LaTeX等号中间加问号

\documentclass{ctexart}
\usepackage{amsmath,amssymb}
\usepackage{mathtools}

\makeatletter
\newcommand\fhcd[3][bin]{
\csname math#1\endcsname{\mathchoice
{\@fhcd\displaystyle{#2}{#3}}
{\@fhcd\textstyle{#2}{#3}}
{\@fhcd\scriptstyle{#2}{#3}}
{\@fhcd\scriptscriptstyle{#2}{#3}}}}
\newcommand\@fhcd[3]{
\settowidth\@tempdima{$\m@th#1{#2}$}
\settowidth\@tempdimb{$\m@th#1{#3}$}
\ifdim\@tempdimb>\@tempdima\@tempdima\@tempdimb\fi
\kern.5\@tempdima\mathclap{#2}\mathclap{#3}\kern.5\@tempdima}
\makeatother

%自定义 等号与问号重叠
\newcommand\dengwen{\fhcd[bin] ? =}

\begin{document}

例如 $f \dengwen g=h$?
\begin{equation*}
  A \dengwen B
\end{equation*}


\end{document}

%%\usepackage{extarrows}：
$$ A \xlongequal{\quad\quad}B $$
$$ A\xlongequal[sub-script]{super-script}B $$