\renewcommand with only one argument, inside \newcommand - why does this work?

Question

I have seen the following definition in a class file (which we use internally):

\newcommand{\@TitleBackground}{default}
\newcommand{\titlebackground}{\renewcommand{\@TitleBackground}}

This provides \titlebackground as a command for changing \@TitleBackground, but how/why does it work? I would understand it if the second line was

\newcommand{\titlebackground}[1]{\renewcommand{\@TitleBackground}{#1}}

but why does it work also without the argument, i.e, with \renewcommand having onle one parameter?

And does the shorter version has some advantages or disadvantages over the longer one?

Answer 1

With \newcommand{\titlebackground}{\renewcommand{\@TitleBackground}}, the following happens when \titlebackground{whatever} is processed: \titlebackground is replaced by its expansion, so TeX will restart from

\renewcommand{\@TitleBackground}{whatever}

and eventually do \def\@TitleBackground.

With \newcommand{\titlebackground}[1]{\renewcommand{\@TitleBackground}{#1}}, upon finding \titlebackground{whatever} TeX will start expanding \titlebackground; since now this is a macro with an argument, TeX absorbs it and replaces the whole \titlebackground{whatever} with the replacement text, getting at

\renewcommand{\@TitleBackground}{whatever}

as before. So the difference is that in the second case {whatever} is looked at as an argument one more time than in the first version.

The difference is negligible. On my machine, doing a job like in the second case one million times require 1/100 of a second longer than the first way.