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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.

documentclass{article}

usepackage{showframe}

usepackage{tikz}

usepackage{subcaption}



begin{document}



begin{figure}[ht]

hfill

begin{subfigure}[b]{0.1textwidth}

centering

begin{tikzpicture}

node[draw] (pd1) {a};

end{tikzpicture}

caption{}

end{subfigure}%

hfill

begin{subfigure}[b]{0.9textwidth}

centering

begin{tikzpicture}

node[draw] (b) at (0,0) {b};

node[draw] (c) at (5,0) {c};



draw (b) to (c);

end{tikzpicture}

caption{}

end{subfigure}%

hfill

caption{}

end{figure}



end{document}

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.

documentclass{article}

usepackage{showframe}

usepackage{tikz}

usepackage{subcaption}



newsavebox{mybox}

newlength{mylength}



begin{document}



savebox{mybox}{begin{tikzpicture}

node[draw] (b) at (0,0) {b};

node[draw] (c) at (5,0) {c};



draw (b) to (c);

end{tikzpicture}}



settowidth{mylength}{usebox{mybox}}



begin{figure}[ht]

hfill

begin{subfigure}[b]{0.1textwidth}

centering

begin{tikzpicture}

node[draw] (pd1) {a};

end{tikzpicture}

caption{}

end{subfigure}%

hfill

begin{subfigure}[b]{mylength}

centering

usebox{mybox}

caption{}

end{subfigure}%

hfill

caption{}

end{figure}



end{document}

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.

documentclass{article}

usepackage{showframe}

usepackage{tikz}

usetikzlibrary{calc}

usepackage{subcaption}

ifdefinedfigwidthA

else

xdeffigwidthA{2cm}

fi

ifdefinedfigwidthB

else

xdeffigwidthB{2cm}

fi

tikzset{save tikzpic width in/.style={execute at end picture={

path let p1=($(current bounding box.east)-(current bounding box.west)$)

in pgfextra{xdef#1{x1}};}}}

begin{document}



begin{figure}[ht]

hfill

begin{subfigure}[b]{figwidthA}

centering

begin{tikzpicture}[save tikzpic width in=figwidthA]

node[draw] (pd1) {a};

end{tikzpicture}

caption{}

end{subfigure}%

hfill%

begin{subfigure}[b]{figwidthB}

centering

begin{tikzpicture}[save tikzpic width in=figwidthB]

node[draw] (b) at (0,0) {b};

node[draw] (c) at (5,0) {c};



draw (b) to (c);

end{tikzpicture}

caption{}

end{subfigure}%

hfill{vphantom{x}}

caption{}

end{figure}

makeatletter 

immediatewrite@mainaux{xdefstringfigwidthA{figwidthA}relax} 

immediatewrite@mainaux{xdefstringfigwidthB{figwidthB}relax} 

makeatother 

end{document}