How to access the label text of a node?

Question

What way do you recommend to access the

• the label text

of a named node later in the same tikzpicture?

Small example:

\begin{tikzpicture}   
    \node (curve0) at (0,0) [] {curve $C_0$}; 
    \node (alaternodeinwhichlabeltextofcurveoughttobeused) at (0,2) { The following is a label text of a node we already defined: (curve0.label text) };
\end{tikzpicture}

where, of course, the above use of "(curve0.label text)" is not permissible, but in analogy to existing features like "(curve0.center)".

This must be very usual, but is escaping my reading of the literature.

Feel free to correct if "label text" is not the correct technical term for this particular entity (I am taking it from a certain error message produced by tikz).

Edit: Like recommended in the comments, a workaround is to use a separate macro. For example:

\begin{tikzpicture}   
    \newcommand{\labeltextofcurve0}{curve $C_0$};
    \node (curve0) at (0,0) [] {\labeltextofcurve0}; 
    \node (alaternodeinwhichlabeltextofcurve0oughttobeused) at (0,2) { An isotopy from \labeltextofcurve0 to the curve $C_\infty$ is the following: ... };
\end{tikzpicture}

However, there are at least two issues with this:

• Quantitative: this solution requires one separate line of code for each label text. (In the problem motivating this question, there are many nodes, each of whose label text I would like to access. Imagine (curve0), (curve1), ... , (curve 17), and having one separate \newcommand{\labeltextof...} for each...) It appears more natural to define the label text in the content of the node.

• Qualitative: there is a decision to be made where to place the \newcommmand{\labeltextofcurve0}. Sure, immediately before the definition of (curve0) seems a canonical choice, but there are issues with 'scopes', that appear too complicated and cofusing to describe here.

Therefore, wouldn't it be better to have a way to say (curve0.label text), and have it work whenever (curve0) is defined somewhere else in the code?

An opinion on whether using the additional

  \newcommand{\labeltextofcurve0}{curve $C_0$};

is to be preferred over trying to get a "(curve0.label text)"-feature would be appreciated.

Answer 1

You can hack into the TikZ code and just capture the node content in a macro.

\documentclass{article}
\usepackage{tikz}
\makeatletter
\protected\def\tikz@fig@main#1{%
  \expandafter\gdef\csname labeltextof@\tikz@fig@name\endcsname{#1}%
  \iftikz@node@is@pic%
    \tikz@node@is@picfalse%
    \tikz@subpicture@handle{#1}%
  \else%
    \tikz@@fig@main#1\egroup%
  \fi}
\makeatother

\newcommand\labeltextof[1]{\csname labeltextof@#1\endcsname}
\begin{document}

\begin{tikzpicture}
  \node (curve0) at (0,0) [] {curve $C_0$}; 
    \node (alaternodeinwhichlabeltextofcurve0oughttobeused) at (0,2) {
      An isotopy from \labeltextof{curve0} to the curve
      $C_\infty$ is the following: ... };
\end{tikzpicture}

\end{document}

Answer 2

TikZ, when parsing the node syntax, gets the brace pair content puts it in a hbox or a minipage and forgets it. So it will not remember the contents. This remembering is tough because it can be anything including tables or images. Thus assigning to a key is by no means that straightforward.

But you can use macros as comments and Henri's answer mentioned or using node contents,name and at keys to define the node.

\documentclass{article}
\usepackage{tikz}

\tikzset{nodelist/.is family,
nadd/.code 2 args= {%
    \tikzset{node contents=#1,name=#2}%
    \begingroup\globaldefs=1\relax%
    \pgfkeyssetvalue{/nodelist/#2}{#1}%
    \endgroup%
  }
}
\begin{document}    
\begin{tikzpicture}[]
  \node[nadd={My text}{a},at={(4,2)}]; % Add this to "my remembered nodes list"
  \node[draw] at (1,1) {\pgfkeysvalueof{/nodelist/a}};
\end{tikzpicture}
\end{document}

Note however, if you use node contents key then you won't use the brace syntax for the contents and TikZ uses a different way lloking for arguments. Hence you get a little different behavior. But still in my opinion, this is not as productive as you would think in the long run.