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### Then There’s Boost.Asio

Since the simplest form of Boost.Asio asynchronous operation completion token is a callback function, we could apply the same tactics for Asio as for our hypothetical `AsyncAPI` asynchronous operations.

Fortunately we need not. Boost.Asio incorporates a mechanism[5] by which the caller can customize the notification behavior of any async operation. Therefore we can construct a completion token which, when passed to a Boost.Asio async operation, requests blocking for the calling fiber.

A typical Asio async function might look something like this:[6]

    template < ..., class CompletionToken >
    deduced_return_type
    async_something( ... , CompletionToken&& token)
    {
        // construct handler_type instance from CompletionToken
        handler_type<CompletionToken, ...>::type handler(token);
        // construct async_result instance from handler_type
        async_result<decltype(handler)> result(handler);

        // ... arrange to call handler on completion ...
        // ... initiate actual I/O operation ...

        return result.get();
    }

We will engage that mechanism, which is based on specializing Asio’s `handler_type<>` template for the `CompletionToken` type and the signature of the specific callback. The remainder of this discussion will refer back to `async_something()` as the Asio async function under consideration.

The implementation described below uses lower-level facilities than `promise` and `future` because the `promise` mechanism interacts badly with `io_service::stop()`. It produces `broken_promise` exceptions.

`boost::fibers::asio::yield` is a completion token of this kind. `yield` is an instance of `yield_t`:

    class yield_t {
    public:
        yield_t() = default;

        /**
         * @code
         * static yield_t yield;
         * boost::system::error_code myec;
         * func(yield[myec]);
         * @endcode
         * @c yield[myec] returns an instance of @c yield_t whose @c ec_ points
         * to @c myec. The expression @c yield[myec] "binds" @c myec to that
         * (anonymous) @c yield_t instance, instructing @c func() to store any
         * @c error_code it might produce into @c myec rather than throwing @c
         * boost::system::system_error.
         */
        yield_t operator[]( boost::system::error_code & ec) const {
            yield_t tmp;
            tmp.ec_ = & ec;
            return tmp;
        }

    //private:
        // ptr to bound error_code instance if any
        boost::system::error_code   *   ec_{ nullptr };
    };

`yield_t` is in fact only a placeholder, a way to trigger Boost.Asio customization. It can bind a `boost::system::error_code` for use by the actual handler.

`yield` is declared as:

    // canonical instance
    thread_local yield_t yield{};

Asio customization is engaged by specializing `boost::asio::handler_type<>` for `yield_t`:

    // Handler type specialisation for fibers::asio::yield.
    // When 'yield' is passed as a completion handler which accepts only
    // error_code, use yield_handler<void>. yield_handler will take care of the
    // error_code one way or another.
    template< typename ReturnType >
    struct handler_type< fibers::asio::yield_t, ReturnType( boost::system::error_code) >
    { typedef fibers::asio::detail::yield_handler< void >    type; };

(There are actually four different specializations in detail/yield.hpp, one for each of the four Asio async callback signatures we expect.)

The above directs Asio to use `yield_handler` as the actual handler for an async operation to which `yield` is passed. There’s a generic `yield_handler<T>` implementation and a `yield_handler<void>` specialization. Let’s start with the `<void>` specialization:

    // yield_handler<void> is like yield_handler<T> without value_. In fact it's
    // just like yield_handler_base.
    template<>
    class yield_handler< void >: public yield_handler_base {
    public:
        explicit yield_handler( yield_t const& y) :
            yield_handler_base{ y } {
        }

        // nullary completion callback
        void operator()() {
            ( * this)( boost::system::error_code() );
        }

        // inherit operator()(error_code) overload from base class
        using yield_handler_base::operator();
    };

`async_something()`, having consulted the `handler_type<>` traits specialization, instantiates a `yield_handler<void>` to be passed as the actual callback for the async operation. `yield_handler`’s constructor accepts the `yield_t` instance (the `yield` object passed to the async function) and passes it along to `yield_handler_base`:

