Read other pgfkeys' value

Question

I've spend a couple of days trying to understand how the pgfkeys work. As an example, I'd like to read a key's value and then use it to set another key.

In this case, I want to set a node's label to the same content as its text:

% attempt 1
\pgfkeys{/tikz/labeltext/.style={label=\pgfkeysvalueof{/tikz/node contents}}}

% attempt 2
\pgfkeys{/tikz/node contents/.store in=\nc}
\pgfkeys{/tikz/labeltext/.style={label=\nc}}

% attempt 3
\pgfkeys{/tikz/labeltext/.code={\pgfkeyssetvalue{\pgfkeysvalueof{/tikz/node contents}}}}

\begin{tikzpicture}

    \node [labeltext] {test};

\end{tikzpicture}

What I expect to get is

but nothing happens.

Answer 1

The normal content of a node can't be retrieve from a key. The special node contents key uses some code to store its argument in \tikz@node@content macro (line 896 of tikz.code.tex file).

You may use:

\documentclass[tikz]{standalone}
\makeatletter
\tikzset{
  labeltext/.code={\tikzset{label/.expand once=\tikz@node@content}},
}
\makeatother
\begin{document}
\begin{tikzpicture}
  \node[node contents=test,labeltext];
\end{tikzpicture}
\end{document}

But a simpler solution is:

\documentclass[tikz]{standalone}
\tikzset{
  labeltext/.style={node contents={#1},label={#1}},
}
\begin{document}
\begin{tikzpicture}
  \node[labeltext=test];
\end{tikzpicture}
\end{document}