boost::thread and boost::promise
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Thread: boost::thread and boost::promise

  1. #1
    Join Date
    Dec 2010
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    boost::thread and boost::promise

    I have some question about
    Code:
    boost::promise
    After studying the website of boost and "C++ concurrency in action"
    I still can't get the idea of
    Code:
    boost::promise
    According to the book(if I have no make any mistake)
    boost::future is designed for one off event
    you could wrap the task by packaged_task and let boost::unique_future
    to gain the result of the task. When you launch the thread, you could
    just return the boost::unique_future and wait for the result of the task.
    After the task is finish, you could continue your job.
    It is very like boost::condition_variable but design for one off event.
    Do I make any mistake about it?

    Assume that I didn't make any mistake about the idea of future, I still can't
    get the idea of
    Code:
    boost::promise
    , could you show me an easier example of it?

    Thanks a lot

  2. #2
    Join Date
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    Netherlands
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    Re: boost::thread and boost::promise

    Quote Originally Posted by stereoMatching View Post
    According to the book(if I have no make any mistake)
    boost::future is designed for one off event
    you could wrap the task by packaged_task and let boost::unique_future
    to gain the result of the task. When you launch the thread, you could
    just return the boost::unique_future and wait for the result of the task.
    After the task is finish, you could continue your job.
    It is very like boost::condition_variable but design for one off event.
    Do I make any mistake about it?
    The difference between a future and a condition variable is that a future is a pull-driven mechanism and a condition variable is a push-driven mechanism. When you ask the value of a future, the call waits until the value of the future is set (from another thread). If the value was already set before the call, the function returns immediately. With a condition variable, you wait until it is notified. Whether a condition variable was notified before you call wait on it has no influence; you'll wait until it is notified again.
    Quote Originally Posted by stereoMatching View Post
    Assume that I didn't make any mistake about the idea of future, I still can't
    get the idea of
    Code:
    boost::promise
    , could you show me an easier example of it?
    See http://www.codeguru.com/forum/showthread.php?t=514204
    Cheers, D Drmmr

    Please put [code][/code] tags around your code to preserve indentation and make it more readable.

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  3. #3
    Join Date
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    77

    Re: boost::thread and boost::promise

    With a condition variable, you wait until it is notified. Whether a condition variable was notified before you call wait on it has no influence; you'll wait until it is notified again.
    Thanks, looks like I make a mistake again, that means we have to design carefully to make sure
    the condition_variable would be notify?

  4. #4
    Lindley is offline Elite Member Power Poster
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    Re: boost::thread and boost::promise

    I've always been a bit hazy about futures and promises too. Let me see if I have a handle on it now.

    It seems to me that it is similar to the thread-pool pattern, in that if a packaged_task is sent to a worker thread to be executed, then the future is the handle which can be used to retrieve the result.

    How does a promise fit into this framework?

  5. #5
    Join Date
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    Re: boost::thread and boost::promise

    Quote Originally Posted by Lindley View Post
    How does a promise fit into this framework?
    promise<T> is the provider of future<T>; the promise is managed by the "worker" thread, that sets its value or its exception; the future is managed by the "boss" thread that waits for the value (or the exception) of the future to be set.

    from another POV, you can see them as "splitting" a value T into its two asynchronous parts: set (=promise<T>) and retrieval(=future<T>); in this sense the framework exhausts with promises and futures.

    for example, the "asynchronous version" of

    Code:
    int main()
    {
    	int some_int = 42;
    
    	std::cout << some_int << std::endl;
    }
    is

    Code:
    int main()
    {
    	std::promise<int> 	int_promise;
    	std::future<int>	future_int = int_promise.get_future();
    
    	std::thread worker( [&]{ int_promise.set_value( 42 ); } );
    
    	std::cout << future_int.get() << std::endl;
    	worker.join();
    }
    finally, packaged tasks are just helpers used to automate the creation of the promise associated to the return value of an asynchronous task.

  6. #6
    Lindley is offline Elite Member Power Poster
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    Re: boost::thread and boost::promise

    Hmm. I suppose there's some value in creating those "primitives", but they seem fairly meaningless outside of the thread-pool pattern usage so far. Perhaps I just haven't encountered the right scenario to require them.

  7. #7
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    Re: boost::thread and boost::promise

    well, they are useful whenever you have an asynchronous call, with the benefit of having a uniform interface supporting, among other things, exception handling in a natural way. For example, suppose you are developing a library implementing, say, an actor model with asynchronous messages/actions; with futures ( and promises, hidden behind some call mechanism ) you can return a value to user code with asynchronous semantics in a way he can understand, indipendently of the specific library interface, in a generic way; this is valuable, I think.

  8. #8
    Lindley is offline Elite Member Power Poster
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    Re: boost::thread and boost::promise

    Hmm. I suppose I can see the value there.

  9. #9
    Join Date
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    Re: boost::thread and boost::promise

    Quote Originally Posted by Lindley View Post
    Hmm. I suppose there's some value in creating those "primitives", but they seem fairly meaningless outside of the thread-pool pattern usage so far. Perhaps I just haven't encountered the right scenario to require them.
    What a frustrating and discouraging comment. It's lucky of me that I am in a good mood, otherwise I'll get mad about this. How about a broken promise ?
    You mark them as Async then export them as a service and call them in your C/C++ code.
    hi,,,

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