    // This class encapsulates common elements between yield_handler<T> (capturing
    // a value to return from asio async function) and yield_handler<void> (no
    // such value). See yield_handler<T> and its <void> specialization below. Both
    // yield_handler<T> and yield_handler<void> are passed by value through
    // various layers of asio functions. In other words, they're potentially
    // copied multiple times. So key data such as the yield_completion instance
    // must be stored in our async_result<yield_handler<>> specialization, which
    // should be instantiated only once.
    class yield_handler_base {
    public:
        yield_handler_base( yield_t const& y) :
            // capture the context* associated with the running fiber
            ctx_{ boost::fibers::context::active() },
            // capture the passed yield_t
            yt_( y ) {
        }

        // completion callback passing only (error_code)
        void operator()( boost::system::error_code const& ec) {
            BOOST_ASSERT_MSG( ycomp_,
                              "Must inject yield_completion* "
                              "before calling yield_handler_base::operator()()");
            BOOST_ASSERT_MSG( yt_.ec_,
                              "Must inject boost::system::error_code* "
                              "before calling yield_handler_base::operator()()");
            // If originating fiber is busy testing state_ flag, wait until it
            // has observed (completed != state_).
            yield_completion::lock_t lk{ ycomp_->mtx_ };
            yield_completion::state_t state = ycomp_->state_;
            // Notify a subsequent yield_completion::wait() call that it need not
            // suspend.
            ycomp_->state_ = yield_completion::complete;
            // set the error_code bound by yield_t
            * yt_.ec_ = ec;
            // unlock the lock that protects state_
            lk.unlock();
            // If ctx_ is still active, e.g. because the async operation
            // immediately called its callback (this method!) before the asio
            // async function called async_result_base::get(), we must not set it
            // ready.
            if ( yield_completion::waiting == state) {
                // wake the fiber
                fibers::context::active()->schedule( ctx_);
            }
        }

    //private:
        boost::fibers::context      *   ctx_;
        yield_t                         yt_;
        // We depend on this pointer to yield_completion, which will be injected
        // by async_result.
        yield_completion::ptr_t         ycomp_{};
    };

`yield_handler_base` stores a copy of the `yield_t` instance — which, as shown above, contains only an `error_code*`. It also captures the `context`* for the currently-running fiber by calling `context::active()`.

You will notice that `yield_handler_base` has one more data member (`ycomp_`) that is initialized to `nullptr` by its constructor — though its `operator()()` method relies on `ycomp_` being non-null. More on this in a moment.

Having constructed the `yield_handler<void>` instance, `async_something()` goes on to construct an `async_result` specialized for the `handler_type<>::type`: in this case, `async_result<yield_handler<void>>`. It passes the `yield_handler<void>` instance to the new `async_result` instance.

    // Without the need to handle a passed value, our yield_handler<void>
    // specialization is just like async_result_base.
    template<>
    class async_result< boost::fibers::asio::detail::yield_handler< void > > :
        public boost::fibers::asio::detail::async_result_base {
    public:
        typedef void type;

        explicit async_result( boost::fibers::asio::detail::yield_handler< void > & h):
            boost::fibers::asio::detail::async_result_base{ h } {
        }
    };

Naturally that leads us straight to `async_result_base`:

    // Factor out commonality between async_result<yield_handler<T>> and
    // async_result<yield_handler<void>>
    class async_result_base {
    public:
        explicit async_result_base( yield_handler_base & h) :
                ycomp_{ new yield_completion{} } {
            // Inject ptr to our yield_completion instance into this
            // yield_handler<>.
            h.ycomp_ = this->ycomp_;
            // if yield_t didn't bind an error_code, make yield_handler_base's
            // error_code* point to an error_code local to this object so
            // yield_handler_base::operator() can unconditionally store through
            // its error_code*
            if ( ! h.yt_.ec_) {
                h.yt_.ec_ = & ec_;
            }
        }

        void get() {
            // Unless yield_handler_base::operator() has already been called,
            // suspend the calling fiber until that call.
            ycomp_->wait();
            // The only way our own ec_ member could have a non-default value is
            // if our yield_handler did not have a bound error_code AND the
            // completion callback passed a non-default error_code.
            if ( ec_) {
                throw_exception( boost::system::system_error{ ec_ } );
            }
        }

    private:
        // If yield_t does not bind an error_code instance, store into here.
        boost::system::error_code       ec_{};
        yield_completion::ptr_t         ycomp_;
    };

This is how `yield_handler_base::ycomp_` becomes non-null: `async_result_base`’s constructor injects a pointer back to its own `yield_completion` member.

