use variadic macros/templates in IPC message implementation

ipc/ipc_message_templates.h

Index: ipc/ipc_message_templates.h
diff --git a/ipc/ipc_message_templates.h b/ipc/ipc_message_templates.h
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+// Copyright 2015 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 IPC_IPC_MESSAGE_TEMPLATES_H_
+#define IPC_IPC_MESSAGE_TEMPLATES_H_
+
+#include <stdint.h>
+
+#include <type_traits>
+
+#include "base/logging.h"
+#include "base/tuple.h"
+#include "build/build_config.h"
+#include "ipc/ipc_message.h"
+#include "ipc/ipc_message_utils.h"
+
+namespace IPC {
+
+// This function is for all the async IPCs that don't pass an extra parameter
+// using IPC_BEGIN_MESSAGE_MAP_WITH_PARAM.
+template <typename ObjT, typename Method, typename P, typename Tuple>
+void DispatchToMethod(ObjT* obj, Method method, P*, const Tuple& tuple) {
+  base::DispatchToMethod(obj, method, tuple);
+}
+
+template <typename ObjT,
+          typename Method,
+          typename P,
+          typename Tuple,
+          size_t... Ns>
+void DispatchToMethodImpl(ObjT* obj,
+                          Method method,
+                          P* parameter,
+                          const Tuple& tuple,
+                          base::IndexSequence<Ns...>) {
+  // TODO(mdempsky): Apply UnwrapTraits like base::DispatchToMethod?
+  (obj->*method)(parameter, base::get<Ns>(tuple)...);
+}
+
+// The following function is for async IPCs which have a dispatcher with an
+// extra parameter specified using IPC_BEGIN_MESSAGE_MAP_WITH_PARAM.
+template <typename ObjT, typename P, typename... Args, typename... Ts>
+typename std::enable_if<sizeof...(Args) == sizeof...(Ts)>::type
+DispatchToMethod(ObjT* obj,
+                 void (ObjT::*method)(P*, Args...),
+                 P* parameter,
+                 const base::Tuple<Ts...>& tuple) {
+  DispatchToMethodImpl(obj, method, parameter, tuple,
+                       base::MakeIndexSequence<sizeof...(Ts)>());
+}
+
+enum class MessageKind {
+  CONTROL,
+  ROUTED,
+};
+
+// Routing is a helper struct so MessageT's private common constructor has a
+// different type signature than the public "int32_t routing_id" one.
+struct Routing {
+  int32_t id;
+};
+
+// We want to restrict MessageT's constructors so that a routing_id is always
+// provided for ROUTED messages and never provided for CONTROL messages, so
+// use the SFINAE technique from N4387's "Implementation Hint" section.
+#if defined(COMPILER_MSVC)
+// MSVC 2013 doesn't support default arguments for template member functions
+// of templated classes, so there we have to rely on the DCHECKs instead.
+// TODO(mdempsky): Reevaluate once MSVC 2015.

[vmpstr, 2016/02/04 23:43:27]

I'm not really up to date, but is MSVC2015 here? Or very close to here?

Also,
https://code.google.com/p/chromium/codesearch#chromium/src/base/memory/scoped...

has default argument for a template member (ctor) of a templated class, and it
seems to work on MSVC. Does the ctor behave differently from regular functions?

[mdempsky, 2016/02/05 00:32:32]

No idea.
I don't recall anything special about constructors vs other member functions. 
If someone can easily confirm that it works in MSVC 2015 now, I'll change this
CL.  Otherwise, I'd rather try playing with that in a separate followup CL.

[danakj, 2016/02/05 00:41:04]

https://groups.google.com/a/chromium.org/forum/#!topic/chromium-dev/6Wd16Iqhy9Y

[vmpstr, 2016/02/05 01:24:31]

My point was simply that this seems to already work in MSVC (since scoped_ptr is
compiled by whatever the current compiler is). But maybe there's something else
that's subtle going on.

[mdempsky, 2016/02/05 02:06:10]

Understood.  I'm going to lean towards something more subtle is going on,
because those lines from scoped_ptr.h are from Nov 2015, and I know this was
failing on the Windows trybots more recently than that.

+#define IPC_MESSAGET_SFINAE(x)
+#else
+#define IPC_MESSAGET_SFINAE(x) \
+  template <bool X = (x), typename std::enable_if<X, bool>::type = false>

[vmpstr, 2016/02/04 23:43:27]

Is the extra bool X = (x) just convenience? Or does something not work if you
just pass x to enable_if?

[mdempsky, 2016/02/05 00:32:32]

I'm pretty sure it's necessary.  I think it's because if you just did

    template <typename std::enable_if<(x), bool>::type = false>

then the std::enable_if<(x), bool>::type type will be expanded when the class
template is instantiated.  But since (x) is false, it won't evaluate to a valid
type, and the entire class will be ill-formed.

By using

    template <typename bool X = (x), ...>

the value of X is potentially user-controlled, so the compiler defers
determining std::enable_if<X, bool>::type until the function is actually used,
which allows SFINAE to actually work.

[vmpstr, 2016/02/05 01:24:31]

I guess my initial concern was that if one explicitly specifies "true" for the
first parameter, then it enables either one of the ctors. However, TIL that it's
actually impossible to explicitly specify ctor template parameters, so this
looks fine to me :)

Although I would've hoped that

template <typename std::enable_if<Meta::kKind == MessageKind::CONTROL,
bool>::type = false>

would also work (where that equality is what "x" should be)

[mdempsky, 2016/02/05 02:06:10]

I did too, but alas, that's not what the C++ committee decided. :(

+#endif
+
+// MessageT is the common template used for all user-defined message types.
+// It's intended to be used via the macros defined in ipc_message_macros.h.
+template <typename Meta,
+          typename InTuple = typename Meta::InTuple,
+          typename OutTuple = typename Meta::OutTuple>
+class MessageT;
+
+// Asynchronous message partial specialization.
+template <typename Meta, typename... Ins>
+class MessageT<Meta, base::Tuple<Ins...>, void> : public Message {
+ public:
+  using Param = base::Tuple<Ins...>;
+  enum { ID = Meta::ID };
+
+  // TODO(mdempsky): Remove.

