Suppose you need to know that the argument of your command is empty:
that is, to distinguish between \cmd{}
and \cmd{blah}. This is pretty simple:
\def\cmd#1{%
\def\tempa{}%
\def\tempb{#1}%
\ifx\tempa\tempb
\else
\fi
}
The case where you want to ignore an argument that consists of nothing
but spaces, rather than something completely empty, is more tricky.
It’s solved in the code fragment ifmtarg, which defines
commands \@ifmtarg and \@ifnotmtarg, which examine their
first argument, and select (in opposite directions) their second or
third argument. The package’s code also appears in the LaTeX
memoir class.
Ifmtarg makes challenging reading; there’s also a discussion of the issue in number two of the “around the bend” articles by the late lamented Mike Downes.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=empty