I recently discovered that the Boost.Thread library implements some
extensions to boost::future from the 'Technical Specification for C++
Extensions for Concurrency' [1]. The std::experimental interfaces are
documented on cppreference.com [2], but neither gcc nor clang provide them.
The coolest new feature is future::then(), which takes a continuation to
be called once the future is ready, and returns a future for the
continuation's result. This allows futures to be composed into
higher-level asynchronous operations - an ability that is sorely lacking
in the c++11 std::future.
While these extensions are not enabled in boost by default, they can be
turned on with a series of BOOST_THREAD_PROVIDES_FUTURE* defines (since
boost 1.57). The boost documentation for 'Conformance and Extension'
[3]describes the status of this implementation, listing some features as
'partial'. I wrote a set of unit tests to explore this, and was happy
with the results. The only unfortunate piece missing is 'implicit
unwrapping' for future::then() - if you pass it a continuation that
returns a future<T>, then() will return a future<future<T>>. This
-should- be implicitly converted to future<T>, but boost::future
requires you to do it explicitly by calling future::unwrap().
I'm curious to see how people feel about this. Would you consider using
boost::futures? Are there concerns about using boost extensions? About
slight changes to interfaces? I opened a pull request [4] for discussion
- take a look at the unit tests for examples, and let me know what you
think.
Thanks,
Casey
[1] http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/p0159r0.html
[2] http://en.cppreference.com/w/cpp/experimental/concurrency
[3]
http://www.boost.org/doc/libs/1_63_0/doc/html/thread/compliance.html#thread.compliance.cxx1y
[4] https://github.com/ceph/ceph/pull/14987
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