Add the void(...) asio handler overload to the use_continuable initiation token

* Makes it possible to use the initiation token with dispatch/post
* Ref #44

dep/asio/asio

@@ -1 +1 @@
-Subproject commit 8087252a0c3c2f0baad96ddbd6554db17a846376
+Subproject commit 6c054e98f3f53352d12b6cd46d63b6d404cc044b

examples/example-asio/example-asio-integration.cpp

@@ -50,18 +50,22 @@ void unexpected_error(cti::exception_t);
 // Check that the failure was an aborted operation, as expected.
 void check_aborted_operation(cti::exception_t);
 
+// Use a strand as executor
+void using_strand();
+
 int main(int, char**) {
   daytime_service();
 
   successful_async_wait();
   cancelled_async_wait();
 
+  using_strand();
+
   return 0;
 }
 
 void daytime_service() {
   using asio::ip::tcp;
-
   asio::io_context ioc(1);
   tcp::resolver resolver(ioc);
   tcp::socket socket(ioc);
@@ -146,3 +150,27 @@ void check_aborted_operation(cti::exception_t ex) {
     puts("Continuation failed due to aborted async operation, as expected.");
   }
 }
+
+template <typename T>
+auto through_post(T& postable) {
+  return [&postable](auto&& work) mutable {
+    asio::post(postable, std::forward<decltype(work)>(work));
+  };
+}
+
+void using_strand() {
+  asio::io_context ioc(1);
+  asio::io_context::strand strand(ioc);
+
+  asio::post(strand, cti::use_continuable).then([]() {
+    puts("Dispatched through initiation token");
+  });
+
+  cti::async_on(
+      []() mutable {
+        puts("Dispatched through executor token");
+      },
+      through_post(strand));
+
+  ioc.run();
+}

include/continuable/detail/external/asio.hpp

@@ -109,31 +109,50 @@ using system_error_t = ::boost::system::system_error;
 #  endif
 #endif
 
-// Binds `promise` to the first argument of a continuable resolver, giving it
-// the signature of an ASIO handler.
 template <typename Promise, typename Token>
-auto promise_resolver_handler(Promise&& promise, Token&& token) noexcept {
-  return [promise = std::forward<Promise>(promise),
-          token = std::forward<Token>(token)](error_code_t e,
-                                              auto&&... args) mutable noexcept {
+class promise_resolver {
+public:
+  explicit promise_resolver(Promise promise, Token token)
+    : promise_(std::move(promise))
+    , token_(std::move(token)) {}
+
+  template <typename... T>
+  void operator()(T&&... args) noexcept {
+    promise_.set_value(std::forward<T>(args)...);
+  }
+
+  template <typename... T>
+  void operator()(error_code_t e, T&&... args) noexcept {
     if (e) {
-      if (!token.is_ignored(e)) {
-        if (token.is_cancellation(e)) {
-          promise.set_canceled();
+      if (!token_.is_ignored(e)) {
+        if (token_.is_cancellation(e)) {
+          promise_.set_canceled();
           return;
         } else {
 #if defined(CONTINUABLE_HAS_EXCEPTIONS)
-          promise.set_exception(
+          promise_.set_exception(
               std::make_exception_ptr(system_error_t(std::move(e))));
 #else
-          promise.set_exception(exception_t(e.value(), e.category()));
+          promise_.set_exception(exception_t(e.value(), e.category()));
 #endif
           return;
         }
       }
     }
-    promise.set_value(std::forward<decltype(args)>(args)...);
-  };
+    promise_.set_value(std::forward<T>(args)...);
+  }
+
+private:
+  Promise promise_;
+  Token token_;
+};
+
+// Binds `promise` to the first argument of a continuable resolver, giving it
+// the signature of an ASIO handler.
+template <typename Promise, typename Token>
+auto promise_resolver_handler(Promise&& promise, Token&& token) noexcept {
+  return promise_resolver<std::decay_t<Promise>, std::decay_t<Token>>(
+      std::forward<Promise>(promise), std::forward<Token>(token));
 }
 
 // Helper struct wrapping a call to `cti::make_continuable` and, if needed,
@@ -141,6 +160,19 @@ auto promise_resolver_handler(Promise&& promise, Token&& token) noexcept {
 template <typename Signature>
 struct initiate_make_continuable;
 
+template <typename... Args>
+struct initiate_make_continuable<void(Args...)> {
+#if defined(CTI_DETAIL_ASIO_HAS_EXPLICIT_RET_TYPE_INTEGRATION)
+  using erased_return_type = continuable<Args...>;
+#endif
+
+  template <typename Continuation>
+  auto operator()(Continuation&& continuation) {
+    return base::attorney::create_from(std::forward<Continuation>(continuation),
+                                       identity<Args...>{}, util::ownership{});
+  }
+};
+
 template <typename... Args>
 struct initiate_make_continuable<void(error_code_t, Args...)> {
 #if defined(CTI_DETAIL_ASIO_HAS_EXPLICIT_RET_TYPE_INTEGRATION)