[Base] Use variadic template in base/callback.h (wave 1)

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 63 matching lines @@
   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) &&
                             !is_const<T>::value;
 };
 
+// Returns Then as |type| if condition is true. Otherwise returns Else.

[Aaron Boodman, 2014/09/30 21:05:37]

The pipes are usually used around param names in chromium, so i think:

Returns |Then| as SelectType::type when |condition| is true. Otherwise returns
|Else|.

[tzik, 2014/10/01 01:22:40]

Done.

+template <bool condition, typename Then, typename Else>
+struct SelectType {
+  typedef Then type;
+};
+
+template <typename Then, typename Else>
+struct SelectType<false, Then, Else> {
+  typedef Else type;
+};
+
+template <typename>
+struct CallbackParamTraitsForMoveOnlyType;
+
+template <typename>
+struct CallbackParamTraitsForNonMoveOnlyType;

[Nico, 2014/09/30 11:41:04]

Should there be a "TODO: Use a default parameter once MSVS supports variadic
templates with default values"?

[tzik, 2014/09/30 14:43:10]

Done.

+
 // This is a typetraits object that's used to take an argument type, and
 // extract a suitable type for storing and forwarding arguments.
 //
 // In particular, it strips off references, and converts arrays to
 // pointers for storage; and it avoids accidentally trying to create a
 // "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, bool is_move_only = IsMoveOnlyType<T>::value>
-struct CallbackParamTraits {
+template <typename T>
+struct CallbackParamTraits
+    : SelectType<IsMoveOnlyType<T>::value,
+         CallbackParamTraitsForMoveOnlyType<T>,
+         CallbackParamTraitsForNonMoveOnlyType<T> >::type {
+};
+
+template <typename T>
+struct CallbackParamTraitsForNonMoveOnlyType {
   typedef const T& ForwardType;
   typedef T StorageType;
 };
 
 // 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 CallbackParamTraits<T&, false> {
+struct CallbackParamTraitsForNonMoveOnlyType<T&> {
   typedef T& ForwardType;
   typedef T StorageType;
 };
 
 // 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 CallbackParamTraits<T[n], false> {
+struct CallbackParamTraitsForNonMoveOnlyType<T[n]> {
   typedef const T* ForwardType;
   typedef const T* StorageType;
 };
 
 // See comment for CallbackParamTraits<T[n]>.
 template <typename T>
-struct CallbackParamTraits<T[], false> {
+struct CallbackParamTraitsForNonMoveOnlyType<T[]> {
   typedef const T* ForwardType;
   typedef const T* StorageType;
 };
 
 // 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 CallbackParamTraits<T, true> {
+struct CallbackParamTraitsForMoveOnlyType {
   typedef T ForwardType;
   typedef T StorageType;
 };
 
 // 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
@@ skipping 14 matching lines @@
 
 template <typename T>
 typename enable_if<IsMoveOnlyType<T>::value, T>::type CallbackForward(T& t) {
   return t.Pass();
 }
 
 }  // namespace internal
 }  // namespace base
 
 #endif  // BASE_CALLBACK_INTERNAL_H_