Set font family default after loading it

Question

fontspec offers the commands \setmainfont, \setsansfont and \setmonofont to load a font and set it as the default roman, sans serif or monospaced font, respectively. It also offers the command \newfontfamily, which loads a font and defines a command that activates it:

% loads the font family 'Latin Modern Roman' and defines the command `\latinmodernroman`,
% which activates it
\newfontfamily\latinmodernroman{Latin Modern Roman}
\latinmodernroman% activates the Latin Modern Roman font family

% loads the font family 'Latin Modern Roman' and *also* sets it as the roman family default
\setmainfont{Latin Modern Roman}

How would one go about decoupling loading and setting, i.e., first loading a font with \newfontfamily and then setting it as family default?

\newfontfamily\latinmodernroman{Latin Modern Roman}

\def\rmdefault{  what?  }
% My understanding is that it's 'lmr' for 'Latin Modern Roman'
% But how about arbitrary fonts? How to know their "LaTeX identifiers" when using fontspec?

I've been trying to understand fontspec.dtx, but I'm not familiar with the "expl" syntax.

Answer 1

What you are looking for is called "NFSS family name" (where "NFSS" stands for New Font Selection Scheme).

Section 5.2 of the fontspec manual:

In fontspec, the family names are auto-generated based on the fontname of the font; for example, writing \fontspec{Times New Roman} for the first time would generate an internal font family name of ‘TimesNewRoman(1)’.

The solution is provided in the same section: use the NFSSFamily feature when loading the font:

\newfontfamily\verbatimfont[NFSSFamily=myverbatimfont]{Inconsolata}

% Now, you can do:
\def\rmdefault{myverbatimfont}
\fontfamily{myverbatimfont}\selectfont

Beware, however:

Only use this feature when necessary; the in-built font switching commands that fontspec generates (such as \verbatimfont in the example above) are recommended in all other cases.

The fontspec manual (again, in the same section) mentions a mechanism that's part of the package's programming interface and which allows to know the NFSS family name that's auto-generated. However, it makes use of "expl" syntax, which I'm not familiar with.