Extend base::Callback to have a move-only variant

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"
 #include "base/template_util.h"
 
 namespace base {
 namespace internal {
+template <CopyMode copy_mode>
 class CallbackBase;
 
 // BindStateBase is used to provide an opaque handle that the Callback
 // class can use to represent a function object with bound arguments.  It
 // behaves as an existential type that is used by a corresponding
 // DoInvoke function to perform the function execution.  This allows
 // us to shield the Callback class from the types of the bound argument via
 // "type erasure."
 // At the base level, the only task is to add reference counting data. Don't use
 // RefCountedThreadSafe since it requires the destructor to be a virtual method.
 // Creating a vtable for every BindState template instantiation results in a lot
 // of bloat. Its only task is to call the destructor which can be done with a
 // function pointer.
 class BindStateBase {
  protected:
   explicit BindStateBase(void (*destructor)(BindStateBase*))
       : ref_count_(0), destructor_(destructor) {}
   ~BindStateBase() = default;
 
  private:
   friend class scoped_refptr<BindStateBase>;
+  template <CopyMode copy_mode>
   friend class CallbackBase;
 
   void AddRef();
   void Release();
 
   AtomicRefCount ref_count_;
 
   // Pointer to a function that will properly destroy |this|.
   void (*destructor_)(BindStateBase*);
 
   DISALLOW_COPY_AND_ASSIGN(BindStateBase);
 };
 
 // Holds the Callback methods that don't require specialization to reduce
 // template bloat.
-class BASE_EXPORT CallbackBase {
+// 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(const CallbackBase& c);
-  CallbackBase& operator=(const CallbackBase& c);
+  CallbackBase(CallbackBase&& c);
+  CallbackBase& operator=(CallbackBase&& c);
 
   // Returns true if Callback is null (doesn't refer to anything).
   bool is_null() const { return bind_state_.get() == NULL; }
 
   // Returns the Callback into an uninitialized state.
   void Reset();
 
  protected:
   // In C++, it is safe to cast function pointers to function pointers of
   // another type. It is not okay to use void*. We create a InvokeFuncStorage
   // that that can store our function pointer, and then cast it back to
   // the original type on usage.
   using InvokeFuncStorage = void(*)();
 
   // Returns true if this callback equals |other|. |other| may be null.
-  bool Equals(const CallbackBase& other) const;
+  bool EqualsInternal(const CallbackBase& other) const;
 
   // Allow initializing of |bind_state_| via the constructor to avoid default
   // initialization of the scoped_refptr.  We do not also initialize
   // |polymorphic_invoke_| here because doing a normal assignment in the
   // derived Callback templates makes for much nicer compiler errors.
   explicit CallbackBase(BindStateBase* bind_state);
 
   // Force the destructor to be instantiated inside this translation unit so
   // that our subclasses will not get inlined versions.  Avoids more template
   // bloat.
   ~CallbackBase();
 
   scoped_refptr<BindStateBase> bind_state_;
-  InvokeFuncStorage polymorphic_invoke_;
+  InvokeFuncStorage polymorphic_invoke_ = nullptr;
 };
 
+// CallbackBase<Copyable> is a direct base class of Copyable Callbacks.
+template <>
+class BASE_EXPORT CallbackBase<CopyMode::Copyable>
+    : public CallbackBase<CopyMode::MoveOnly> {
+ public:
+  CallbackBase(const CallbackBase& c);
+  CallbackBase(CallbackBase&& c);
+  CallbackBase& operator=(const CallbackBase& c);
+  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>;

[Nico, 2016/03/09 03:24:53]

extern templates tend to always cause lots of problems. Hopefully these will
behave better – if so, maybe we can use these as model going forward.

+
 // 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 {
@@ skipping 101 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_