Myujth

Using savebox within a tikzpicture environment seems wipe out the content of the savebox.
In the MWE below, the following macros define, measure and display the size of a savebox:

newcommand*DefineBoxAndItSize%
 saveboxtempboxAHg%
 %% https://tex.stackexchange.com/a/11945/4301
 setlengthSizeOfBoxdimexprhttempboxA+dptempboxArelax%
%
newcommand*ShowBoxAndSize%
 Box fboxuseboxtempboxA is printlengthSizeOfBox%

As long as DefineBoxAndItSize is invoked outside of a tikzpicture, using ShowBoxAndSize both outside (first line of output) and inside a tikzpicture environment (second line of output) works fine.

However, the third line invokes DefineBoxAndItSize inside a tikzpicture before ShowBoxAndSize, which ends up making the box empty.

Question:

How can I define a savebox within a tikzpicture environment?

Code:

documentclass[10pt]article
usepackagetikz
usepackageprintlen

newsaveboxtempboxA
newlengthSizeOfBox

fboxsep=0pt

newcommand*DefineBoxAndItSize%
 saveboxtempboxAHg%
 %% https://tex.stackexchange.com/a/11945/4301
 setlengthSizeOfBoxdimexprhttempboxA+dptempboxArelax%
%
newcommand*ShowBoxAndSize%
 Box fboxuseboxtempboxA is printlengthSizeOfBox%


begindocument
 DefineBoxAndItSize%
 noindent
 Outside TikZ: ShowBoxAndSize.

 medskipnoindent
 begintikzpicture[inner sep=0pt, anchor=east]
 node at (0,0) Use inside TikZ: ShowBoxAndSize.;
 endtikzpicture%

 medskipnoindent
 begintikzpicture[inner sep=0pt, anchor=east]
 DefineBoxAndItSize% <--- What is wrong with this?

 node at (0,-1) Define inside TikZ: ShowBoxAndSize.;
 endtikzpicture%
enddocument

Background: TikZ happens to gobble text unless it is an argument of a node. Even though this is sort of well known, this also has less obvious implications. In particular, saveboxes may end up being empty. This has already lead to confusion here and here. In the latter post, the OP solves the issue by wrapping their macro in beginpgfinterruptpicture and endpgfinterruptpicture.

The next step of sophistication, which may or may not deserve a new answer, might then be to wrap the savebox definition in an beginpgfinterruptpicture and endpgfinterruptpicture only if the macro is called inside a tikzpicture environment. To this end, the tests of this post and this post are employed. (Let me remark that the IfInTikzPic is a bit unusual in that its first argument is the action that is to be taken if one is not in a tikzpicture environment. Whether or not this is related to the author driving on the other side of the road, I don't know. ;-)

documentclass[10pt]article
usepackagetikz
usepackageprintlen

newsaveboxtempboxA
newlengthSizeOfBox

fboxsep=0pt

makeatletter% from https://tex.stackexchange.com/a/458855/121799
newcommandIfInTikzPic% https://tex.stackexchange.com/a/121309/4301
 ifxpgfpictureid@undefined
 expandafter@firstoftwo
 else
 expandafter@secondoftwo
 fi

makeatother


newcommand*DefineBoxAndItSize%
IfInTikzPicsaveboxtempboxAHg%
% https://tex.stackexchange.com/a/11945/4301
setlengthSizeOfBoxdimexprhttempboxA+dptempboxArelax%
beginpgfinterruptpicture%
saveboxtempboxAHg%
% https://tex.stackexchange.com/a/11945/4301
setlengthSizeOfBoxdimexprhttempboxA+dptempboxArelax%
endpgfinterruptpicture
%
newcommand*ShowBoxAndSize%
 Box fboxuseboxtempboxA is printlengthSizeOfBox%


begindocument
 DefineBoxAndItSize%
 noindent
 Outside TikZ: ShowBoxAndSize.

 medskipnoindent
 begintikzpicture[inner sep=0pt, anchor=east]
 node at (0,0) Use inside TikZ: ShowBoxAndSize.;
 endtikzpicture%

 medskipnoindent
 begintikzpicture[inner sep=0pt, anchor=east]
 DefineBoxAndItSize% <--- What is wrong with this?

 node at (0,-1) Define inside TikZ: ShowBoxAndSize.;
 endtikzpicture%
enddocument

I'll be happy to remove this answer if people feel that this question is a duplicate and thus does not deserve a separate answer. (However, at the time of writing this, I am reputation capped, so the reason is certainly not to harvest reputation points, which it is in none of my posts.)

EDIT: Fixed a space, big thanks to Phelype Oleinik!