Make file_sink flushable

[Andrew Schweitzer <a.schweitzer.grps@…>]

file_sink is hard to use as a debug log, since it does not flush.

For example, when used when pushed into a filtering_ostream with cout, if flush is not called, characters don't appear on console or in file in a timely fashion. If flush is called, the characters on the console appear but not those in the file, because file_sink does not implement Flushable concept, and doesn't flush.

This code:

filtering_ostream fo;

file_sink fsink("out.txt"); tee_filter<file_sink> tee_fsink(fsink); fo.push(tee_fsink);

fo.push(cout);

fo << "some text"; fo.flush();

is very useful for arbitrary chains of debug logs, but not currently usable.

The same issue applies to file, file_descriptor, and file_descriptor_sink.

If there is some reason these classes should not be modified to made flushable, then it seems easy to make flushable_* versions. Example of this attached, which was copied from file.hpp.

I also tried to add flushable_file_descriptor, by privately inheriting from file_descriptor, and adding a flush call which calls ::FlushFileBuffers on handle()... but so far that doesn't seem to work, so not attaching.

Discussion at ​http://groups.google.com/group/boost-list/browse_thread/thread/4c07e7c255e8143a/b9003b1b83c5a4db?lnk=raot&pli=1

[Andrew Schweitzer <a.schweitzer.grps@…>]

sample implementation of flushable file_sink, copied from boost/iostreams/device/file_sink.hpp

[Steven Watanabe]

I don't think that file_descriptor_sink should be made flushable. It looks like FlushFileBuffers is the Windows equivalent of fsync, and I rather doubt that standard streams use it.

[Steven Watanabe]

(In [63039]) Make basic_file and basic_file_sync Flushable. Fixes #2998.