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Updated Implementation of Futures for C++

Sunday, 11 May 2008

I have updated my prototype futures library implementation in light of various comments received, and my own thoughts.

The new version is available for download, again under the Boost Software License. It still needs to be compiled against the Boost Subversion Trunk, as it uses the Boost Exception library, which is not available in an official boost release.

Sample usage can be seen in the test harness. The support for alternative allocators is still missing.


  • I have removed the try_get/timed_get functions, as they can be replaced with a combination of wait() or timed_wait() and get(), and they don't work with unique_future<R&> or unique_future<void>.
  • I've also removed the move() functions on unique_future. Instead, get() returns an rvalue-reference to allow moving in those types with move support. Yes, if you call get() twice on a movable type then the second get() returns an empty shell of an object, but I don't really think that's a problem: if you want to call get() multiple times, use a shared_future. I've implemented this with both rvalue-references and the boost.thread move emulation, so you can have a unique_future<boost::thread> if necessary. test_unique_future_for_move_only_udt() in test_futures.cpp shows this in action with a user-defined movable-only type X.
  • Finally, I've added a set_wait_callback() function to both promise and packaged_task. This allows for lazy-futures which don't actually run the operation to generate the value until the value is needed: no threading required. It also allows for a thread pool to do task stealing if a pool thread waits for a task that's not started yet. The callbacks must be thread-safe as they are potentially called from many waiting threads simultaneously. At the moment, I've specified the callbacks as taking a non-const reference to the promise or packaged_task for which they are set, but I'm open to just making them be any callable function, and leaving it up to the user to call bind() to do that.

I've left the wait operations as wait() and timed_wait(), but I've had a suggestion to use wait()/wait_for()/wait_until(), which I'm actively considering.

Please download this prototype, put it through its paces, and let me know what you think.

Posted by Anthony Williams
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1 Comment

Hi Anthony,

Your implementation of futures is very interesting. For some of us, additional examples are needed, to make use of it.

In your test examples is always used callback method without parameters. e.g. pi.set_wait_callback(wait_callback);

where method wait_callback() is defined as follows: void wait_callback(jss::promise<int>& pi) { //... }

=> Can you please show how to pass parameters to callback method so that these values could be used in calculation (processing) that happens within callback method?

=> Can you please show how to make future callback method as a class member?

Thank you in advance.

Best regards Nenad

by Nenad at 15:00:33 on Monday, 21 January 2019

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