Hide NSAutoreleasePool from ARC translation units.

base/message_loop/message_pump_mac.h

Index: base/message_loop/message_pump_mac.h
diff --git a/base/message_loop/message_pump_mac.h b/base/message_loop/message_pump_mac.h
index 424cb70938d058a90288ceb5314bf73c8ae3200c..c075b28cc941305800d6fb4d2e33ab495220dc5e 100644
--- a/base/message_loop/message_pump_mac.h
+++ b/base/message_loop/message_pump_mac.h
@@ -38,9 +38,7 @@
 
 #include "base/memory/weak_ptr.h"
 
-#if !defined(__OBJC__)
-class NSAutoreleasePool;
-#else  // !defined(__OBJC__)
+#if defined(__OBJC__)
 #if defined(OS_IOS)
 #import <Foundation/Foundation.h>
 #else
@@ -55,7 +53,7 @@ class NSAutoreleasePool;
 - (BOOL)isHandlingSendEvent;
 @end
 #endif  // !defined(OS_IOS)
-#endif  // !defined(__OBJC__)
+#endif  // defined(__OBJC__)
 
 namespace base {
 
@@ -63,6 +61,21 @@ class MessagePumpInstrumentation;
 class RunLoop;
 class TimeTicks;
 
+// AutoreleasePoolType is a proxy type for autorelease pools. Its definition
+// depends on the TU in which this header appears. In pure C++ TUs, it is

[Mark Mentovai, 2014/04/29 02:07:03]

TU is not a commonly-understood abbreviation. Say “translation unit” the first
time, and you can leave the rest as TU.

+// defined as a forward C++ class declaration (that is never defined), because
+// autorelease pools are an Objective-C concept. In Automatic Reference Counting
+// (ARC) Objective-C TUs, it is similarly defined as a forward C++ class
+// declaration, because clang will not allow the type "NSAutoreleasePool" in
+// such TUs. Finally, in Manual Retain Release (MRR) Objective-C TUs, it is a
+// type alias for NSAutoreleasePool. In all cases, a method that takes or
+// returns an NSAutoreleasePool* can use AutoreleasePoolType* instead.
+#if !defined(__OBJC__) || __has_feature(objc_arc)
+class AutoreleasePoolType;
+#else   // !defined(__OBJC__) || __has_feature(objc_arc)
+typedef NSAutoreleasePool AutoreleasePoolType;
+#endif  // !defined(__OBJC__) || __has_feature(objc_arc)
+
 class MessagePumpCFRunLoopBase : public MessagePump {
   // Needs access to CreateAutoreleasePool.
   friend class MessagePumpScopedAutoreleasePool;
@@ -94,7 +107,7 @@ class MessagePumpCFRunLoopBase : public MessagePump {
   // In some cases, CreateAutoreleasePool may return nil intentionally to
   // preventing an autorelease pool from being created, allowing any
   // objects autoreleased by work to fall into the current autorelease pool.
-  virtual NSAutoreleasePool* CreateAutoreleasePool();
+  virtual AutoreleasePoolType* CreateAutoreleasePool();
 
   // Enables instrumentation of the MessagePump. See MessagePumpInstrumentation
   // in the implementation for details.
@@ -296,7 +309,7 @@ class MessagePumpCrApplication : public MessagePumpNSApplication {
  protected:
   // Returns nil if NSApp is currently in the middle of calling
   // -sendEvent.  Requires NSApp implementing CrAppProtocol.
-  virtual NSAutoreleasePool* CreateAutoreleasePool() OVERRIDE;
+  virtual AutoreleasePoolType* CreateAutoreleasePool() OVERRIDE;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(MessagePumpCrApplication);