where this mistake came from? I've read the documentation of both packages but I'm not very advanced. . .

This is the first time I write my own document class, it's quite different from a "classical" use of LaTeX, so please excuse me if my class is a makeshift job. Thanks

PS : Here is link for the document class : MyClass.cls

The figure environment is defined on lines 382 to 410 and the caption on lines 452 to 510.

And here is the link for .tex file : Test.tex

Answer 1

• When loading subcaption, it automatically calls \DeclareCaptionSubType{figure} if counter figure is defined.
• Inside \DeclareCaptionSubType{figure},
  • command \ext@figure is expanded. (Actually this happens inside the expansion of \caption@@@@declaresublistentry, defined in caption3.sty)
  • new counter subfigure is defined, hence the corresponding command \thesubfigure is defined.

For counter table, similar things happen.

So the key is to provide counter <float type> and command \ext@<float type> before calling \DeclareCaptionSubType[*]{<float type>}.

A full example

\documentclass{minimal}

\newcounter{figure}
\newcounter{table}

\makeatletter
\def\ext@figure{lof}
\def\ext@table {lot}
\makeatother

\RequirePackage{subcaption}

\renewcommand{\thesubfigure}{...}
\renewcommand{\thesubtable} {...}

\begin{document}
abc
\end{document}