[Sync] Add type conversion constructor for WeakHandle

chrome/browser/sync/util/weak_handle.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.
 
 // Weak handles provides a way to refer to weak pointers from another
 // thread.  This is useful because it is not safe to reference a weak
 // pointer from a thread other than the thread on which it was
 // created.
 //
 // Weak handles can be passed across threads, so for example, you can
@@ skipping 36 matching lines @@
 #ifndef CHROME_BROWSER_SYNC_UTIL_WEAK_HANDLE_H_
 #define CHROME_BROWSER_SYNC_UTIL_WEAK_HANDLE_H_
 #pragma once
 
 #include <cstddef>
 
 #include "base/basictypes.h"
 #include "base/bind.h"
 #include "base/callback.h"
 #include "base/compiler_specific.h"
+#include "base/gtest_prod_util.h"
 #include "base/location.h"
 #include "base/logging.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/weak_ptr.h"
 
 namespace base {
 class MessageLoopProxy;
 }  // namespace base
 
 namespace tracked_objects {
@@ skipping 206 matching lines @@
 template <typename T>
 class WeakHandle {
  public:
   // Creates an uninitialized WeakHandle.
   WeakHandle() {}
 
   // Creates an initialized WeakHandle from |ptr|.
   explicit WeakHandle(const base::WeakPtr<T>& ptr)
       : core_(new internal::WeakHandleCore<T>(ptr)) {}
 
+  // Allow conversion from WeakHandle<U> to WeakHandle<T> if U is
+  // convertible to T, but we *must* be on |other|'s owner thread.
+  // Note that this doesn't override the regular copy constructor, so
+  // that one can be called on any thread.
+  template <typename U>
+  WeakHandle(const browser_sync::WeakHandle<U>& other)  // NOLINT
+      : core_(
+          other.IsInitialized() ?
+          new internal::WeakHandleCore<T>(other.Get()) :
+          NULL) {}
+
   // Returns true iff this WeakHandle is initialized.  Note that being
   // initialized isn't a guarantee that the underlying object is still
   // alive.
   bool IsInitialized() const {
     return core_.get() != NULL;
   }
 
   // Resets to an uninitialized WeakHandle.
   void Reset() {
     core_ = NULL;
@@ skipping 48 matching lines @@
             void (U::*fn)(A1, A2, A3, A4),
             typename internal::ParamTraits<A1>::ForwardType a1,
             typename internal::ParamTraits<A2>::ForwardType a2,
             typename internal::ParamTraits<A3>::ForwardType a3,
             typename internal::ParamTraits<A4>::ForwardType a4) const {
     CHECK(IsInitialized());
     core_->Call(from_here, fn, a1, a2, a3, a4);
   }
 
  private:
+  FRIEND_TEST_ALL_PREFIXES(WeakHandleTest,
+                           TypeConversionConstructor);
+  FRIEND_TEST_ALL_PREFIXES(WeakHandleTest,
+                           TypeConversionConstructorAssignment);
+
   scoped_refptr<internal::WeakHandleCore<T> > core_;
 };
 
 // Makes a WeakHandle from a WeakPtr.
 template <typename T>
 WeakHandle<T> MakeWeakHandle(const base::WeakPtr<T>& ptr) {
   return WeakHandle<T>(ptr);
 }
 
 }  // namespace browser_sync
 
 #endif  // CHROME_BROWSER_SYNC_UTIL_WEAK_HANDLE_H_