How to get the font size parameter passed to the main standard classes?

Question

For complex length manipulation (see here), I would need to get the font size that was passed as an option to the document class:

\documentclass[12pt]{article}
\newlength{\mainfontsize}
\setlength{\mainfontsize}{\@ptsize}

I would like it to work on all the main classes including beamer and extarticle. Currently this fails on beamer. How to make it work? (and I don't want the exact size of the font like 10.95pt for 11pt, I want the option that was passed).

Answer 1

Since most classes define their own option handling system you need one different macro for each.

For example, for 12pt, article (and the standard LaTeX classes) store just 2 in \@ptsize. extarticle stores 12. beamer stores {size12.clo} in a \beamer@size macro, and memoir stores 12 in \@memptsize. Not exactly standardised.

Here's a macro that checks for these classes and uses the correct macro accordingly, then stores the font size (from the option, not the actual font size) in a length:

\documentclass[14pt]{extarticle}
\makeatletter
\newcommand{\deffontsize}[1]{%
  \setlength#1%
    {%
      \vincent@ifclassloaded {beamer}
        \get@beamersize
        {%
          \vincent@ifclassloaded {memoir}
            \get@memoirsize
            {%
              \vincent@ifclassloaded {extarticle}
                \get@extartsize
                \get@standardsize
            }%
        }%
    }%
}
\def\vincent@ifclassloaded#1{%
  \expandafter\ifx\csname ver@#1.cls\endcsname\relax
    \expandafter\@secondoftwo
  \else
    \expandafter\@firstoftwo
  \fi}
\def\get@beamersize{%
  \expandafter\expandafter\expandafter\get@@beamersize
  \expandafter\@firstofone\beamer@size}
\def\get@@beamersize size#1.clo{#1 pt}
\def\get@memoirsize{\@memptsize pt}
\def\get@extartsize{\@ptsize pt}
\def\get@standardsize{1\@ptsize pt}
\makeatother
\begin{document}
\newlength\mainfontsize
\deffontsize{\mainfontsize}
\the\mainfontsize
\end{document}

For other classes you'd need to add another conditional and the corresponding macro to get the option.