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How can I set the width of a subfigure to fit its content, i. e. a tikzpicture.

In my MWE I do not want to specify 0.1textwidth respectively 0.9textwidth, but instead I want the whitespace to be distributed equally around both subfigures.

documentclassarticle
usepackageshowframe
usepackagetikz
usepackagesubcaption

begindocument

beginfigure[ht]
hfill
beginsubfigure[b]0.1textwidth
centering
begintikzpicture
node[draw] (pd1) a;
endtikzpicture
caption
endsubfigure%
hfill
beginsubfigure[b]0.9textwidth
centering
begintikzpicture
node[draw] (b) at (0,0) b;
node[draw] (c) at (5,0) c;

draw (b) to (c);
endtikzpicture
caption
endsubfigure%
hfill
caption
endfigure

enddocument

I found this answer How can I access the size of a tikzpicture's bounding box outside the tikzpicture? but didn't read it in detail until now. I was wondering if my aim could be achieved easier.

You could place the tikzpicture in a savebox and measure its size. This size can then be used for the subfigure. The following example shows how to do this for the right hand image. In the same way you could create another box for the other image.

documentclassarticle
usepackageshowframe
usepackagetikz
usepackagesubcaption

newsaveboxmybox
newlengthmylength

begindocument

saveboxmyboxbegintikzpicture
node[draw] (b) at (0,0) b;
node[draw] (c) at (5,0) c;

draw (b) to (c);
endtikzpicture

settowidthmylengthuseboxmybox

beginfigure[ht]
hfill
beginsubfigure[b]0.1textwidth
centering
begintikzpicture
node[draw] (pd1) a;
endtikzpicture
caption
endsubfigure%
hfill
beginsubfigure[b]mylength
centering
useboxmybox
caption
endsubfigure%
hfill
caption
endfigure

enddocument

I think that @samcarter's nice proposal is the way to go. This is only if you need to access the nodes later (with remember picture), in which case saveboxes may be inconvenient. This proposal measures the widths of the tikzpictures and writes them to the aux files such that they get used in the next run. Conceptually it is the same as e.g. this nice answer.

documentclassarticle
usepackageshowframe
usepackagetikz
usetikzlibrarycalc
usepackagesubcaption
ifdefinedfigwidthA
else
xdeffigwidthA2cm
fi
ifdefinedfigwidthB
else
xdeffigwidthB2cm
fi
tikzsetsave tikzpic width in/.style=execute at end picture=
path let p1=($(current bounding box.east)-(current bounding box.west)$)
in pgfextraxdef#1x1;
begindocument

beginfigure[ht]
hfill
beginsubfigure[b]figwidthA
centering
begintikzpicture[save tikzpic width in=figwidthA]
node[draw] (pd1) a;
endtikzpicture
caption
endsubfigure%
hfill%
beginsubfigure[b]figwidthB
centering
begintikzpicture[save tikzpic width in=figwidthB]
node[draw] (b) at (0,0) b;
node[draw] (c) at (5,0) c;

draw (b) to (c);
endtikzpicture
caption
endsubfigure%
hfillvphantomx
caption
endfigure
makeatletter 
immediatewrite@mainauxxdefstringfigwidthAfigwidthArelax 
immediatewrite@mainauxxdefstringfigwidthBfigwidthBrelax 
makeatother 
enddocument