Do Perfect Forwarding from base::Bind to BindState storage

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 111 matching lines @@
 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 storing and forwarding arguments.

[danakj, 2016/01/28 23:01:00]

Comment is out of date now

[tzik, 2016/02/01 15:00:16]

Done.

 //
 // In particular, it strips off references, and converts arrays to
 // pointers for storage; and it avoids accidentally trying to create a

[danakj, 2016/01/28 23:01:00]

And here

[tzik, 2016/02/01 15:00:16]

Done.

 // "reference of a reference" if the argument is a reference type.
 //
 // This array type becomes an issue for storage because we are passing bound
 // parameters by const reference. In this case, we end up passing an actual
 // array type in the initializer list which C++ does not allow.  This will
 // break passing of C-string literals.
 template <typename T>
 struct CallbackParamTraits
     : std::conditional<IsMoveOnlyType<T>::value,
          CallbackParamTraitsForMoveOnlyType<T>,
          CallbackParamTraitsForNonMoveOnlyType<T>>::type {
 };
 
 template <typename T>
 struct CallbackParamTraitsForNonMoveOnlyType {
   using ForwardType = const T&;
-  using StorageType = T;
 };
 
 // The Storage should almost be impossible to trigger unless someone manually
 // specifies type of the bind parameters.  However, in case they do,
 // this will guard against us accidentally storing a reference parameter.
 //
 // The ForwardType should only be used for unbound arguments.
 template <typename T>
 struct CallbackParamTraitsForNonMoveOnlyType<T&> {

[danakj, 2016/01/28 23:01:00]

Do we still need this specialization?

[tzik, 2016/02/01 15:00:16]

Done.

   using ForwardType = T&;
-  using StorageType = 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*;
-  using StorageType = const T*;
 };
 
 // See comment for CallbackParamTraits<T[n]>.
 template <typename T>
 struct CallbackParamTraitsForNonMoveOnlyType<T[]> {
   using ForwardType = const T*;
-  using StorageType = 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;
-  using StorageType = 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.
 //
@@ skipping 14 matching lines @@
 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_