What does the command \@dblarg do in a class?

It essentially checks if the token after \CJ@title is [ or not, in order to pass to the main command the same argument twice, if [ is not found. The command \CJ@title must be defined by

\def\CJ@title[#1]#2{...}

so that, with a call such as

\title{xyz}

the expansions will be (successively}

\@dblarg\CJ@title{xyz}
...<some complex action>...
\CJ@title[{xyz}]{xyz}

and with a call such as

\title[abc]{xyz}

the expansions will be (successively}

\@dblarg\CJ@title[abc]{xyz}
...<some complex action>...
\CJ@title[abc]{xyz}

One can follow ...<some complex action>... by looking at the definition of \@dblarg:

\long\def\@dblarg#1{\kernel@ifnextchar[{#1}{\@xdblarg{#1}}}
\long\def\@xdblarg#1#2{#1[{#2}]{#2}}

So with \title{xyz} one has (one step on each line, I omit the expansions of \kernel@ifnextchar that are irrelevant here)

\@dblarg\CJ@title{xyz}
\kernel@ifnextchar[{\CJ@title}{\@xdblarg{\CJ@title}}{xyz}
\@xdblarg{\CJ@title}{xyz}
\CJ@title[{xyz}]{xyz}

With \title[abc]{xyz} one has

\@dblarg\CJ@title{xyz}
\kernel@ifnextchar[{\CJ@title}{\@xdblarg{\CJ@title}}[abc]{xyz}
\CJ@title[abc]{xyz}

There's an extra pair of braces in the first case, but it will be removed by rule of TeX; it’s needed in case the title contains a ].

How to do the same with xparse

With the xparse package one can obtain the same functionality with a clearer syntax (I assume this is done in a class or package file)

\RequirePackage{xparse}

\NewDocumentCommand{\title}{om}{%
  \IfNoValueTF{#1}
    {\CJ@title{#2}{#2}}
    {\CJ@title{#1}{#2}}%
}
\newcommand{\CJ@title}[2]{...}

So no delimited argument is necessary.

With a recent version of xparse one can do even better:

\RequirePackage{xparse}

\NewDocumentCommand{\title}{O{#2}m}{%
  \CJ@title{#1}{#2}%
}
\newcommand{\CJ@title}[2]{...}

because with this code the value supplied for a missing optional argument is the same as the mandatory argument.


This is an easy way of allowing optional arguments to some other function/macro.

For example, using your case of \def\title{\@dblarg\CJ@title}, you can either use

\title{some title}

without an optional argument, or use

\title[some optional title]{some title}

with an optional argument. "Easy" here refers to the fact that if you don't specify the optional component (like in the former example), it defaults to

\title[some title]{some title}

thereby converting single-argument responses into double-arguments. The macro \CJ@title is appropriately defined to manage both arguments and could have the form:

\def\CJ@title[#1]#2{%
  %...
}

This is typically the case with sectional commands that have an optional argument: \section[<ToC entry>]{<body entry>}. If you don't specify the optional argument, the <body entry> still makes its way into the ToC. How? \@dblarg.

Here are the formal definitions contained within latex.ltx - you'll find its use inside \@startsection, just as an example:

\long\def\@dblarg#1{\kernel@ifnextchar[{#1}{\@xdblarg{#1}}}
\long\def\@xdblarg#1#2{#1[{#2}]{#2}}

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