Replace undefined reference ?? with custom text?

Question

I'm using luatex and the cleverref package while writing a large document. I often only want to compile only parts of it, but I'd like my output to be a little nicer when I only compile a single chapter.

Instead of an undefined reference being replaced with ??, I would like to insert my own custom text.

Currently my solution is to define new commands \ucref and \ucite and use them instead of \cite and \cref for any cross-chapter reference. When I compile only a specific chapter I define these commands to be:

\newif\ifpublishversion{}
\publishversionfalse{}
%\publishversiontrue{}

\ifpublishversion{}        
    \newcommand{\ucref}[1]{\color{red} ref{(#1)} \color{black}}
    \newcommand{\UCref}[1]{\color{red} ref{(#1)} \color{black}}
    \newcommand{\ucite}[1]{[\color{green}#1\color{black}]}
\else
    \newcommand{\ucref}[1]{\cref{#1}}
    \newcommand{\UCref}[1]{\Cref{#1}}
    \newcommand{\ucite}[1]{\cite{#1}}
\fi

And then when I compile the whole document I change them to point to \cref and \cite to get default behavior.

It would be nice if I could just use \cref and \cite everywhere, but when a reference is undefined have latex spit out a warning and then insert this custom "nice" text instead of a ?? double question mark.

Is there a way to write something like ...

% PSEUDOCODE
% Do ways to do something like this exist?
\setundefinedreftext[1]{\ucref{#1}}
\setundefinedcitetext[1]{\ucite{#1}}

Answer 1

As I mentioned in the comments, it is impossible to suggest how to patch your \cite command because we don't know how you are 'doing' your bibliography stuff. But the cref stuff can be fairly easy (depending on what parts of the package you are using). For example:

\documentclass{article}
\parindent 0pt
\usepackage[T1]{fontenc}
\usepackage{etoolbox, xcolor, cleveref}

\newcommand*{\undeffmt}[1]{\textcolor{red}{ref (#1)}}%
\newcommand*{\patchsuccess}[1]{\typeout{Etoolbox patch: Success for \string#1}}
\newcommand*{\patchfailure}[1]{\typeout{Etoolbox patch: Failure for \string#1}}

\makeatletter

\patchcmd{\@setcref}%
  {\nfss@text{\reset@font\bfseries ??}}
  {\nfss@text{\reset@font\undeffmt{#1}}}
  {\patchsuccess{\@setcref}}
  {\patchfailure{\@setcref}}

\patchcmd{\@setcrefrange}%
  {\nfss@text{\reset@font\bfseries ??}}%
  {\nfss@text{\reset@font\undeffmt{#1}}}
  {\patchsuccess{\@setcrefrange}}
  {\patchfailure{\@setcrefrange}}
\patchcmd{\@setcrefrange}%
  {\nfss@text{\reset@font\bfseries ??}}%
  {\nfss@text{\reset@font\undeffmt{#2}}}
  {\patchsuccess{\@setcrefrange}}
  {\patchfailure{\@setcrefrange}}
\patchcmd{\@setcrefrange}%
  {\nfss@text{\reset@font\bfseries ??}}%
  {\nfss@text{\reset@font\undeffmt{#1}}}
  {\patchsuccess{\@setcrefrange}}
  {\patchfailure{\@setcrefrange}}
\patchcmd{\@setcrefrange}%
  {\nfss@text{\reset@font\bfseries ??}}%
  {\nfss@text{\reset@font\undeffmt{#2}}}
  {\patchsuccess{\@setcrefrange}}
  {\patchfailure{\@setcrefrange}}

\patchcmd{\@setnamecref}%
  {\nfss@text{\reset@font\bfseries ??}}%
  {\nfss@text{\reset@font\undeffmt{#1}}}
  {\patchsuccess{\@setnamecref}}
  {\patchfailure{\@setnamecref}}

\patchcmd{\@setcpageref}%
  {\nfss@text{\reset@font\bfseries ??}}%
  {\nfss@text{\reset@font\undeffmt{#1}}}
  {\patchsuccess{\@setcpageref}}
  {\patchfailure{\@setcpageref}}

\patchcmd{\@setcpagerefrange}%
  {\nfss@text{\reset@font\bfseries ??}}%
  {\nfss@text{\reset@font\undeffmt{#1}}}
  {\patchsuccess{\@setcpagerefrange}}
  {\patchfailure{\@setcpagerefrange}}
\patchcmd{\@setcpagerefrange}%
  {\nfss@text{\reset@font\bfseries ??}}%
  {\nfss@text{\reset@font\undeffmt{#2}}}
  {\patchsuccess{\@setcpagerefrange}}
  {\patchfailure{\@setcpagerefrange}}
\patchcmd{\@setcpagerefrange}%
  {\nfss@text{\reset@font\bfseries ??}}%
  {\nfss@text{\reset@font\undeffmt{#1}}}
  {\patchsuccess{\@setcpagerefrange}}
  {\patchfailure{\@setcpagerefrange}}
\patchcmd{\@setcpagerefrange}%
  {\nfss@text{\reset@font\bfseries ??}}%
  {\nfss@text{\reset@font\undeffmt{#2}}}
  {\patchsuccess{\@setcpagerefrange}}
  {\patchfailure{\@setcpagerefrange}}

\makeatother

\begin{document}

\section{One}

defined cref:   \cref{aaa}   \par
undefined cref: \cref{undef} \par
\bigskip

defined crefrange:        \crefrange{aaa}{ccc} \par
1st undefined crefrange:  \crefrange{und}{ccc} \par
2nd undefined crefrange:  \crefrange{aaa}{und} \par
both undefined crefrange: \crefrange{un1}{un2} \par
\bigskip

defined cpageref:   \cpageref{aaa} \par
undefined cpageref: \cpageref{bbb} \par
\bigskip

defined cpagerefrange:        \cpagerefrange{aaa}{ccc} \par
1st undefined cpagerefrange:  \cpagerefrange{und}{ccc} \par
2nd undefined cpagerefrange:  \cpagerefrange{aaa}{und} \par
both undefined cpagerefrange: \cpagerefrange{unf1}{un2} \par
\bigskip

defined nameref:   \namecref{aaa} \par
undefined nameref: \namecref{und} \par


\newpage
\section{Two}

\label{aaa}%
% \label{bbb}%
It would be nice if I could just use \Cref{ccc} and cite everywhere,
but when a reference is undefined have latex spit out a warning and
then insert this custom ``nice'' (e.g., \Cref{bbb}) text instead of a
??  double question mark.

\newpage
\section{Three}
\label{ccc}

\end{document}

As you can see, it is set up so that you define your desired string in the definition for \undeffmt. (I tried to keep it similar to your original example, but I'd probably choose something different if it were up to me.)

Finally, if this does not suit your needs, this is most likely because a question of this sort really needs a 'minimal working example' so others can properly see what your implicit choices and assumptions are.