Introduce base::IsWeakReceiver for base::Bind to support external weak pointers

base/bind_helpers.h

 // Copyright (c) 2011 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.
 
 // This defines a set of argument wrappers and related factory methods that
 // can be used specify the refcounting and reference semantics of arguments
 // that are bound by the Bind() function in base/bind.h.
 //
 // It also defines a set of simple functions and utilities that people want
 // when using Callback<> and Bind().
@@ skipping 152 matching lines @@
 #include <stddef.h>
 
 #include <type_traits>
 #include <utility>
 
 #include "base/callback.h"
 #include "base/memory/weak_ptr.h"
 #include "build/build_config.h"
 
 namespace base {
+
+template <typename T>
+struct IsWeakReceiver;
+
 namespace internal {
 
 // Use the Substitution Failure Is Not An Error (SFINAE) trick to inspect T
 // for the existence of AddRef() and Release() functions of the correct
 // signature.
 //
 // http://en.wikipedia.org/wiki/Substitution_failure_is_not_an_error
 // http://stackoverflow.com/questions/257288/is-it-possible-to-write-a-c-template-to-check-for-a-functions-existence
 // http://stackoverflow.com/questions/4358584/sfinae-approach-comparison
 // http://stackoverflow.com/questions/1966362/sfinae-to-check-for-inherited-member-functions
@@ skipping 269 matching lines @@
 }
 
 // IsWeakMethod is a helper that determine if we are binding a WeakPtr<> to a
 // method.  It is used internally by Bind() to select the correct
 // InvokeHelper that will no-op itself in the event the WeakPtr<> for
 // the target object is invalidated.
 //
 // The first argument should be the type of the object that will be received by
 // the method.
 template <bool IsMethod, typename... Args>
-struct IsWeakMethod : public std::false_type {};
+struct IsWeakMethod : std::false_type {};
 
 template <typename T, typename... Args>
-struct IsWeakMethod<true, WeakPtr<T>, Args...> : public std::true_type {};
-
-template <typename T, typename... Args>
-struct IsWeakMethod<true, ConstRefWrapper<WeakPtr<T>>, Args...>
-    : public std::true_type {};
-
+struct IsWeakMethod<true, T, Args...> : IsWeakReceiver<T> {};
 
 // Packs a list of types to hold them in a single type.
 template <typename... Types>
 struct TypeList {};
 
 // Used for DropTypeListItem implementation.
 template <size_t n, typename List>
 struct DropTypeListItemImpl;
 
 // Do not use enable_if and SFINAE here to avoid MSVC2013 compile failure.
@@ skipping 139 matching lines @@
   return internal::IgnoreResultHelper<Callback<T> >(data);
 }
 
 BASE_EXPORT void DoNothing();
 
 template<typename T>
 void DeletePointer(T* obj) {
   delete obj;
 }
 
+// An injection point to control |this| pointer behavior on a method invocation.
+// If IsWeakReceiver<> is true_type for |T| and |T| is used for a receiver of a
+// method, base::Bind cancels the method invocation if the receiver is tested as
+// false.
+// E.g. Foo::bar() is not called:
+//   struct Foo : base::SupportsWeakPtr<Foo> {
+//     void bar() {}
+//   };
+//
+//   WeakPtr<Foo> oo = nullptr;
+//   base::Bind(&Foo::bar, oo).Run();
+template <typename T>
+struct IsWeakReceiver : std::false_type {};
+
+template <typename T>
+struct IsWeakReceiver<internal::ConstRefWrapper<T>> : IsWeakReceiver<T> {};
+
+template <typename T>
+struct IsWeakReceiver<WeakPtr<T>> : std::true_type {};
+
 }  // namespace base
 
 #endif  // BASE_BIND_HELPERS_H_