Recall that the canonical `yield_t` instance `yield` initializes its `error_code*` member `ec_` to `nullptr`. If this instance is passed to `async_something()` (`ec_` is still `nullptr`), the copy stored in `yield_handler_base` will likewise have null `ec_`. `async_result_base`’s constructor sets `yield_handler_base`’s `yield_t`’s `ec_` member to point to its own `error_code` member.

The stage is now set. `async_something()` initiates the actual async operation, arranging to call its `yield_handler<void>` instance on completion. Let’s say, for the sake of argument, that the actual async operation’s callback has signature `void(error_code)`.

But since it’s an async operation, control returns at once to `async_something()`. `async_something()` calls `async_result<yield_handler<void>>::get()`, and will return its return value.

`async_result<yield_handler<void>>::get()` inherits `async_result_base::get()`.

`async_result_base::get()` immediately calls `yield_completion::wait()`.

    // Bundle a completion bool flag with a spinlock to protect it.
    struct yield_completion {
        enum state_t {
            init,
            waiting,
            complete
        };

        typedef fibers::detail::spinlock                    mutex_t;
        typedef std::unique_lock< mutex_t >                 lock_t;
        typedef boost::intrusive_ptr< yield_completion >    ptr_t;

        std::atomic< std::size_t >  use_count_{ 0 };
        mutex_t                     mtx_{};
        state_t                     state_{ init };

        void wait() {
            // yield_handler_base::operator()() will set state_ `complete` and
            // attempt to wake a suspended fiber. It would be Bad if that call
            // happened between our detecting (complete != state_) and suspending.
            lock_t lk{ mtx_ };
            // If state_ is already set, we're done here: don't suspend.
            if ( complete != state_) {
                state_ = waiting;
                // suspend(unique_lock<spinlock>) unlocks the lock in the act of
                // resuming another fiber
                fibers::context::active()->suspend( lk);
            }
        }

        friend void intrusive_ptr_add_ref( yield_completion * yc) noexcept {
            BOOST_ASSERT( nullptr != yc);
            yc->use_count_.fetch_add( 1, std::memory_order_relaxed);
        }

        friend void intrusive_ptr_release( yield_completion * yc) noexcept {
            BOOST_ASSERT( nullptr != yc);
            if ( 1 == yc->use_count_.fetch_sub( 1, std::memory_order_release) ) {
                std::atomic_thread_fence( std::memory_order_acquire);
                delete yc;
            }
        }
    };

Supposing that the pending async operation has not yet completed, `yield_completion::completed_` will still be `false`, and `wait()` will call `context::suspend()` on the currently-running fiber.

Other fibers will now have a chance to run.

Some time later, the async operation completes. It calls `yield_handler<void>::operator()(error_code const&)` with an `error_code` indicating either success or failure. We’ll consider both cases.

`yield_handler<void>` explicitly inherits `operator()(error_code const&)` from `yield_handler_base`.

`yield_handler_base::operator()(error_code const&)` first sets `yield_completion::completed_` `true`. This way, if `async_something()`’s async operation completes immediately — if `yield_handler_base::operator()` is called even before `async_result_base::get()` — the calling fiber will not suspend.

The actual `error_code` produced by the async operation is then stored through the stored `yield_t::ec_` pointer. If `async_something()`’s caller used (e.g.) `yield[my_ec]` to bind a local `error_code` instance, the actual `error_code` value is stored into the caller’s variable. Otherwise, it is stored into `async_result_base::ec_`.

If the stored fiber context `yield_handler_base::ctx_` is not already running, it is marked as ready to run by passing it to `context::schedule()`. Control then returns from `yield_handler_base::operator()`: the callback is done.

In due course, that fiber is resumed. Control returns from `context::suspend()` to `yield_completion::wait()`, which returns to `async_result_base::get()`.

