boost_1_45_0/libs/thread/doc/futures.qbk

[/
  (C) Copyright 2008-9 Anthony Williams.
  Distributed under the Boost Software License, Version 1.0.
  (See accompanying file LICENSE_1_0.txt or copy at
  http://www.boost.org/LICENSE_1_0.txt).
]

[section:futures Futures]

[template future_state_link[link_text] [link thread.synchronization.futures.reference.future_state [link_text]]]
[def __uninitialized__ [future_state_link `boost::future_state::uninitialized`]]
[def __ready__ [future_state_link `boost::future_state::ready`]]
[def __waiting__ [future_state_link `boost::future_state::waiting`]]

[def __future_uninitialized__ `boost::future_uninitialized`]
[def __broken_promise__ `boost::broken_promise`]
[def __future_already_retrieved__ `boost::future_already_retrieved`]
[def __task_moved__ `boost::task_moved`]
[def __task_already_started__ `boost::task_already_started`]
[def __promise_already_satisfied__ `boost::promise_already_satisfied`]

[def __thread_interrupted__ `boost::thread_interrupted`]


[template unique_future_link[link_text] [link thread.synchronization.futures.reference.unique_future [link_text]]]
[def __unique_future__ [unique_future_link `boost::unique_future`]]

[template unique_future_get_link[link_text] [link thread.synchronization.futures.reference.unique_future.get [link_text]]]
[def __unique_future_get__ [unique_future_get_link `boost::unique_future<R>::get()`]]

[template unique_future_wait_link[link_text] [link thread.synchronization.futures.reference.unique_future.wait [link_text]]]
[def __unique_future_wait__ [unique_future_wait_link `boost::unique_future<R>::wait()`]]

[template unique_future_is_ready_link[link_text] [link thread.synchronization.futures.reference.unique_future.is_ready [link_text]]]
[def __unique_future_is_ready__ [unique_future_is_ready_link `boost::unique_future<R>::is_ready()`]]

[template unique_future_has_value_link[link_text] [link thread.synchronization.futures.reference.unique_future.has_value [link_text]]]
[def __unique_future_has_value__ [unique_future_has_value_link `boost::unique_future<R>::has_value()`]]

[template unique_future_has_exception_link[link_text] [link thread.synchronization.futures.reference.unique_future.has_exception [link_text]]]
[def __unique_future_has_exception__ [unique_future_has_exception_link `boost::unique_future<R>::has_exception()`]]

[template unique_future_get_state_link[link_text] [link thread.synchronization.futures.reference.unique_future.get_state [link_text]]]
[def __unique_future_get_state__ [unique_future_get_state_link `boost::unique_future<R>::get_state()`]]

[template shared_future_link[link_text] [link thread.synchronization.futures.reference.shared_future [link_text]]]
[def __shared_future__ [shared_future_link `boost::shared_future`]]

[template shared_future_get_link[link_text] [link thread.synchronization.futures.reference.shared_future.get [link_text]]]
[def __shared_future_get__ [shared_future_get_link `boost::shared_future<R>::get()`]]

[template shared_future_wait_link[link_text] [link thread.synchronization.futures.reference.shared_future.wait [link_text]]]
[def __shared_future_wait__ [shared_future_wait_link `boost::shared_future<R>::wait()`]]

[template shared_future_is_ready_link[link_text] [link thread.synchronization.futures.reference.shared_future.is_ready [link_text]]]
[def __shared_future_is_ready__ [shared_future_is_ready_link `boost::shared_future<R>::is_ready()`]]

[template shared_future_has_value_link[link_text] [link thread.synchronization.futures.reference.shared_future.has_value [link_text]]]
[def __shared_future_has_value__ [shared_future_has_value_link `boost::shared_future<R>::has_value()`]]

[template shared_future_has_exception_link[link_text] [link thread.synchronization.futures.reference.shared_future.has_exception [link_text]]]
[def __shared_future_has_exception__ [shared_future_has_exception_link `boost::shared_future<R>::has_exception()`]]

[template shared_future_get_state_link[link_text] [link thread.synchronization.futures.reference.shared_future.get_state [link_text]]]
[def __shared_future_get_state__ [shared_future_get_state_link `boost::shared_future<R>::get_state()`]]

[template promise_link[link_text] [link thread.synchronization.futures.reference.promise [link_text]]]
[def __promise__ [promise_link `boost::promise`]]

[template packaged_task_link[link_text] [link thread.synchronization.futures.reference.packaged_task [link_text]]]
[def __packaged_task__ [packaged_task_link `boost::packaged_task`]]

[template wait_for_any_link[link_text] [link thread.synchronization.futures.reference.wait_for_any [link_text]]]
[def __wait_for_any__ [wait_for_any_link `boost::wait_for_any()`]]

[template wait_for_all_link[link_text] [link thread.synchronization.futures.reference.wait_for_all [link_text]]]
[def __wait_for_all__ [wait_for_all_link `boost::wait_for_all()`]]


[section:overview Overview]

The futures library provides a means of handling synchronous future values, whether those values are generated by another thread, or
on a single thread in response to external stimuli, or on-demand.

