added composed.

doc/reference/async_for.adoc

@@ -21,3 +21,16 @@ cobalt::main co_main(int argc, char * argv[])
 The requirement is that the <<awaitable,awaitable>> used in the for loop has an `operator bool` to check if it
 can be awaited again. This is the case for <<generator, generator>> and <<promise,promise>>.
 
+That is, `BOOST_COBALT_FOR(__expression__, __declaration__) __statement__`  the above statement is roughly equivalent to:
+
+[source,cpp,subs=+quotes]
+----
+{
+  auto && __aw__ = co_await __expression__;
+  while (__aw__)
+  {
+    __expression__ = co_await __aw__;
+    __statement__
+  }
+}
+----

include/boost/cobalt/composed.hpp

@@ -0,0 +1,139 @@
+//
+// Copyright (c) 2023 Klemens Morgenstern ([email protected])
+//
+// 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)
+//
+
+#ifndef BOOST_COBALT_COMPOSED_HPP
+#define BOOST_COBALT_COMPOSED_HPP
+
+#include <boost/cobalt/detail/handler.hpp>
+
+namespace boost::cobalt::detail
+{
+
+template<typename ... Args>
+struct composed_promise_return
+{
+  completion_handler<Args...> & handler;
+
+  void return_value(std::tuple<Args...> && tup)
+  {
+    handler.result.emplace(std::move(tup));
+  }
+};
+
+
+template<>
+struct composed_promise_return<>
+{
+  completion_handler<> & handler;
+
+  void return_void()
+  {
+    handler.result.emplace();
+  }
+};
+
+template<typename ... Args>
+struct composed_promise : composed_promise_return<Args...>
+{
+#if !defined(BOOST_COBALT_NO_PMR)
+  template<typename ... Args_>
+  void * operator new(const std::size_t size, Args_ & ... args)
+  {
+    auto res = std::get<sizeof...(Args_) - 1>(std::tie(args...)).get_allocator().resource();
+    const auto p = res->allocate(size + sizeof(pmr::memory_resource *), alignof(pmr::memory_resource *));
+    auto pp = static_cast<pmr::memory_resource**>(p);
+    *pp = res;
+    return pp + 1;
+  }
+
+  void operator delete(void * raw, const std::size_t size) noexcept
+  {
+    const auto p = static_cast<pmr::memory_resource**>(raw) - 1;
+    pmr::memory_resource * res = *p;
+    res->deallocate(p, size + sizeof(pmr::memory_resource *), alignof(pmr::memory_resource *));
+  }
+#endif
+
+  template<typename ... Args_>
+  composed_promise(Args_ & ... args)
+        : composed_promise_return<Args...>(std::get<sizeof...(Args_) - 1u>(std::tie(args...))) {}
+
+  using executor_type = executor;
+  const executor_type & get_executor() const {return this->handler.get_executor();}
+
+#if !defined(BOOST_COBALT_NO_PMR)
+  using allocator_type = pmr::polymorphic_allocator<void>;
+  allocator_type get_allocator() const {return this->handler.get_allocator();}
+#endif
+
+  using cancellation_slot_type = asio::cancellation_slot;
+  cancellation_slot_type get_cancellation_slot() const noexcept {return this->handler.get_cancellation_slot();}
+
+  void get_return_object() {}
+  std::suspend_never initial_suspend()        {return {};}
+
+  auto final_suspend() noexcept
+  {
+    struct awaitable
+    {
+      completion_handler<Args...> & handler;
+      constexpr bool await_ready() noexcept
+      {
+        return handler.completed_immediately != nullptr
+           && *handler.completed_immediately == detail::completed_immediately_t::maybe;
+      }
+
+      std::coroutine_handle<void> await_suspend(std::coroutine_handle<composed_promise> h) noexcept
+      {
+        auto tmp = handler.self.release();
+        detail::self_destroy(h);
+        return tmp;
+      }
+
+      void await_resume() noexcept
+      {
+        *handler.completed_immediately = detail::completed_immediately_t::yes;
+        handler.self.release();
+      }
+
+    };
+
+    return awaitable{this->handler};
+  }
+
+  void unhandled_exception() {throw;}
+
+};
+
+template<typename ... Args>
+composed_promise<Args...> completion_handler_probe(const completion_handler<Args...> & );
+
+template<typename T>
+concept is_completion_handler = requires (const T & h) {{completion_handler_probe(h)};};
+
+}
+
+
+namespace std
+{
+
+template<typename ... Args>
+  requires  boost::cobalt::detail::is_completion_handler<
+      boost::mp11::mp_back<boost::mp11::mp_list<Args...>>>
+struct coroutine_traits<void, Args...>
+{
+  using promise_type = decltype(completion_handler_probe(
+      std::declval<const boost::mp11::mp_back<boost::mp11::mp_list<Args...>>&>()));
+
+};
+
+
+}
+
+
+
+#endif //BOOST_COBALT_COMPOSED_HPP

