Report the offset of an input stream error

[gennaroprota]

archive_exception said only that the input failed, without giving a clue as to where the error occurred. It now names the offset at which the input went wrong, for text, binary and XML input archives alike.

[robertramey]

Note that the archive interface doesn't presume that an archive is a type of io stream. This opens the way to certain special purpose archives which might do something like implement a deep copy, calculate a CRC, etc, etc, There might be an example of such a custom archive in the documentation. Consider this when adding to stream behavior.

[gennaroprota]

Hmm, thanks for the heads-up. However, the archive interface is untouched: basic_iarchive, common_iarchive, interface_iarchive and archive_exception are all unchanged. The offset is produced in three places that already hold a stream or a stream buffer of their own:

• basic_binary_iprimitive has std::basic_streambuf<Elem, Tr> & m_sb as a member.
• basic_text_iprimitive<IStream> already needs a real stream for ios_flags_saver and basic_istream_locale_saver.
• basic_xml_iarchive already needs get_is(), and the grammar calls get(), fail() and eof() on it. The two implementations return std::istream & and std::wistream &.

demo_trivial_archive.cpp, the example archive_reference.html points at, has a save_binary and no stream at all, and includes none of these headers. A CRC or deep copy archive would never reach this code, and if it threw archive_exception it would get the message it always got.

But one point is worth mentioning: an archive deriving from basic_xml_iarchive whose get_is() returned something stream-like but not a std::basic_istream would now fail to compile, since the offset comes from get_is().rdbuf(). Both in-tree implementations return real streams but, in general, this might break some code. What do you think?