futures.qbk

[/
  (C) Copyright 2008-11 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 `future`]]
[def __unique_future `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::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::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::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::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::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::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`]]
[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]

[section:async Executing asynchronously]
[endsect]

[section:make_future Making immediate futures easier]

Some functions may know the value at the point of construction. In these cases the value is immediately available, 
but needs to be returned as a future or shared_future. By using make_future (make_shared_future) a future (shared_future) 
can be created which holds a pre-computed result in its shared state.
 
Without these features it is non-trivial to create a future directly from a value. 
First a promise must be created, then the promise is set, and lastly the future is retrieved from the promise. 
This can now be done with one operation.

[heading make_future / make_shared_future]

This function creates a future for a given value. If no value is given then a future<void> is returned. 
This function is primarily useful in cases where sometimes, the return value is immediately available, but sometimes 
it is not. The example below illustrates, that in an error path the value is known immediately, however in other paths 
the function must return an eventual value represented as a future.


  boost::future<int> compute(int x)
  {
    if (x == 0) return boost::make_future(0);
    if (x < 0) return boost::make_future(-1);
    boost::future<int> f1 = boost::async([]() { return x+1; });
    return f1;
  }
  
There are two variations of this function. The first takes a value of any type, and returns a future of that type. 
The input value is passed to the shared state of the returned future. The second version takes no input and returns a future<void>.
make_shared_future has the same functionality as make_future, except has a return type of shared_future.
  

[endsect]

[section:then Associating future continuations]

In asynchronous programming, it is very common for one asynchronous operation, on completion, to invoke a second 
operation and pass data to it. The current C++ standard does not allow one to register a continuation to a future. 
With .then, instead of waiting for the result, a continuation is "attached" to the asynchronous operation, which is 
invoked when the result is ready. Continuations registered using the .then function will help to avoid blocking waits 
or wasting threads on polling, greatly improving the responsiveness and scalability of an application.

future.then provides the ability to sequentially compose two futures by declaring one to be the continuation of another. 
With .then the antecedent future is ready (has a value or exception stored in the shared state) before the continuation 
starts as instructed by the lambda function.

In the example below the future<int> f2 is registered to be a continuation of future<int> f1 using the .then member 
function. This operation takes a lambda function which describes how f2 should proceed after f1 is ready.


  #include <boost/thread/future.hpp>
  using namespace boost;
  int main() 
  {
    future<int> f1 = async([]() { return 123; });
    future<string> f2 = f1.then([](future<int> f) { return f.get().to_string(); // here .get() won't block });
  }

One key feature of this function is the ability to chain multiple asynchronous operations. In asynchronous programming, 
it's common to define a sequence of operations, in which each continuation executes only when the previous one completes. 
In some cases, the antecedent future produces a value that the continuation accepts as input. By using future.then, 
creating a chain of continuations becomes straightforward and intuitive: 

  myFuture.then(...).then(...).then(...). 
  
Some points to note are:

* Each continuation will not begin until the preceding has completed.
* If an exception is thrown, the following continuation can handle it in a try-catch block


Input Parameters:

* Lambda function2: One option which was considered was to follow JavaScript's approach and take two functions, one for 
success and one for error handling. However this option is not viable in C++ as there is no single base type for 
exceptions as there is in JavaScript. The lambda function takes a future as its input which carries the exception 
through. This makes propagating exceptions straightforward. This approach also simplifies the chaining of continuations.
* Scheduler: Providing an overload to .then, to take a scheduler reference places great flexibility over the execution 
of the future in the programmer's hand. As described above, often taking a launch policy is not sufficient for powerful 
asynchronous operations. The lifetime of the scheduler must outlive the continuation.
* Launch policy: if the additional flexibility that the scheduler provides is not required.


Return values: The decision to return a future was based primarily on the ability to chain multiple continuations using
.then. This benefit of composability gives the programmer incredible control and flexibility over their code. Returning 
a future object rather than a shared_future is also a much cheaper operation thereby improving performance. A 
shared_future object is not necessary to take advantage of the chaining feature. It is also easy to go from a future 
to a shared_future when needed using future::share().


[endsect]


[include future_ref.qbk]

[endsect]