\pnum
\indextext{\idxcode{struct}!\tcode{class} versus}%
\indextext{structure}%
\indextext{\idxcode{union}!\tcode{class} versus}%
A \defn{union} is a class defined with the \grammarterm{class-key}
\tcode{union};
\indextext{access control!\idxcode{union} default member}%
it holds at most one data member at a time\iref{class.union}.
\begin{note}
Aggregates of class type are described in~\ref{dcl.init.aggr}.
\end{note}