boxeslengthstex-core
One can specify a box in LaTeX, using various methods for example:
\newsavebox{\Abox}
\savebox{\Abox}{\parbox{4cm}{\lorem}}
\usebox{\Abox}
How can I retrieve the width, height and depth of the box. TeX has the primitives \wd, \ht and \dp. However in order for these to be used it is necessary to know the number of the box for example \box13.
You can't \unhcopy an hbox in math mode, that's simple! Indeed
$\unhcopy\strutbox$
gives
! Incompatible list can't be unboxed.
<*> $\unhcopy\strutbox
$
The command \unhcopy is used outside of math mode because of two facts:
A similar definition is that of \leavevmode:
\def\leavevmode{\unhbox\voidb@x}
where \voidb@x is a box register that should always remain void. The difference between \unhbox and \unhcopy is that the former destroys the contents of the box register, while the latter produces only a copy of it and leaves the register intact.
Among the duties of \fontsize is to provide a convenient \strut; the strut's height and depth are always 7/10 and 3/10 of the baselineskip, which is set by the second argument to \fontsize. So when a command such as \large is executed, also the strut is changed; it will restored when the group ends or another \fontsize command (possibly implicit) is executed.
(For the rest you're right.)
The register \voidb@x can be used to ensure that a box register is void, by saying \setbox\mybox=\box\voidb@x, which destroys the contents of \mybox and is more efficient than the equivalent {\setbox0=\box\mybox}.
If you only have regular text in the box then TikZ can measure it too, which is what you are trying to do manually anyways. It just uses its own temporary box to measure. But \textbf etc. stuff is not so easy to use. So use it with caution.
\begin{tikzpicture}
\pgfmathsetmacro\mylen{width("some text")}
\node[anchor=west,inner sep=0] {some text};
\draw[red,thick] (0,0) -- (\mylen pt,0);
\end{tikzpicture}
Oh and height works too.
Best Answer
You really shouldn't be referring to boxes by number even with the plain format. For example,
will work fine. Both
\newboxand\newsaveboxdo a\chardefto assign the name you give as a number. So in LaTeX you can happily doand all will be fine.