[vmpstr, 2016/02/04 23:43:27]

Can you elaborate on this todo?

[mdempsky, 2016/02/05 00:32:32]

Done.

+  using Schema = MessageT;
+
+  IPC_MESSAGET_SFINAE(Meta::kKind == MessageKind::CONTROL)
+  MessageT(const Ins&... ins) : MessageT(Routing{MSG_ROUTING_CONTROL}, ins...) {

[vmpstr, 2016/02/04 23:43:27]

Is this syntax allowed? I keep being confused between initializer_lists and
uniform initialization syntax and all that...

Routing{MSG_ROUTING_CONTROL} is what I'm referring to. 

[mdempsky, 2016/02/05 00:32:32]

Blah, looks like still no.  Done.

+    DCHECK(Meta::kKind == MessageKind::CONTROL) << Meta::kName;
+  }
+
+  IPC_MESSAGET_SFINAE(Meta::kKind == MessageKind::ROUTED)
+  MessageT(int32_t routing_id, const Ins&... ins)
+      : MessageT(Routing{routing_id}, ins...) {
+    DCHECK(Meta::kKind == MessageKind::ROUTED) << Meta::kName;
+  }
+
+  static bool Read(const Message* msg, Param* p);
+  static void Log(std::string* name, const Message* msg, std::string* l);
+
+  template <class T, class S, class P, class Method>
+  static bool Dispatch(const Message* msg,
+                       T* obj,
+                       S* sender,
+                       P* parameter,
+                       Method func) {
+    Param p;
+    if (Read(msg, &p)) {
+      DispatchToMethod(obj, func, parameter, p);
+      return true;
+    }
+    return false;
+  }
+
+ private:
+  MessageT(Routing routing, const Ins&... ins);
+};
+
+// Synchronous message partial specialization.
+template <typename Meta, typename... Ins, typename... Outs>
+class MessageT<Meta, base::Tuple<Ins...>, base::Tuple<Outs...>>
+    : public SyncMessage {
+ public:
+  using SendParam = base::Tuple<Ins...>;
+  using ReplyParam = base::Tuple<Outs...>;
+  enum { ID = Meta::ID };
+
+  // TODO(mdempsky): Remove.
+  using Schema = MessageT;
+
+  IPC_MESSAGET_SFINAE(Meta::kKind == MessageKind::CONTROL)
+  MessageT(const Ins&... ins, Outs*... outs)
+      : MessageT(Routing{MSG_ROUTING_CONTROL}, ins..., outs...) {
+    DCHECK(Meta::kKind == MessageKind::CONTROL) << Meta::kName;
+  }
+
+  IPC_MESSAGET_SFINAE(Meta::kKind == MessageKind::ROUTED)
+  MessageT(int32_t routing_id, const Ins&... ins, Outs*... outs)
+      : MessageT(Routing{routing_id}, ins..., outs...) {
+    DCHECK(Meta::kKind == MessageKind::ROUTED) << Meta::kName;
+  }
+
+  static bool ReadSendParam(const Message* msg, SendParam* p);
+  static bool ReadReplyParam(const Message* msg, ReplyParam* p);
+  static void WriteReplyParams(Message* reply, const Outs&... outs);
+  static void Log(std::string* name, const Message* msg, std::string* l);
+
+  template <class T, class S, class P, class Method>
+  static bool Dispatch(const Message* msg,
+                       T* obj,
+                       S* sender,
+                       P* parameter,
+                       Method func) {
+    SendParam send_params;
+    bool ok = ReadSendParam(msg, &send_params);
+    Message* reply = SyncMessage::GenerateReply(msg);
+    if (ok) {
+      ReplyParam reply_params;
+      base::DispatchToMethod(obj, func, send_params, &reply_params);
+      WriteParam(reply, reply_params);
+      LogReplyParamsToMessage(reply_params, msg);
+    } else {
+      NOTREACHED() << "Error deserializing message " << msg->type();
+      reply->set_reply_error();
+    }
+    sender->Send(reply);
+    return ok;
+  }
+
+  template <class T, class P, class Method>
+  static bool DispatchDelayReply(const Message* msg,
+                                 T* obj,
+                                 P* parameter,
+                                 Method func) {
+    SendParam send_params;
+    bool ok = ReadSendParam(msg, &send_params);
+    Message* reply = SyncMessage::GenerateReply(msg);
+    if (ok) {
+      base::Tuple<Message&> t = base::MakeRefTuple(*reply);
+      ConnectMessageAndReply(msg, reply);
+      base::DispatchToMethod(obj, func, send_params, &t);
+    } else {
+      NOTREACHED() << "Error deserializing message " << msg->type();
+      reply->set_reply_error();
+      obj->Send(reply);
+    }
+    return ok;
+  }
+
+ private:
+  MessageT(Routing routing, const Ins&... ins, Outs*... outs);
+};
+
+}  // namespace IPC
+
+#if defined(IPC_MESSAGE_IMPL)
+#include "ipc/ipc_message_templates_impl.h"
+#endif
+
+#endif  // IPC_IPC_MESSAGE_TEMPLATES_H_