How the order you define commands in a class file changes the output?

Question

I'm working with a class file, trying to adjust it to my needs, and somewhere in the middle of it is defined a function that writes the title of the paper. What I have is this:

\title{
\vspace{-30pt}
\begin{flushleft}
\HorRule
\\
\fontsize{35}{40}
\usefont{T1}{phv}{b}{n}
\color{titlecolor}
\@TheTitle
\\
\end{flushleft}
}

\renewcommand{\title}[2]{\def\@TheTitle{#1}\def\@ShortTitle{#2}}

Let us begin with the last line of code. It defines a command \title with two arguments. When you call \title inside your .tex file what it does is to define two text replacements inside the .cls file. The \@ShortTitle will be used in the footer and \@TheTitle in the title. Now, going back to the first block of code, it calls a function \title which uses the value stored in \@TheTitle. My doubt is this: how can \title be called before it is defined (in the last line of the code)? And how can it use something that was not defined yet (the \@TheTitle)?

I tried to pass the line

\renewcommand{\title}[2]{\def\@TheTitle{#1}\def\@ShortTitle{#2}}

to the begining of the code but it simply does not work that way. What is going on?

Answer 1

Typical document classes only store the contents of \title inside a macro, let's say, \@title. The common procedure for this resembles

\def\title#1{\def\@title{#1}}

This allows users to specify \title inside the preamble (say) and only later issue \maketitle (within the document environment) to set the title. Whenever a call to \title is made, it can contain any "garbage" that may or may not exist - the problem (of undefined control sequences) will only realise itself when you actually set the title via \maketitle.

So, in your case, \title probably only stores it's contents inside \@title like the usual classes do, even though \@TheTitle is undefined at the time \title is called. When the user of your document class calls \title, it uses the redefined version of \title which sets the values of \@TheTitle and \@ShortTitle.

The reason for this ordering is probably to ensure that the default \title procedure (set via \maketitle) is still usable in the traditional sense. One issue may be that \title typically only takes a single argument, while the redefinition clearly requires two mandatory arguments.

Answer 2

You have (again) only posted fragments, not a reproducible test case, so it is not possible to give a full answer but

my doubt is this: how can \title be called before it is defined (in the last line of the code)?

the clue is in the last line

\renewcommand{\title}..

which is not a definition of \title but a redefinition. This necessarily means that \title must already have been defined (presumably with a different definition) and it is that (unshown) definition that is used in the call to \title at the start of your fragment.

The previous, unshown, definition presumably defined \title with one argument not two, looking at the use here.

Unrelated to your question but the use of \title containing negative spacing, low level font calls and unwanted space tokens is just horrible if you are learning latex, you really shouldn't be learning from code like this!