Redo r113722 - Add Pass(), which implements move semantics, to scoped_ptr, scoped_array....

base/bind_helpers.h

Index: base/bind_helpers.h
diff --git a/base/bind_helpers.h b/base/bind_helpers.h
index 6e0f8fe72742a06f7875d5f73a4b9e0ec97f6c45..df8cf82bca5994f41db931999410f0223040642c 100644
--- a/base/bind_helpers.h
+++ b/base/bind_helpers.h
@@ -6,19 +6,29 @@
 // can be used specify the refcounting and reference semantics of arguments
 // that are bound by the Bind() function in base/bind.h.
 //
-// The public functions are base::Unretained(), base::Owned(),
-// base::ConstRef(), and base::IgnoreReturn().
+// The public functions are base::Unretained(), base::Owned(), bass::Passed(),
+// base::ConstRef(), and base::IgnoreResult().
 //
 // Unretained() allows Bind() to bind a non-refcounted class, and to disable
 // refcounting on arguments that are refcounted objects.
+//
 // Owned() transfers ownership of an object to the Callback resulting from
 // bind; the object will be deleted when the Callback is deleted.
+//
+// Passed() is for transferring movable-but-not-copyable types (eg. scoped_ptr)
+// through a Callback. Logically, this signifies a destructive transfer of
+// the state of the argument into the target function.  Invoking
+// Callback::Run() twice on a Callback that was created with a Passed()
+// argument will CHECK() because the first invocation would have already
+// transferred ownership to the target function.
+//
 // ConstRef() allows binding a constant reference to an argument rather
 // than a copy.
-// IgnoreReturn() is used to adapt a 0-argument Callback with a return type to
-// a Closure. This is useful if you need to PostTask with a function that has
-// a return value that you don't care about.
 //
+// IgnoreResult() is used to adapt a function or Callback with a return type to
+// one with a void return. This is most useful if you have a function with,
+// say, a pesky ignorable bool return that you want to use with PostTask or
+// something else that expect a Callback with a void return.
 //
 // EXAMPLE OF Unretained():
 //
@@ -75,13 +85,45 @@
 // its bound callbacks.
 //
 //
-// EXAMPLE OF IgnoreReturn():
+// EXAMPLE OF IgnoreResult():
 //
 //   int DoSomething(int arg) { cout << arg << endl; }
-//   Callback<int(void)> cb = Bind(&DoSomething, 1);
-//   Closure c = IgnoreReturn(cb);  // Prints "1"
-//       or
-//   ml->PostTask(FROM_HERE, IgnoreReturn(cb));  // Prints "1" on |ml|
+//
+//   // Assign to a Callback with a void return type.
+//   Callback<void(int)> cb = Bind(IgnoreResult(&DoSomething));
+//   cb->Run(1);  // Prints "1".
+//
+//   // Prints "1" on |ml|.
+//   ml->PostTask(FROM_HERE, Bind(IgnoreResult(&DoSomething), 1);
+//
+//
+// EXAMPLE OF Passed():
+//
+//   void TakesOwnership(scoped_ptr<Foo> arg) { }
+//   scoped_ptr<Foo> CreateFoo() { return scoped_ptr<Foo>(new Foo()); }
+//
+//   scoped_ptr<Foo> f(new Foo());
+//
+//   // |cb| is given ownership of Foo(). |f| is now NULL.
+//   // You can use f.Pass() in place of &f, but it's more verbose.
+//   Closure cb = Bind(&TakesOwnership, Passed(&f));
+//
+//   // Run was never called so |cb| still owns Foo() and deletes
+//   // it on Reset().
+//   cb.Reset();
+//
+//   // |cb| is given a new Foo created by CreateFoo().
+//   cb = Bind(&TakesOwnership, Passed(CreateFoo()));
+//
+//   // |arg| in TakesOwnership() is given ownership of Foo(). |cb|
+//   // no longer owns Foo() and, if reset, would not delete Foo().
+//   cb.Run();  // Foo() is now transferred to |arg| and deleted.
+//   cb.Run();  // This CHECK()s since Foo() already been used once.
+//
+// Passed() is particularly useful with PostTask() when you are transferring
+// ownership of an argument into a task, but don't necessarily know if the
+// task will always be executed. This can happen if the task is cancellable
+// or if it is posted to a MessageLoopProxy.
 
