Creating an error message

Is there a way to create my own error message that is given if the main font size isn't set to 11pt and then the compilation of the document begin stopped?

Update

I'm trying to write a package, and I use:

\documentclass{scrreprt}%or scrbook or scrartcl

\usepackage{classicthesis}

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Isn't it easier to just set the font size, since you want to stop the compilation if it is something else? –  Peter Grill Sep 26 '12 at 21:17

If you're writing a package, say foo.sty then

\AtBeginDocument{
\if\@ptsize1\else
\PackageError{foo}
{Illegal font size option detected.\MessageBreak
This is a fatal error, I'll stop the run}
{You have specified a point size different from\MessageBreak
the unique size this package supports, which is 11pt.\MessageBreak
\expandafter\@@end
\fi}


will do.

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if could help, I have updated my question. Another thing, I haven't understand why I need to ensure that the 11pt option is correctly passed to the main class. And exactly where I must put that code. Thanks. –  FormlessCloud Sep 26 '12 at 22:01
@FormlessCloud The code should be in your package. I've removed the references to a class. –  egreg Sep 26 '12 at 22:10
Sure, but I was referring to the deleted code: \LoadClassWithOptions... –  FormlessCloud Sep 26 '12 at 22:14
@FormlessCloud Since you're not writing a class, that code means nothing. –  egreg Sep 26 '12 at 22:18
Ok thanks, it was only to know something more. –  FormlessCloud Sep 26 '12 at 22:28

If you're using one of the standard document classes (article, book or report), the macro \@ptsize contains 1 if you're using 11pt:

\documentclass[12pt]{article}
\makeatletter
\AtBeginDocument{%
\newcommand{\@ptsizetest}{1}% Equivalent to 11pt for a document class option.
\ifx\@ptsizetest\@ptsize\relax\else
\@latex@error
{You need to use 11pt font size}% Error message
{Add this as a document class option [11pt]}% Help text
\fi%
}
\makeatother
\begin{document}
Test document
\end{document}​


This creates the error

LaTeX Error: You need to use 11pt font size.

If you're using other packages that define the font size, some more work is required.

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