Boost C++ Libraries of the most highly regarded and expertly designed C++ library projects in the world. Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

This is the documentation for an old version of Boost. Click here to view this page for the latest version.

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 >
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 {
    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;

    // 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.
class yield_handler< void >: public yield_handler_base {
    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 {
    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 completed_ flag, wait until it
        // has observed (! completed_).
        yield_completion::lock_t lk{ ycomp_->mtx_ };
        // Notify a subsequent yield_completion::wait() call that it need not
        // suspend.
        ycomp_->completed_ = true;
        // set the error_code bound by yield_t
        * yt_.ec_ = ec;
        // 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 ( fibers::context::active() != ctx_ ) {
            // wake the fiber
            fibers::context::active()->schedule( ctx_);

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

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.
class async_result< boost::fibers::asio::detail::yield_handler< void > > :
    public boost::fibers::asio::detail::async_result_base {
    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 {
    explicit async_result_base( yield_handler_base & h) {
        // 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.
        // 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_ } );

    // If yield_t does not bind an error_code instance, store into here.
    boost::system::error_code       ec_{};
    // async_result_base owns the yield_completion because, unlike
    // yield_handler<>, async_result<> is only instantiated once.
    yield_completion                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 {
    typedef fibers::detail::spinlock    mutex_t;
    typedef std::unique_lock< mutex_t > lock_t;

    mutex_t mtx_{};
    bool    completed_{ false };

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

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::set_ready(). 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().

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 {
    // 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);

    // 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 {
    // 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() {
        return std::move( value_);

    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.