How can I change text, depending on whether it is \scshape or not?

Question

What would be the simplest way to create a macro like \ifscshape{sc case}{else} (similar to e.g. \ifdraft{}{}), so that this

\newcommand{\cec}{\ifscshape{CEC}{\textsc{Cec}}}

In normal text, the acronym \cec{} should be printed in small caps.

\textsc{But if surrounded by small caps, \cec{} should be printed in uppercase.}

would output something like this

similarly to how \emph interacts with italics?

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Edit:

I tried to use \scacronym (an adaptation from @TiMauzi's macro in their answer), which works like a charm in regular text, but breaks inside titles.

MWE

\documentclass{scrbook}

\usepackage{microtype,xspace,libertine}

\setkomafont{disposition}{\normalcolor\rmfamily}
\addtokomafont{chapter}{\scshape\lsstyle}

\usepackage{ifthen}
\makeatletter
\newcommand{\scacronym}[1]{%
    \ifthenelse{\equal{\f@shape}{sc}}{\uppercase{#1}}{\textsc{#1}}\xspace}
\makeatother

\begin{document}
    \chapter{Shapes \& Types of \scacronym{Cec}}
    
    In normal text, the acronym \scacronym{Cec} should be printed in small caps.\\
    \textsc{But if surrounded by small caps, \scacronym{Cec} should be printed in uppercase.}
\end{document}

When I try to compile this, I get a huge number of similar errors, some shown below. What's happening here and how can I fix this?

---

Edit 2:

@TiMauzi's updated solution using etoolbox solved above issue:

However, if this macro should apply in the TOC entries and headers/footers, it's better to define it using \DeclareRobustCommand instead of \newcommand.

\makeatletter
\DeclareRobustCommand{\scacronym}[1]{%
    \ifdefstring{\f@shape}{sc}{\uppercase{#1}}{\textsc{#1}}\xspace}
\makeatother

Answer 1

Please take a look at the following MWE:

\documentclass{scrartcl}

\usepackage{ifthen}

\makeatletter
\newcommand{\ifscshape}[2]{%
    \ifthenelse{\equal{\f@shape}{sc}}{#1}{#2}%
}
\makeatother

\newcommand{\cec}{\ifscshape{\uppercase{Cec}}{\textsc{Cec}}}

\begin{document}
In normal text, the acronym \cec{} should be printed in small caps.

\textsc{But if surrounded by small caps, \cec{} should be printed in uppercase.}
\end{document}

I used the package ifthen here, comparing \f@shape (with the current font shape as output) to the string sc by using the \equal command. Within your text, you should put {} after the \cec command, so a space is placed after your emphasized text.

Note: Within the command \ifscshape, I changed your original first argument CEC to \uppercase{Cec} for better presentation and distinction from \textsc. It works either way, though.

Edit 1:

If the command should be working within sectioning commands, the usage of the package etoolbox is the better alternative:

\documentclass{scrartcl}

\usepackage{etoolbox}

\makeatletter
\newcommand{\ifscshape}[2]{%
    \ifdefstring{\f@shape}{sc}{#1}{#2}%
}
\makeatother

\newcommand{\cec}{\ifscshape{CEC}{\textsc{Cec}}}

\begin{document}
\section{The Acronym \cec}
This is the section's content using \cec{} or \textsc{also \cec}.
\end{document}

Instead of \ifthenelse, the command \ifdefstring is used, comparing the content of a command to a given string. In general, this approach seems to be more stable than the approach with the \ifthen package. Of course, a small caps variant should be supported by the used section font.

Edit 2:

Adjusting the second solution to the second MWE of @schoekling, the following should work:

\documentclass{scrbook}

\usepackage{microtype,xspace,libertine}

\setkomafont{disposition}{\normalcolor\rmfamily}
\addtokomafont{chapter}{\scshape\lsstyle}

\usepackage{etoolbox}
\makeatletter
\newcommand{\scacronym}[1]{%
    \ifdefstring{\f@shape}{sc}{\uppercase{#1}}{\textsc{#1}}\xspace}
\makeatother

\begin{document}
    \chapter{Shapes \& Types of \scacronym{Cec}}
    
    In normal text, the acronym \scacronym{Cec} should be printed in small caps.\\
    \textsc{But if surrounded by small caps, \scacronym{Cec} should be printed in uppercase.}
\end{document}

Note: This approach seems to work if the document is compiled with LuaLaTeX, but not with e.g. XeLaTeX or PDFLaTeX.