{

cppcoro::coroutine_handle<> m_continuation;

shared_task_waiter* m_next;

{

friend struct final_awaiter;

struct final_awaiter

{

bool await_ready() const noexcept { return false; }

template<typename PROMISE>

void await_suspend(cppcoro::coroutine_handle<PROMISE> h) noexcept

{

shared_task_promise_base& promise = h.promise();

// Exchange operation needs to be 'release' so that subsequent awaiters have

// visibility of the result. Also needs to be 'acquire' so we have visibility

// of writes to the waiters list.

void* const valueReadyValue = &promise;

void* waiters = promise.m_waiters.exchange(valueReadyValue, std::memory_order_acq_rel);

if (waiters != nullptr)

{

shared_task_waiter* waiter = static_cast<shared_task_waiter*>(waiters);

while (waiter->m_next != nullptr)

{

// Read the m_next pointer before resuming the coroutine

// since resuming the coroutine may destroy the shared_task_waiter value.

auto* next = waiter->m_next;

waiter->m_continuation.resume();

waiter = next;

}

// Resume last waiter in tail position to allow it to potentially

// be compiled as a tail-call.

waiter->m_continuation.resume();

}

}

void await_resume() noexcept {}

};

public:

shared_task_promise_base() noexcept

: m_refCount(1)

, m_waiters(&this->m_waiters)

, m_exception(nullptr)

{}

cppcoro::suspend_always initial_suspend() noexcept { return {}; }

final_awaiter final_suspend() noexcept { return {}; }

void unhandled_exception() noexcept

{

m_exception = std::current_exception();

}

bool is_ready() const noexcept

{

const void* const valueReadyValue = this;

return m_waiters.load(std::memory_order_acquire) == valueReadyValue;

}

void add_ref() noexcept

{

m_refCount.fetch_add(1, std::memory_order_relaxed);

}

/// Decrement the reference count.

///

/// \return

/// true if successfully detached, false if this was the last

/// reference to the coroutine, in which case the caller must

/// call destroy() on the coroutine handle.

bool try_detach() noexcept

{

return m_refCount.fetch_sub(1, std::memory_order_acq_rel) != 1;

}

/// Try to enqueue a waiter to the list of waiters.

///

/// \param waiter

/// Pointer to the state from the waiter object.

/// Must have waiter->m_coroutine member populated with the coroutine

/// handle of the awaiting coroutine.

///

/// \param coroutine

/// Coroutine handle for this promise object.

///

/// \return

/// true if the waiter was successfully queued, in which case

/// waiter->m_coroutine will be resumed when the task completes.

/// false if the coroutine was already completed and the awaiting

/// coroutine can continue without suspending.

bool try_await(shared_task_waiter* waiter, cppcoro::coroutine_handle<> coroutine)

{

void* const valueReadyValue = this;

void* const notStartedValue = &this->m_waiters;

constexpr void* startedNoWaitersValue = static_cast<shared_task_waiter*>(nullptr);

// NOTE: If the coroutine is not yet started then the first waiter

// will start the coroutine before enqueuing itself up to the list

// of suspended waiters waiting for completion. We split this into

// two steps to allow the first awaiter to return without suspending.

// This avoids recursively resuming the first waiter inside the call to

// coroutine.resume() in the case that the coroutine completes

// synchronously, which could otherwise lead to stack-overflow if

// the awaiting coroutine awaited many synchronously-completing

// tasks in a row.

// Start the coroutine if not already started.

void* oldWaiters = m_waiters.load(std::memory_order_acquire);

if (oldWaiters == notStartedValue &&

m_waiters.compare_exchange_strong(

oldWaiters,

startedNoWaitersValue,

std::memory_order_relaxed))

{

// Start the task executing.

coroutine.resume();

oldWaiters = m_waiters.load(std::memory_order_acquire);

}

// Enqueue the waiter into the list of waiting coroutines.

do

{

if (oldWaiters == valueReadyValue)

{

// Coroutine already completed, don't suspend.

return false;

}

waiter->m_next = static_cast<shared_task_waiter*>(oldWaiters);

} while (!m_waiters.compare_exchange_weak(

oldWaiters,

static_cast<void*>(waiter),

std::memory_order_release,

std::memory_order_acquire));

return true;

}

protected:

bool completed_with_unhandled_exception()

{

return m_exception != nullptr;

}

void rethrow_if_unhandled_exception()

{

if (m_exception != nullptr)

{

std::rethrow_exception(m_exception);

}

}

private:

std::atomic<std::uint32_t> m_refCount;

// Value is either

// - nullptr - indicates started, no waiters

// - this - indicates value is ready

// - &this->m_waiters - indicates coroutine not started

// - other - pointer to head item in linked-list of waiters.

