Explicitly delete some Callback constructors to improve errors.

base/callback_internal.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.
 
 // This file contains utility functions and classes that help the
 // implementation, and management of the Callback objects.
 
 #ifndef BASE_CALLBACK_INTERNAL_H_
 #define BASE_CALLBACK_INTERNAL_H_
 
@@ skipping 61 matching lines @@
 // Holds the Callback methods that don't require specialization to reduce
 // template bloat.
 // CallbackBase<MoveOnly> is a direct base class of MoveOnly callbacks, and
 // CallbackBase<Copyable> uses CallbackBase<MoveOnly> for its implementation.
 template <>
 class BASE_EXPORT CallbackBase<CopyMode::MoveOnly> {
  public:
   CallbackBase(CallbackBase&& c);
   CallbackBase& operator=(CallbackBase&& c);
 
+  CallbackBase(const CallbackBase&) = delete;
+  CallbackBase& operator=(const CallbackBase&) = delete;

[dcheng, 2016/12/19 11:07:40]

Deleting it in the Callback template helper itself would improve the error even
further, but it's tricky to do this:
http://stackoverflow.com/questions/27073082/conditionally-disabling-a-copy-co...

I'm not even sure it's possible to apply that pattern here, as it defines a
inheritance relation between the partial specialization and the base template...
but the Callback template already inherits from other things.

[brucedawson, 2016/12/19 19:16:00]

Simple is probably better in that case.

+
   explicit CallbackBase(const CallbackBase<CopyMode::Copyable>& c);
   CallbackBase& operator=(const CallbackBase<CopyMode::Copyable>& c);
 
   // Returns true if Callback is null (doesn't refer to anything).
   bool is_null() const { return bind_state_.get() == NULL; }
   explicit operator bool() const { return !is_null(); }
 
   // Returns true if the callback invocation will be nop due to an cancellation.
   // It's invalid to call this on uninitialized callback.
   bool IsCancelled() const;
@@ skipping 38 matching lines @@
   ~CallbackBase() {}
 };
 
 extern template class CallbackBase<CopyMode::MoveOnly>;
 extern template class CallbackBase<CopyMode::Copyable>;
 
 }  // namespace internal
 }  // namespace base
 
 #endif  // BASE_CALLBACK_INTERNAL_H_