added noop utility

closes #3

include/boost/cobalt/detail/generator.hpp

@@ -15,6 +15,7 @@
 #include <boost/cobalt/detail/this_thread.hpp>
 #include <boost/cobalt/unique_handle.hpp>
 #include <boost/cobalt/detail/wrapper.hpp>
+#include <boost/cobalt/noop.hpp>
 
 #include <boost/asio/bind_allocator.hpp>
 #include <boost/core/exchange.hpp>
@@ -92,6 +93,8 @@ struct generator_receiver : generator_receiver_base<Yield, Push>
 
   bool ready() { return exception || result || done; }
 
+  generator_receiver(noop<Yield> n) : result(std::move(n.value)), done(true) {}
+
   generator_receiver() = default;
   generator_receiver(generator_receiver && lhs)
   : generator_receiver_base<Yield, Push>{std::move(lhs.pushed_value)},

include/boost/cobalt/detail/promise.hpp

@@ -12,6 +12,7 @@
 #include <boost/cobalt/detail/forward_cancellation.hpp>
 #include <boost/cobalt/detail/wrapper.hpp>
 #include <boost/cobalt/detail/this_thread.hpp>
+#include <boost/cobalt/noop.hpp>
 #include <boost/cobalt/unique_handle.hpp>
 
 #include <boost/asio/cancellation_signal.hpp>
@@ -65,6 +66,8 @@ struct promise_value_holder
     result.emplace(ret);
     static_cast<promise_receiver<T>*>(this)->set_done();
   }
+  constexpr promise_value_holder() = default;
+  constexpr promise_value_holder(noop<T> value) noexcept(std::is_nothrow_move_constructible_v<T>) : result(std::move(value.value)) {}
 
 };
 
@@ -79,6 +82,9 @@ struct promise_value_holder<void>
   }
 
   inline void return_void();
+
+  constexpr promise_value_holder() = default;
+  constexpr promise_value_holder(noop<void>) {}
 };
 
 
@@ -113,6 +119,7 @@ struct promise_receiver : promise_value_holder<T>
   }
 
   promise_receiver() = default;
+  promise_receiver(noop<T> value) : promise_value_holder<T>(std::move(value)), done(true) {}
   promise_receiver(promise_receiver && lhs) noexcept
       : promise_value_holder<T>(std::move(lhs)),
         exception(std::move(lhs.exception)), done(lhs.done), awaited_from(std::move(lhs.awaited_from)),

include/boost/cobalt/detail/task.hpp

@@ -12,6 +12,7 @@
 #include <boost/cobalt/detail/forward_cancellation.hpp>
 #include <boost/cobalt/detail/wrapper.hpp>
 #include <boost/cobalt/detail/this_thread.hpp>
+#include <boost/cobalt/noop.hpp>
 
 #include <boost/asio/bind_allocator.hpp>
 #include <boost/asio/cancellation_signal.hpp>
@@ -59,6 +60,9 @@ struct task_value_holder
     result.emplace(ret);
     static_cast<task_receiver<T>*>(this)->set_done();
   }
+
+  constexpr task_value_holder() noexcept = default;
+  constexpr task_value_holder(noop<T> n) noexcept(std::is_nothrow_move_constructible_v<T>) : result(std::move(n.value)) {}
 };
 
 template<>
@@ -72,6 +76,9 @@ struct task_value_holder<void>
   }
 
   inline void return_void();
+
+  constexpr task_value_holder() noexcept = default;
+  constexpr task_value_holder(noop<void> n) noexcept {}
 };
 
 
@@ -114,6 +121,7 @@ struct task_receiver : task_value_holder<T>
       promise->signal.emit(ct);
   }
 
+  task_receiver(noop<T> n) : task_value_holder<T>(std::move(n)), done(true) {}
   task_receiver() = default;
   task_receiver(task_receiver && lhs)
       : task_value_holder<T>(std::move(lhs)),

include/boost/cobalt/generator.hpp

@@ -47,6 +47,7 @@ struct [[nodiscard]] generator
   generator(const generator &) = delete;
   generator& operator=(const generator &) = delete;
 
+  constexpr generator(noop<Yield> n) : receiver_(std::move(n)){}
 
  private:
   template<typename, typename>

include/boost/cobalt/noop.hpp

@@ -0,0 +1,55 @@
+//
+// Copyright (c) 2024 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_NOOP_HPP
+#define BOOST_COBALT_NOOP_HPP
+
+#include <type_traits>
+#include <utility>
+
+namespace boost::cobalt
+{
+
+
+// tag::outline[]
+// This is a tag type allowing the creation of promises or generators without creating a coroutine.
+template<typename T = void>
+struct noop
+{
+  template<typename ... Args>
+  constexpr noop(Args && ... args) noexcept(std::is_nothrow_constructible_v<T, Args&&...>)
+    : value(std::forward<Args>(args)...)
+  {
+  }
+  // end::outline[]
+  T value;
+
+  constexpr static bool await_ready() {return true;}
+  template<typename P>
+  constexpr static void await_suspend(std::coroutine_handle<P>) {}
+  constexpr T await_resume() {return std::move(value);}
+
+  // tag::outline[]
+};
+// end::outline[]
+
+template<> struct noop<void>
+{
+  constexpr static bool await_ready() {return true;}
+  template<typename P>
+  constexpr static void await_suspend(std::coroutine_handle<P>) {}
+  constexpr static void await_resume() {}
+};
+
+
+template<typename T> noop(      T &&) -> noop<T>;
+template<typename T> noop(const T & ) -> noop<T>;
+noop() -> noop<void>;
+
+}
+
+#endif //BOOST_COBALT_NOOP_HPP

include/boost/cobalt/promise.hpp

@@ -41,6 +41,10 @@ struct [[nodiscard]] promise
     // end::outline[]
 
     /* tag::outline[]
+    // Create an already completed promimse
+
+    static promise
+
     // Get the return value. If !ready() this function has undefined behaviour.
     Return get();
     end::outline[] */
@@ -60,6 +64,8 @@ struct [[nodiscard]] promise
       if (attached_)
         cancel();
     }
+
+    constexpr promise(noop<Return> n) : receiver_(std::move(n)), attached_(false) {}
   private:
     template<typename>
     friend struct detail::cobalt_promise;

include/boost/cobalt/task.hpp

@@ -34,6 +34,8 @@ struct [[nodiscard]] task
 
     using promise_type = detail::task_promise<Return>;
 
+    constexpr task(noop<Return> n) : receiver_(std::move(n)){}
+
  private:
     template<typename>
     friend struct detail::task_promise;

test/generator.cpp

@@ -296,11 +296,14 @@ cobalt::generator<int, int> detached_push()
   co_return i;
 }
 
+cobalt::generator<int> np() {return cobalt::noop(42);}
+
 CO_TEST_CASE(detached_push_)
 {
   auto g = detached_push();
 
   co_await g(1);
+  co_await np();
 }
 
 BOOST_AUTO_TEST_SUITE_END();

test/promise.cpp

@@ -65,7 +65,7 @@ BOOST_AUTO_TEST_CASE(unwind)
 
 cobalt::promise<int> return_(std::size_t ms)
 {
-  co_return 1234u;
+  return cobalt::noop(1234);
 }
 
 cobalt::promise<int> return_(std::size_t ms, asio::executor_arg_t,

test/task.cpp

@@ -34,11 +34,12 @@ namespace
 
 cobalt::task<void> test0()
 {
-    co_return;
+    return cobalt::noop<void>();
 }
 
 cobalt::task<double> test2(int i)
 {
+    co_await cobalt::noop();
     co_await test0();
     co_return i;
 }