Convert :class:`~ctfile.ctfile.CTfile` into JSON string.

        :return: ``JSON`` formatted string.
        :rtype: :py:class:`str`.

def _to_json(self, sort_keys=False, indent=4):
        """Convert :class:`~ctfile.ctfile.CTfile` into JSON string.

        :return: ``JSON`` formatted string.
        :rtype: :py:class:`str`.
        """
        return json.dumps(self, sort_keys=sort_keys, indent=indent, cls=CtabAtomBondEncoder)