include/boost/cobalt/detail/handler.hpp

@@ -23,6 +23,13 @@ namespace boost::cobalt
 namespace detail
 {
 
+template<typename ... >
+struct composed_promise_return;
+
+
+template<typename ... >
+struct composed_promise;
+
 enum class completed_immediately_t
 {
   no, maybe, yes, initiating
@@ -285,6 +292,11 @@ struct completion_handler : detail::completion_handler_base
 #if defined(BOOST_ASIO_ENABLE_HANDLER_TRACKING)
     boost::source_location loc_;
 #endif
+
+  template<typename ... >
+  friend struct detail::composed_promise_return;
+  template<typename ... >
+  friend struct detail::composed_promise;
 };
 
 };

test/CMakeLists.txt

@@ -8,7 +8,7 @@ add_executable(boost_cobalt_main_compile main_compile.cpp)
 add_executable(boost_cobalt_basic_tests
       async_for.cpp doctest.cpp promise.cpp with.cpp op.cpp handler.cpp join.cpp race.cpp this_coro.cpp leaf.cpp
       channel.cpp generator.cpp run.cpp task.cpp gather.cpp wait_group.cpp wrappers.cpp left_race.cpp
-      strand.cpp fork.cpp thread.cpp any_completion_handler.cpp detached.cpp monotonic_resource.cpp sbo_resource.cpp)
+      strand.cpp fork.cpp thread.cpp any_completion_handler.cpp detached.cpp monotonic_resource.cpp sbo_resource.cpp composed.cpp)
 
 target_link_libraries(boost_cobalt_main         Boost::cobalt OpenSSL::SSL)
 target_link_libraries(boost_cobalt_main_compile Boost::cobalt OpenSSL::SSL)

test/composed.cpp

@@ -0,0 +1,61 @@
+//
+// Copyright (c) 2023 Klemens Morgenstern ([email protected])
+//
+// 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)
+//
+
+#include <boost/cobalt/composed.hpp>
+#include <boost/cobalt/op.hpp>
+
+#include "doctest.h"
+#include "test.hpp"
+
+using namespace boost;
+
+
+TEST_SUITE_BEGIN("composed");
+
+struct post_composed_op : cobalt::op<>
+{
+
+  post_composed_op() {}
+
+  void initiate(cobalt::completion_handler<> complete)
+  {
+    co_await asio::post(cobalt::use_op);
+  }
+};
+
+
+struct ec_composed_op : cobalt::op<system::error_code>
+{
+  ec_composed_op() {}
+
+  void initiate(cobalt::completion_handler<system::error_code> complete)
+  {
+    co_return asio::error::already_open;
+  }
+};
+
+struct ec_n_composed_op : cobalt::op<system::error_code, std::size_t>
+{
+
+  ec_n_composed_op() {}
+
+  void initiate(cobalt::completion_handler<system::error_code, std::size_t> complete)
+  {
+    co_await asio::post(cobalt::use_op);
+    co_return std::make_tuple(system::error_code{}, 2u);
+  }
+};
+
+
+CO_TEST_CASE("composed-op")
+{
+  co_await post_composed_op();
+  CHECK_THROWS(co_await ec_composed_op(), system::system_error(asio::error::already_open));
+  CHECK(co_await ec_n_composed_op() == 2u);
+}
+
+TEST_SUITE_END();