- If the original caller passed `yield[my_ec]` to `async_something()` to bind a local `error_code` instance, then `yield_handler_base::operator()` stored its `error_code` to the caller’s `my_ec` instance, leaving `async_result_base::ec_` initialized to success.
- If the original caller passed `yield` to `async_something()` without binding a local `error_code` variable, then `yield_handler_base::operator()` stored its `error_code` into `async_result_base::ec_`. If in fact that `error_code` is success, then all is well.
- Otherwise — the original caller did not bind a local `error_code` and `yield_handler_base::operator()` was called with an `error_code` indicating error — `async_result_base::get()` throws `system_error` with that `error_code`.

The case in which `async_something()`’s completion callback has signature `void()` is similar. `yield_handler<void>::operator()()` invokes the machinery above with a “success” `error_code`.

A completion callback with signature `void(error_code, T)` (that is: in addition to `error_code`, callback receives some data item) is handled somewhat differently. For this kind of signature, `handler_type<>::type` specifies `yield_handler<T>` (for `T` other than `void`).

A `yield_handler<T>` reserves a `value_` pointer to a value of type `T`:

    // asio uses handler_type<completion token type, signature>::type to decide
    // what to instantiate as the actual handler. Below, we specialize
    // handler_type< yield_t, ... > to indicate yield_handler<>. So when you pass
    // an instance of yield_t as an asio completion token, asio selects
    // yield_handler<> as the actual handler class.
    template< typename T >
    class yield_handler: public yield_handler_base {
    public:
        // asio passes the completion token to the handler constructor
        explicit yield_handler( yield_t const& y) :
            yield_handler_base{ y } {
        }

        // completion callback passing only value (T)
        void operator()( T t) {
            // just like callback passing success error_code
            (*this)( boost::system::error_code(), std::move(t) );
        }

        // completion callback passing (error_code, T)
        void operator()( boost::system::error_code const& ec, T t) {
            BOOST_ASSERT_MSG( value_,
                              "Must inject value ptr "
                              "before caling yield_handler<T>::operator()()");
            // move the value to async_result<> instance BEFORE waking up a
            // suspended fiber
            * value_ = std::move( t);
            // forward the call to base-class completion handler
            yield_handler_base::operator()( ec);
        }

    //private:
        // pointer to destination for eventual value
        // this must be injected by async_result before operator()() is called
        T   *   value_{ nullptr };
    };

This pointer is initialized to `nullptr`.

When `async_something()` instantiates `async_result<yield_handler<T>>`:

    // asio constructs an async_result<> instance from the yield_handler specified
    // by handler_type<>::type. A particular asio async method constructs the
    // yield_handler, constructs this async_result specialization from it, then
    // returns the result of calling its get() method.
    template< typename T >
    class async_result< boost::fibers::asio::detail::yield_handler< T > > :
        public boost::fibers::asio::detail::async_result_base {
    public:
        // type returned by get()
        typedef T type;

        explicit async_result( boost::fibers::asio::detail::yield_handler< T > & h) :
            boost::fibers::asio::detail::async_result_base{ h } {
            // Inject ptr to our value_ member into yield_handler<>: result will
            // be stored here.
            h.value_ = & value_;
        }

        // asio async method returns result of calling get()
        type get() {
            boost::fibers::asio::detail::async_result_base::get();
            return std::move( value_);
        }

    private:
        type                            value_{};
    };

this `async_result<>` specialization reserves a member of type `T` to receive the passed data item, and sets `yield_handler<T>::value_` to point to its own data member.

`async_result<yield_handler<T>>` overrides `get()`. The override calls `async_result_base::get()`, so the calling fiber suspends as described above.

`yield_handler<T>::operator()(error_code, T)` stores its passed `T` value into `async_result<yield_handler<T>>::value_`.

Then it passes control to `yield_handler_base::operator()(error_code)` to deal with waking the original fiber as described above.

When `async_result<yield_handler<T>>::get()` resumes, it returns the stored `value_` to `async_something()` and ultimately to `async_something()`’s caller.

The case of a callback signature `void(T)` is handled by having `yield_handler<T>::operator()(T)` engage the `void(error_code, T)` machinery, passing a “success” `error_code`.

The source code above is found in yield.hpp and detail/yield.hpp.

[5] This mechanism has been proposed as a conventional way to allow the caller of an arbitrary async function to specify completion handling: N4045.

[6] per N4045