Where does fontspec store newfontfamily name?

Question

I am using LuaTeX with TeXlive 2016, Ubuntu. Using fontspec to create a new font family. MWE:

% !TeX program = LuaLaTeX
% !TeX encoding = UTF-8
\documentclass{minimal}
\usepackage{fontspec}
\newfontfamily\MyPrivate{Latin Modern Roman} % or some other font
\begin{document}
Your private font is % \whatisMyPrivate
\end{document}

I seek \whatisMyPrivate, so that I can uncomment that line and have the document print out the name of the font (in this case, Latin Modern Roman). I see that the \newfontfamily command is defined in file fontspec-luatex.sty, but alas I cannot decipher where it stores the information for subsequent read-back.

I am hoping to do this without actually setting any text in the private font. But if the only way to do this is to use the font to set some text, then read-back from within that text, then that's OK.

Answer 1

You could for example apply the font in a group and extract the altered definition of \f@family which holds the font name. I wrapped this into a higher level macro.

\documentclass{article}
\usepackage{fontspec}
\newfontfamily\MyPrivate{Latin Modern Roman} % or some other font
\def\getfontname#1{%
  {#1\expandafter\xdef\csname private_font_name\endcsname{\csname f@family\endcsname}}%
  \csname private_font_name\endcsname
}
\begin{document}
Your private font is \getfontname\MyPrivate
\end{document}

Answer 2

You can get the family name by pure expansion.

The trick is knowing that the assigned family name is stored in the internal variable \g__fontspec_MyPrivate_family:

\documentclass{article}
\usepackage{fontspec}

\ExplSyntaxOn
\DeclareExpandableDocumentCommand{\getfamilyname}{m}
 {
  \use:c { g__fontspec_ \cs_to_str:N #1 _family }
 }
\ExplSyntaxOff

\newfontfamily\MyPrivate{Latin Modern Roman} % or some other font

\begin{document}

Your private font is \getfamilyname\MyPrivate

% emulate a \write operation
\edef\test{\getfamilyname\MyPrivate}

\texttt{\meaning\test}

\end{document}

The final example shows that if you do

\write\baz{\getfamilyname\MyPrivate}

(where \baz is an output stream), the requested family name will be written.

However fontspec doesn't provide a public interface for accessing font family data, so this method might fail in future releases. Actually, recording the particular internal name assigned to the font family is not useful at all. In case an explicit name is needed, the manual explains how (and why) to assign one: see section 5.2.

\newfontfamily\MyPrivate{Latin Modern Roman}[
  NFSSFamily=foo,
  % <other options>
]

The family name assigned will be foo.