How to define an empty parameter for minipage

Question

minipage supports empty values for its optional arguments:

\begin{minipage}[c][][c]{1cm} test \end{minipage}

or

\begin{minipage}[c][\relax][c]{1cm} test \end{minipage}

but it doesn't if that empty values are passed by a macro:

\def\param{}
\begin{minipage}[c][\param][c]{1cm} test \end{minipage}

or

\def\param{\relax}
\begin{minipage}[c][\param][c]{1cm} test \end{minipage}

How can I properly define the macro, so that minipage takes an empty value?

Answer 1

The internal code for minipage uses the optional arguments when processing \endminipage. At a certain point, \@iiiparbox is called, which does \ifx\relax#2, where #2 is the second optional argument (or \relax if the optional argument is empty). Since no expansion is performed by \ifx, your code compares

\ifx\relax\param

which returns false, because \param is a macro expanding to \relax, not the same as \relax (assuming you gave \def\param{\relax}; or it expands to nothing, which again is not the same as being empty when the argument is absorbed if you give \def\param{}.

By the way, \def\param\relax is incorrect syntax; in the two lines in the second example you're defining \param with parameter text \relax\begin, expanding to minipage. But the intended \def\param{\relax} (that I assumed in the answer) will not work, as explained.