Referencing to subfigures in main caption

Question

I'm trying to reference to subfigures in the main caption of my figure. Unfortunately I'm only getting blanks, not even undefined referencing ??. What's going on?

\documentclass{report}

\usepackage[superscript]{cite}
\usepackage{mathtools}
\usepackage{graphicx}
\usepackage{caption}
\usepackage{subcaption}

\begin{document}
Some text.

\begin{figure}[!h]
    \centering
    \begin{subfigure}[b]{0.4\textwidth}
            \centering
            \includegraphics[width=\textwidth]{figure1}
    \label{fig:fig1}
    \end{subfigure}
\begin{subfigure}[b]{0.4\textwidth}
            \centering
            \includegraphics[width=\textwidth]{figure2}
    \label{fig:fig2}
    \end{subfigure}
    \caption{\protect\subref{fig:fig1} shows figure 1 and \protect\subref{fig:fig2} shows figure 2.}
\end{figure}
\end{document}

Answer 1

You need to add a separate \caption for each subfigure in order to display the label (which can then be referenced):

\documentclass{article}
\usepackage{graphicx}% http://ctan.org/pkg/graphicx
\usepackage{caption,subcaption}% http://ctan.org/pkg/{caption,subcaption}

\begin{document}
Some text.

\begin{figure}[ht]
  \centering
  \begin{subfigure}[b]{0.5\linewidth}
    \centering\includegraphics[width=100pt]{example-image-a}
    \caption{\label{fig:fig1}}
  \end{subfigure}%
  \begin{subfigure}[b]{0.5\linewidth}
    \centering\includegraphics[width=100pt]{example-image-b}
    \caption{\label{fig:fig2}}
  \end{subfigure}
  \caption{\subref{fig:fig1} shows Figure~1 and~\subref{fig:fig2} shows Figure~2.}
\end{figure}
\end{document}

Answer 2

Taken from the subcaption documentation, section "Referencing sub-figures without sub-captions":

If you don’t want to give a sub-figure a caption, because the picture itself already contains the caption, or for some other reason, you can use the command \phantomsubcaption instead of \subcaption, or – when inside a subfigure or subtable environ- ment – \phantomcaption instead of \caption. \phantomsubcaption and \phantomcaption do not have any arguments, and they do not generate any output, but give you an anchor for a \label command which can be placed afterwards. Fur- thermore it increases the sub-figure resp. sub-table counter. Please note that – just like \subcaption – the \phantomsubcaption command must be applied inside its own group, box, or environment.

\documentclass{report}

\usepackage[superscript]{cite}
\usepackage{mathtools}
\usepackage[demo]{graphicx}
\usepackage{caption}
\usepackage{subcaption}

\begin{document}
Some text.

\begin{figure}[!h]
    \centering
    \begin{subfigure}[b]{0.4\textwidth}
            \centering
            \includegraphics[width=\textwidth]{figure1}
    \phantomcaption
    \label{fig:fig1}
    \end{subfigure}
    \begin{subfigure}[b]{0.4\textwidth}
            \centering
            \includegraphics[width=\textwidth]{figure2}
    \phantomcaption
    \label{fig:fig2}
    \end{subfigure}
    \caption{\subref{fig:fig1} shows figure 1 and \subref{fig:fig2} shows figure 2.}
\end{figure}
\end{document}

Note: \phantomsubcaption needs subcaption version 1.1 (2011/08/17) or newer.