// values are of type 'cppcoro::shared_task_waiter'.

// indicates that the coroutine has been started.

std::atomic<void*> m_waiters;

std::exception_ptr m_exception;

{

public:

shared_task_promise() noexcept = default;

~shared_task_promise()

{

if (this->is_ready() && !this->completed_with_unhandled_exception())

{

reinterpret_cast<T*>(&m_valueStorage)->~T();

}

}

shared_task<T> get_return_object() noexcept;

template<

typename VALUE,

typename = std::enable_if_t<std::is_convertible_v<VALUE&&, T>>>

void return_value(VALUE&& value)

noexcept(std::is_nothrow_constructible_v<T, VALUE&&>)

{

new (&m_valueStorage) T(std::forward<VALUE>(value));

}

T& result()

{

this->rethrow_if_unhandled_exception();

return *reinterpret_cast<T*>(&m_valueStorage);

}

private:

// Not using std::aligned_storage here due to bug in MSVC 2015 Update 2

// that means it doesn't work for types with alignof(T) > 8.

// See MS-Connect bug #2658635.

alignas(T) char m_valueStorage[sizeof(T)];

{

public:

using promise_type = detail::shared_task_promise<T>;

using value_type = T;

private:

struct awaitable_base

{

cppcoro::coroutine_handle<promise_type> m_coroutine;

detail::shared_task_waiter m_waiter;

awaitable_base(cppcoro::coroutine_handle<promise_type> coroutine) noexcept

: m_coroutine(coroutine)

{}

bool await_ready() const noexcept

{

return !m_coroutine || m_coroutine.promise().is_ready();

}

bool await_suspend(cppcoro::coroutine_handle<> awaiter) noexcept

{

m_waiter.m_continuation = awaiter;

return m_coroutine.promise().try_await(&m_waiter, m_coroutine);

}

};

public:

shared_task() noexcept

: m_coroutine(nullptr)

{}

explicit shared_task(cppcoro::coroutine_handle<promise_type> coroutine)

: m_coroutine(coroutine)

{

// Don't increment the ref-count here since it has already been

// initialised to 2 (one for shared_task and one for coroutine)

// in the shared_task_promise constructor.

}

shared_task(shared_task&& other) noexcept

: m_coroutine(other.m_coroutine)

{

other.m_coroutine = nullptr;

}

shared_task(const shared_task& other) noexcept

: m_coroutine(other.m_coroutine)

{

if (m_coroutine)

{

m_coroutine.promise().add_ref();

}

}

~shared_task()

{

destroy();

}

shared_task& operator=(shared_task&& other) noexcept

{

if (&other != this)

{

destroy();

m_coroutine = other.m_coroutine;

other.m_coroutine = nullptr;

}

return *this;

}

shared_task& operator=(const shared_task& other) noexcept

{

if (m_coroutine != other.m_coroutine)

{

destroy();

m_coroutine = other.m_coroutine;

if (m_coroutine)

{

m_coroutine.promise().add_ref();

}

}

return *this;

}

void swap(shared_task& other) noexcept

{

std::swap(m_coroutine, other.m_coroutine);

}

/// \brief

/// Query if the task result is complete.

///

/// Awaiting a task that is ready will not block.

bool is_ready() const noexcept

{

return !m_coroutine || m_coroutine.promise().is_ready();

}

auto operator co_await() const noexcept

{

struct awaitable : awaitable_base

{

using awaitable_base::awaitable_base;

decltype(auto) await_resume()

{

if (!this->m_coroutine)

{

throw broken_promise{};

}

return this->m_coroutine.promise().result();

}

};

return awaitable{ m_coroutine };

}

/// \brief

/// Returns an awaitable that will await completion of the task without

/// attempting to retrieve the result.

auto when_ready() const noexcept

{

struct awaitable : awaitable_base

{

using awaitable_base::awaitable_base;

void await_resume() const noexcept {}

};

return awaitable{ m_coroutine };

}

private:

template<typename U>

friend bool operator==(const shared_task<U>&, const shared_task<U>&) noexcept;

void destroy() noexcept

{

if (m_coroutine)

{

if (!m_coroutine.promise().try_detach())

{

m_coroutine.destroy();

}

}

}

cppcoro::coroutine_handle<promise_type> m_coroutine;

-> shared_task<detail::remove_rvalue_reference_t<typename awaitable_traits<AWAITABLE>::await_result_t>>

{

co_return co_await static_cast<AWAITABLE&&>(awaitable);