This is done through the provision of four class templates: __unique_future__ and __shared_future__ which are used to retrieve the
asynchronous results, and __promise__ and __packaged_task__ which are used to generate the asynchronous results.

An instance of __unique_future__ holds the one and only reference to a result. Ownership can be transferred between instances using
the move constructor or move-assignment operator, but at most one instance holds a reference to a given asynchronous result. When
the result is ready, it is returned from __unique_future_get__ by rvalue-reference to allow the result to be moved or copied as
appropriate for the type.

On the other hand, many instances of __shared_future__ may reference the same result. Instances can be freely copied and assigned,
and __shared_future_get__ returns a `const` reference so that multiple calls to __shared_future_get__ are safe. You can move an
instance of __unique_future__ into an instance of __shared_future__, thus transferring ownership of the associated asynchronous
result, but not vice-versa.

You can wait for futures either individually or with one of the __wait_for_any__ and __wait_for_all__ functions.

[endsect]

[section:creating Creating asynchronous values]

You can set the value in a future with either a __promise__ or a __packaged_task__. A __packaged_task__ is a callable object that
wraps a function or callable object. When the packaged task is invoked, it invokes the contained function in turn, and populates a
future with the return value. This is an answer to the perennial question: "how do I return a value from a thread?": package the
function you wish to run as a __packaged_task__ and pass the packaged task to the thread constructor. The future retrieved from the
packaged task can then be used to obtain the return value. If the function throws an exception, that is stored in the future in
place of the return value.

    int calculate_the_answer_to_life_the_universe_and_everything()
    {
        return 42;
    }

    boost::packaged_task<int> pt(calculate_the_answer_to_life_the_universe_and_everything);
    boost::unique_future<int> fi=pt.get_future();

    boost::thread task(boost::move(pt)); // launch task on a thread

    fi.wait(); // wait for it to finish
        
    assert(fi.is_ready());
    assert(fi.has_value());
    assert(!fi.has_exception());
    assert(fi.get_state()==boost::future_state::ready);
    assert(fi.get()==42);


A __promise__ is a bit more low level: it just provides explicit functions to store a value or an exception in the associated
future. A promise can therefore be used where the value may come from more than one possible source, or where a single operation may
produce multiple values.

    boost::promise<int> pi;
    boost::unique_future<int> fi;
    fi=pi.get_future();

    pi.set_value(42);
    
    assert(fi.is_ready());
    assert(fi.has_value());
    assert(!fi.has_exception());
    assert(fi.get_state()==boost::future_state::ready);
    assert(fi.get()==42);

[endsect]

[section:lazy_futures Wait Callbacks and Lazy Futures]

Both __promise__ and __packaged_task__ support ['wait callbacks] that are invoked when a thread blocks in a call to `wait()` or
`timed_wait()` on a future that is waiting for the result from the __promise__ or __packaged_task__, in the thread that is doing the
waiting. These can be set using the `set_wait_callback()` member function on the __promise__ or __packaged_task__ in question.

This allows ['lazy futures] where the result is not actually computed until it is needed by some thread. In the example below, the
call to `f.get()` invokes the callback `invoke_lazy_task`, which runs the task to set the value. If you remove the call to
`f.get()`, the task is not ever run.

    int calculate_the_answer_to_life_the_universe_and_everything()
    {
        return 42;
    }

    void invoke_lazy_task(boost::packaged_task<int>& task)
    {
        try
        {
            task();
        }
        catch(boost::task_already_started&)
        {}
    }

    int main()
    {
        boost::packaged_task<int> task(calculate_the_answer_to_life_the_universe_and_everything);
        task.set_wait_callback(invoke_lazy_task);
        boost::unique_future<int> f(task.get_future());

        assert(f.get()==42);
    }


[endsect]

[include future_ref.qbk]

[endsect]