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///////////////////////////////////////////////////////////////////////////////
// Reference implementation of std::generator proposal P2168.
//
// See https://wg21.link/P2168 for details.
//
///////////////////////////////////////////////////////////////////////////////
// Copyright Lewis Baker, Corentin Jabot
//
// Use, modification and distribution is subject to the Boost Software License,
// Version 1.0.
// (See accompanying file LICENSE or http://www.boost.org/LICENSE_1_0.txt)
///////////////////////////////////////////////////////////////////////////////
export module nihil.generator:generator_promise_base;
import nihil.std;
import :elements_of;
import :forward;
import :manual_lifetime;
namespace nihil {
template <typename Ref>
struct generator_promise_base
{
template <typename Ref2, typename Value, typename Alloc>
friend struct generator;
generator_promise_base *m_root;
std::coroutine_handle<> m_parent_or_leaf;
// Note: Using manual_lifetime here to avoid extra calls to exception_ptr
// constructor/destructor in cases where it is not needed (i.e. where this
// generator coroutine is not used as a nested coroutine).
// This member is lazily constructed by the __yield_sequence_awaiter::await_suspend()
// method if this generator is used as a nested generator.
manual_lifetime<std::exception_ptr> m_exception;
manual_lifetime<Ref> m_value;
explicit generator_promise_base(std::coroutine_handle<> this_coro) noexcept
: m_root(this)
, m_parent_or_leaf(this_coro)
{
}
~generator_promise_base()
{
if (m_root != this) {
// This coroutine was used as a nested generator and so will
// have constructed its __exception_ member which needs to be
// destroyed here.
m_exception.destruct();
}
}
auto initial_suspend() noexcept -> std::suspend_always
{
return {};
}
auto return_void() noexcept -> void
{
}
auto unhandled_exception() -> void
{
if (m_root != this)
m_exception.get() = std::current_exception();
else
throw;
}
// Transfers control back to the parent of a nested coroutine
struct final_awaiter
{
auto await_ready() noexcept -> bool
{
return false;
}
template <typename Promise>
auto
await_suspend(std::coroutine_handle<Promise> h) noexcept -> std::coroutine_handle<>
{
Promise &promise = h.promise();
generator_promise_base &root = *promise.m_root;
if (&root != &promise) {
auto parent = promise.m_parent_or_leaf;
root.m_parent_or_leaf = parent;
return parent;
}
return std::noop_coroutine();
}
void await_resume() noexcept
{
}
};
auto final_suspend() noexcept -> final_awaiter
{
return {};
}
auto yield_value(Ref &&x) noexcept(std::is_nothrow_move_constructible_v<Ref>)
-> std::suspend_always
{
m_root->m_value.construct((Ref &&)x);
return {};
}
template <typename T>
requires(!std::is_reference_v<Ref>) && std::is_convertible_v<T, Ref>
auto
yield_value(T &&x) noexcept(std::is_nothrow_constructible_v<Ref, T>) -> std::suspend_always
{
m_root->m_value.construct((T &&)x);
return {};
}
template <typename Gen>
struct yield_sequence_awaiter
{
Gen m_gen;
yield_sequence_awaiter(Gen &&g) noexcept
// Taking ownership of the generator ensures frame are destroyed
// in the reverse order of their execution.
: m_gen((Gen &&)g)
{
}
auto await_ready() noexcept -> bool
{
return false;
}
// set the parent, root and exceptions pointer and
// resume the nested
template <typename Promise>
auto
await_suspend(std::coroutine_handle<Promise> h) noexcept -> std::coroutine_handle<>
{
generator_promise_base ¤t = h.promise();
generator_promise_base &nested = *m_gen.get_promise();
generator_promise_base &root = *current.m_root;
nested.m_root = current.m_root;
nested.m_parent_or_leaf = h;
// Lazily construct the __exception_ member here now that we
// know it will be used as a nested generator. This will be
// destroyed by the promise destructor.
nested.m_exception.construct();
root.m_parent_or_leaf = m_gen.get_coro();
// Immediately resume the nested coroutine (nested generator)
return m_gen.get_coro();
}
void await_resume()
{
generator_promise_base &nested_promise = *m_gen.get_promise();
if (nested_promise.m_exception.get()) {
std::rethrow_exception(nested_promise.m_exception.get());
}
}
};
template <typename OValue, typename OAlloc>
auto yield_value(nihil::elements_of<generator<Ref, OValue, OAlloc>> g) noexcept
-> yield_sequence_awaiter<generator<Ref, OValue, OAlloc>>
{
return std::move(g).get();
}
template <std::ranges::range Rng, typename Allocator>
auto yield_value(nihil::elements_of<Rng, Allocator> &&x)
-> yield_sequence_awaiter<generator<Ref, std::remove_cvref_t<Ref>, Allocator>>
{
return [](std::allocator_arg_t, Allocator,
auto &&rng) -> generator<Ref, std::remove_cvref_t<Ref>, Allocator> {
for (auto &&e : rng)
co_yield static_cast<decltype(e)>(e);
}(std::allocator_arg, x.get_allocator(), std::forward<Rng>(x.get()));
}
void resume()
{
m_parent_or_leaf.resume();
}
// Disable use of co_await within this coroutine.
void await_transform() = delete;
};
} // namespace nihil
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