Disassociate argument type from return type in PostTaskAndReplyWithResult template.

base/task_runner_util.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef BASE_TASK_RUNNER_UTIL_H_
 #define BASE_TASK_RUNNER_UTIL_H_
 
 #include "base/bind.h"
 #include "base/bind_helpers.h"
 #include "base/callback_internal.h"
@@ skipping 14 matching lines @@
 
 // Helper class for TaskRunner::PostTaskAndReplyWithResult.
 template <typename ReturnType>
 Closure ReturnAsParam(const Callback<ReturnType(void)>& func,
                       ReturnType* result) {
   DCHECK(result);
   return Bind(&ReturnAsParamAdapter<ReturnType>, func, result);
 }
 
 // Helper class for TaskRunner::PostTaskAndReplyWithResult.
-template <typename ReturnType>
+template <typename ReturnType, typename ResultType>

[Jeffrey Yasskin, 2012/11/05 22:13:22]

Could you give these template arguments the same names as you're using in
PostTaskAndReplyWithResult?

[awong, 2012/11/19 05:57:09]

Done.

 void ReplyAdapter(const Callback<void(ReturnType)>& callback,
-                  ReturnType* result) {
+                  ResultType* result) {
   DCHECK(result);
   if(!callback.is_null())
     callback.Run(CallbackForward(*result));
 }
 
 // Helper class for TaskRunner::PostTaskAndReplyWithResult.
-template <typename ReturnType, typename OwnedType>
+template <typename ReturnType, typename ResultType>

[Jeffrey Yasskin, 2012/11/05 22:13:22]

Ditto here.

[awong, 2012/11/19 05:57:09]

Done.

[awong, 2012/11/19 05:57:09]

Done.

 Closure ReplyHelper(const Callback<void(ReturnType)>& callback,
-                    OwnedType result) {
-  return Bind(&ReplyAdapter<ReturnType>, callback, result);
+                    ResultType* result) {
+  return Bind(&ReplyAdapter<ReturnType, ResultType>, callback, Owned(result));
 }
 
 }  // namespace internal
 
 // When you have these methods
 //
 //   R DoWorkAndReturn();
 //   void Callback(const R& result);
 //
 // and want to call them in a PostTaskAndReply kind of fashion where the
 // result of DoWorkAndReturn is passed to the Callback, you can use
 // PostTaskAndReplyWithResult as in this example:
 //
 // PostTaskAndReplyWithResult(
 //     target_thread_.message_loop_proxy(),
 //     FROM_HERE,
 //     Bind(&DoWorkAndReturn),
 //     Bind(&Callback));
-template <typename ReturnType>
+template <typename TaskReturnType, typename ReplyArgType>
 bool PostTaskAndReplyWithResult(
     TaskRunner* task_runner,
     const tracked_objects::Location& from_here,
-    const Callback<ReturnType(void)>& task,
-    const Callback<void(ReturnType)>& reply) {
-  ReturnType* result = new ReturnType;
+    const Callback<TaskReturnType(void)>& task,
+    const Callback<void(ReplyArgType)>& reply) {

[Jeffrey Yasskin, 2012/11/05 22:13:22]

Could you double-check that users get a sensible error message when
TaskReturnType isn't convertible to ReplyArgType?

[awong, 2012/11/19 05:57:09]

Define sensible :)

It looks good enough to me. The compile error happens at the CallbackForward()
inside the adapter function below and the conversion shows up first on the error
list which is a bonus.

In file included from ../../base/task_runner_util_unittest.cc:5:
../../base/task_runner_util.h:30:16: error: reference to type 'const double'
could not bind to an lvalue of type 'char *'
  callback.Run(CallbackForward(*result));
               ^~~~~~~~~~~~~~~~~~~~~~~~
../../base/task_runner_util.h:61:18: note: in instantiation of function template
specialization 'base::internal::ReplyAdapter<char *, double>' requested here
      base::Bind(&internal::ReplyAdapter<TaskReturnType, ReplyArgType>, reply,
                 ^
../../base/task_runner_util_unittest.cc:90:3: note: in instantiation of function
template specialization 'base::PostTaskAndReplyWithResult<char *, double>'
requested here
  PostTaskAndReplyWithResult(
  ^
../../base/callback.h:427:65: note: passing argument to parameter 'a1' here
  R Run(typename internal::CallbackParamTraits<A1>::ForwardType a1) const {
                                                                ^
2 errors generated.
ninja: build stopped: subcommand failed.

[Jeffrey Yasskin, 2012/11/19 10:33:46]

LGTM

+  TaskReturnType* result = new TaskReturnType;
   return task_runner->PostTaskAndReply(
       from_here,
-      internal::ReturnAsParam<ReturnType>(task, result),
-      internal::ReplyHelper(reply, Owned(result)));
+      internal::ReturnAsParam<TaskReturnType>(task, result),
+      internal::ReplyHelper(reply, result));
 }
 
 }  // namespace base
 
 #endif  // BASE_TASK_RUNNER_UTIL_H_