correct usage of \@namedef and \@nameuse

Question

In a previous question I asked how to generate multiple saveboxes in a loop. The answer build around csname and the part inside csname is rather long and I need the name several times (create the box, fill the box, get properties and use it). Therefore I would like to shorten this. I tried

\makeatletter
...
\@namedef{saveBox}{<what was inbetween \csname \endcsname>}
...
\expandafter\newsavebox\@nameuse{saveBox}
...
\makeatother

which is not working for me. A short background information on the inner workings/general intended use would be a bonus.

---

The awnser leaves me with one issue, I can't get the \def version to work with a direct version, see example for clarification:

\def\floatRow@currentSaveBox{floatRow@savebox\alph{floatRow@rowImageCount}}
%\expandafter\newsavebox\csname\floatRow@currentSaveBox\endcsname   %works
\expandafter\newsavebox\csname{}floatRow@savebox\alph{floatRow@saveboxCount}\endcsname  %gives mising number error

I assume that my separation of \commandName and the string floatRow@savebox is wrong. Here is a MWE:

\documentclass{report}
\begin{document}
\newcounter{c}
\def\currentSBox{prefix\alph{c}}
\expandafter\newsavebox\csname\currentSBox\endcsname
\expandafter\savebox\csname\currentSBox\endcsname{dummy}
\expandafter\usebox\csname\currentSBox\endcsname
\expandafter\usebox\csname{}prefix\alph{c}\endcsname
\end{document}

Answer 1

Let's look at what is happening here. The construct

\@namedef{saveBox}{<what was inbetween \csname \endcsname>}

is defining \saveBox as a macro with replacement text <what was inbetween \csname \endcsname>. That's because \@namedef is defined as

#1->\expandafter\def\csname #1\endcsname

That means that what it does is first convert #1 (saveBox in your case) into a control sequence (\saveBox here), the apply \def. So it's become the same as doing

\def\saveBox{<what was inbetween \csname \endcsname>}

This is a useful-enough thing to do, but in the current case isn't actually what you want!

The second line

\expandafter\newsavebox\@nameuse{saveBox}

is going to fail badly for several reasons, as comments already mention. \@nameuse is a wrapper around \csname ...\endcsname, and so with only one expansion you get to

\newsavebox\csname saveBox\endscsname

which tries to define \csname as a new box!. If we do two expansions we get to

\newsavebox\saveBox

which is still wrong as you've already defined \saveBox as a macro.

Based on the linked question, what you seem to want is more of the form

\def\my@name{MyString\@alph\count@}
\expandafter\newsavebox\csname\my@name\endcsname
...
\expandafter\usebox\csname\my@name\endcsname

i.e. to save the form of the name in a macro and reuse that.

---

You may wonder what \@nameuse is for, as it seems to make things more not less complex (\@namedef is handy). This is at least partly historical: when LaTex2e was written, tokens were very 'expensive', so the saving of \@nameuse (one token) over \csname ...\endcsname (two tokens) was worth making when you look at all of the uses in the kernel. Today, that's much less important, although there is an argument that

\def\foo#1{% Some function
  ...
  \@nameuse{my@#1}%
}

is clearer than

\def\foo#1{% Some function
  ...
  \csname my@#1\endcsname
}

as it shows up that something is being used rather than say defined by name.

---

Using \@namedef is more common, as it allows easy construction of 'on the fly' names. For example, we can imagine storing some user input:

\newcommand{\mycommand}[2]{%
  \@namedef{mycommand@store@#1}{#2}%
}

This might then say be used to save some user information: I'll use e-mail addresses but it could be anything

\mycommand{Joe Bloggs}{joe.blogs@some.domain}

If you have a bigger set up doing this, say storing several pieces of data about several 'keys', then using \@namedef keeps things a lot more readable than using \expandafter\def\csname .... \mycommand{A. N. Other}{another@place.com}