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

base/callback_internal.h

Index: base/callback_internal.h
diff --git a/base/callback_internal.h b/base/callback_internal.h
index 3e8ee82e4f246c5a875ba97d5ae219097e745f19..fbeac185a1037d163d6d2745bc7d330f6aea0135 100644
--- a/base/callback_internal.h
+++ b/base/callback_internal.h
@@ -22,7 +22,7 @@
 
 namespace base {
 namespace internal {
-class CallbackBase;
+class MoveOnlyCallbackBase;
 
 // BindStateBase is used to provide an opaque handle that the Callback
 // class can use to represent a function object with bound arguments.  It
@@ -43,7 +43,7 @@ class BindStateBase {
 
  private:
   friend class scoped_refptr<BindStateBase>;
-  friend class CallbackBase;
+  friend class MoveOnlyCallbackBase;
 
   void AddRef();
   void Release();
@@ -58,10 +58,12 @@ class BindStateBase {
 
 // Holds the Callback methods that don't require specialization to reduce
 // template bloat.
-class BASE_EXPORT CallbackBase {
+// MoveOnlyCallbackBase is a direct base class of MoveOnly callbacks, and
+// CopyableCallbackBase uses MoveOnlyCallbackBase for its implementation.
+class BASE_EXPORT MoveOnlyCallbackBase {
  public:
-  CallbackBase(const CallbackBase& c);
-  CallbackBase& operator=(const CallbackBase& c);
+  MoveOnlyCallbackBase(MoveOnlyCallbackBase&& c);
+  MoveOnlyCallbackBase& operator=(MoveOnlyCallbackBase&& c);
 
   // Returns true if Callback is null (doesn't refer to anything).
   bool is_null() const { return bind_state_.get() == NULL; }
@@ -77,21 +79,31 @@ class BASE_EXPORT CallbackBase {
   using InvokeFuncStorage = void(*)();
 
   // Returns true if this callback equals |other|. |other| may be null.
-  bool Equals(const CallbackBase& other) const;
+  bool EqualsInternal(const MoveOnlyCallbackBase& 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);
+  void set_bind_state(BindStateBase* bind_state);
+
+  MoveOnlyCallbackBase();
 
   // Force the destructor to be instantiated inside this translation unit so
   // that our subclasses will not get inlined versions.  Avoids more template
   // bloat.
-  ~CallbackBase();
+  ~MoveOnlyCallbackBase();
 
   scoped_refptr<BindStateBase> bind_state_;
-  InvokeFuncStorage polymorphic_invoke_;
+  InvokeFuncStorage polymorphic_invoke_ = nullptr;
+};
+
+// CopyableCallbackBase is a direct base class of Copyable Callbacks.
+class BASE_EXPORT CopyableCallbackBase : public MoveOnlyCallbackBase {
+ public:
+  CopyableCallbackBase(const CopyableCallbackBase& c);
+  CopyableCallbackBase(CopyableCallbackBase&& c);
+  CopyableCallbackBase& operator=(const CopyableCallbackBase& c);
+  CopyableCallbackBase& operator=(CopyableCallbackBase&& c);
+ protected:
+  CopyableCallbackBase() {}
+  ~CopyableCallbackBase() {}
 };
 
 // A helper template to determine if given type is non-const move-only-type,