-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 On Wednesday 28 May 2008 05:55 am, Johan Torp wrote:
Do you propose to remove reference counting from futures and use shared_ptr<future<..>> instead?
No, the mentions I made of shared_ptr were not important. I was only trying to convey that promise::has_future could be easily implemented by imagining a typical implementation of promise/future where they both are implemented by having a shared_ptr to some common object.
I believe the main motivation for a unique_future was to be able to move large/expensive types in the get-function instead of copying them.
shared_future<R> requires it's client to copy the data to move it into their own data structures. It has the following signature: R const & get() const;
unique_future<R> moves the value out of the future and is "consumed" once get() has been called. It has the following signature: R&& get();
I don't like having both shared_future and unique_future either. One could perhaps distinguish between move-get and const_ref-get somehow and only allow the former if use_count == 1. One problem is that the promise typically belongs to some other thread and new futures can be created concurrently after having queried use_count. Another option is declaring whether the future is unique or shared as a parameter in the futures constuctor. An exception could be thrown if you try to create more futures from a uniquely "owned" promise.
Not distuinguishing between unique and shared might remove some type safety and clarity in user code. Some of the N2627 future proposal authors can probably provide more insight.
This seems unrelated to what I was trying to get at with promise::has_future. The idea is a promise-fulfilling thread can query promise::has_future if it likes, and choose to exit early if there are no futures out there that might be interested in its result value. Or, if the promise-fulfilling thread is intended to cause some side-effects independent of setting futures, it can proceed and ignore promise::has_future. -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.6 (GNU/Linux) iD8DBQFIPVTC5vihyNWuA4URAnD6AJ9aIPeYkU59JV5i/XOAlN65rV3w2wCfbJs9 XjYN/k3k3HaOXslKjl24XCw= =YQS7 -----END PGP SIGNATURE-----