Remove CallbackParamTraits and CallbackForward

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_
 
-#include <stddef.h>
-#include <memory>
-#include <type_traits>
-#include <vector>
-
 #include "base/atomic_ref_count.h"
 #include "base/base_export.h"
 #include "base/callback_forward.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/scoped_ptr.h"
 
 namespace base {
 namespace internal {
 template <CopyMode copy_mode>
@@ skipping 84 matching lines @@
   CallbackBase& operator=(CallbackBase&& c);
  protected:
   explicit CallbackBase(BindStateBase* bind_state)
       : CallbackBase<CopyMode::MoveOnly>(bind_state) {}
   ~CallbackBase() {}
 };
 
 extern template class CallbackBase<CopyMode::MoveOnly>;
 extern template class CallbackBase<CopyMode::Copyable>;
 
-// A helper template to determine if given type is non-const move-only-type,
-// i.e. if a value of the given type should be passed via std::move() in a
-// destructive way. Types are considered to be move-only if they have a
-// sentinel MoveOnlyTypeForCPP03 member: a class typically gets this from using
-// the DISALLOW_COPY_AND_ASSIGN_WITH_MOVE_FOR_BIND macro.
-// It would be easy to generalize this trait to all move-only types... but this
-// confuses template deduction in VS2013 with certain types such as
-// std::unique_ptr.
-// TODO(dcheng): Revisit this when Windows switches to VS2015 by default.
-
-template <typename T> struct IsMoveOnlyType {
-  // Types YesType and NoType are guaranteed such that sizeof(YesType) <
-  // sizeof(NoType).
-  using YesType = char;
-  struct NoType { YesType dummy[2]; };
-
-  template <typename U>
-  static YesType Test(const typename U::MoveOnlyTypeForCPP03*);
-
-  template <typename U>
-  static NoType Test(...);
-
-  static const bool value = sizeof((Test<T>(0))) == sizeof(YesType) &&
-                            !std::is_const<T>::value;
-};
-
-// Specialization of IsMoveOnlyType so that std::unique_ptr is still considered
-// move-only, even without the sentinel member.
-template <typename T, typename D>
-struct IsMoveOnlyType<std::unique_ptr<T, D>> : std::true_type {};
-
-// Specialization of std::vector, so that it's considered move-only if the
-// element type is move-only. Allocator is explicitly ignored when determining
-// move-only status of the std::vector.
-template <typename T, typename Allocator>
-struct IsMoveOnlyType<std::vector<T, Allocator>> : IsMoveOnlyType<T> {};
-
-template <typename>
-struct CallbackParamTraitsForMoveOnlyType;
-
-template <typename>
-struct CallbackParamTraitsForNonMoveOnlyType;
-
-// TODO(tzik): Use a default parameter once MSVS supports variadic templates
-// with default values.
-// http://connect.microsoft.com/VisualStudio/feedbackdetail/view/957801/compilation-error-with-variadic-templates
-//
-// This is a typetraits object that's used to take an argument type, and
-// extract a suitable type for forwarding arguments.
-template <typename T>
-struct CallbackParamTraits
-    : std::conditional<IsMoveOnlyType<T>::value,
-         CallbackParamTraitsForMoveOnlyType<T>,
-         CallbackParamTraitsForNonMoveOnlyType<T>>::type {
-};
-
-template <typename T>
-struct CallbackParamTraitsForNonMoveOnlyType {
-  using ForwardType = const T&;
-};
-
-// Note that for array types, we implicitly add a const in the conversion. This
-// means that it is not possible to bind array arguments to functions that take
-// a non-const pointer. Trying to specialize the template based on a "const
-// T[n]" does not seem to match correctly, so we are stuck with this
-// restriction.
-template <typename T, size_t n>
-struct CallbackParamTraitsForNonMoveOnlyType<T[n]> {
-  using ForwardType = const T*;
-};
-
-// See comment for CallbackParamTraits<T[n]>.
-template <typename T>
-struct CallbackParamTraitsForNonMoveOnlyType<T[]> {
-  using ForwardType = const T*;
-};
-
-// Parameter traits for movable-but-not-copyable scopers.
-//
-// Callback<>/Bind() understands movable-but-not-copyable semantics where
-// the type cannot be copied but can still have its state destructively
-// transferred (aka. moved) to another instance of the same type by calling a
-// helper function.  When used with Bind(), this signifies transferal of the
-// object's state to the target function.
-//
-// For these types, the ForwardType must not be a const reference, or a
-// reference.  A const reference is inappropriate, and would break const
-// correctness, because we are implementing a destructive move.  A non-const
-// reference cannot be used with temporaries which means the result of a
-// function or a cast would not be usable with Callback<> or Bind().
-template <typename T>
-struct CallbackParamTraitsForMoveOnlyType {
-  using ForwardType = T;
-};
-
-// CallbackForward() is a very limited simulation of C++11's std::forward()
-// used by the Callback/Bind system for a set of movable-but-not-copyable
-// types.  It is needed because forwarding a movable-but-not-copyable
-// argument to another function requires us to invoke the proper move
-// operator to create a rvalue version of the type.  The supported types are
-// whitelisted below as overloads of the CallbackForward() function. The
-// default template compiles out to be a no-op.
-//
-// In C++11, std::forward would replace all uses of this function.  However, it
-// is impossible to implement a general std::forward without C++11 due to a lack
-// of rvalue references.
-//
-// In addition to Callback/Bind, this is used by PostTaskAndReplyWithResult to
-// simulate std::forward() and forward the result of one Callback as a
-// parameter to another callback. This is to support Callbacks that return
-// the movable-but-not-copyable types whitelisted above.
-template <typename T>
-typename std::enable_if<!IsMoveOnlyType<T>::value, T>::type& CallbackForward(

[Nico, 2016/03/10 21:54:50]

Don't we lose this protection now? Or does std::foward() have this too?

[tzik, 2016/03/14 08:49:42]

Do you mean a protection to prevent moving non-move-only type?
Yes, we'll lose it. We'll no longer check if the type is move-only. std::forward
doesn't check if it's movable. I think it was just needed to do forwarding
without rvalue reference, but no longer needed.

-    T& t) {
-  return t;
-}
-
-template <typename T>
-typename std::enable_if<IsMoveOnlyType<T>::value, T>::type CallbackForward(
-    T& t) {
-  return std::move(t);
-}
-
 }  // namespace internal
 }  // namespace base
 
 #endif  // BASE_CALLBACK_INTERNAL_H_