 #ifndef BASE_BIND_HELPERS_H_
 #define BASE_BIND_HELPERS_H_
@@ -287,6 +329,45 @@ class OwnedWrapper {
   mutable T* ptr_;
 };
 
+// PassedWrapper is a copyable adapter for a scoper that ignores const.
+//
+// It is needed to get around the fact that Bind() takes a const reference to
+// all its arguments.  Because Bind() takes a const reference to avoid
+// unnecessary copies, it is incompatible with movable-but-not-copyable
+// types; doing a destructive "move" of the type into Bind() would violate
+// the const correctness.
+//
+// This conundrum cannot be solved without either C++11 rvalue references or
+// a O(2^n) blowup of Bind() templates to handle each combination of regular
+// types and movable-but-not-copyable types.  Thus we introduce a wrapper type
+// that is copyable to transmit the correct type information down into
+// BindState<>. Ignoring const in this type makes sense because it is only
+// created when we are explicitly trying to do a destructive move.
+//
+// Two notes:
+//  1) PassedWrapper supports any type that has a "Pass()" function.
+//     This is intentional. The whitelisting of which specific types we
+//     support is maintained by CallbackParamTraits<>.
+//  2) is_valid_ is distinct from NULL because it is valid to bind a "NULL"
+//     scoper to a Callback and allow the Callback to execute once.
+template <typename T>
+class PassedWrapper {
+ public:
+  explicit PassedWrapper(T scoper) : is_valid_(true), scoper_(scoper.Pass()) {}
+  PassedWrapper(const PassedWrapper& other)
+      : is_valid_(other.is_valid_), scoper_(other.scoper_.Pass()) {
+  }
+  T Pass() const {
+    CHECK(is_valid_);
+    is_valid_ = false;
+    return scoper_.Pass();
+  }
+
+ private:
+  mutable bool is_valid_;
+  mutable T scoper_;
+};
+
 // Unwrap the stored parameters for the wrappers above.
 template <typename T>
 struct UnwrapTraits {
@@ -330,9 +411,17 @@ struct UnwrapTraits<OwnedWrapper<T> > {
   }
 };
 
+template <typename T>
+struct UnwrapTraits<PassedWrapper<T> > {
+  typedef T ForwardType;
+  static T Unwrap(PassedWrapper<T>& o) {
+    return o.Pass();
+  }
+};
+
 // Utility for handling different refcounting semantics in the Bind()
 // function.
-template <bool, typename T>
+template <bool is_method, typename T>
 struct MaybeRefcount;
 
 template <typename T>
@@ -348,21 +437,15 @@ struct MaybeRefcount<false, T[n]> {
 };
 
 template <typename T>
-struct MaybeRefcount<true, T*> {
-  static void AddRef(T* o) { o->AddRef(); }
-  static void Release(T* o) { o->Release(); }
-};
-
-template <typename T>
-struct MaybeRefcount<true, UnretainedWrapper<T> > {
-  static void AddRef(const UnretainedWrapper<T>&) {}
-  static void Release(const UnretainedWrapper<T>&) {}
+struct MaybeRefcount<true, T> {
+  static void AddRef(const T&) {}
+  static void Release(const T&) {}
 };
 
 template <typename T>
-struct MaybeRefcount<true, OwnedWrapper<T> > {
-  static void AddRef(const OwnedWrapper<T>&) {}
-  static void Release(const OwnedWrapper<T>&) {}
+struct MaybeRefcount<true, T*> {
+  static void AddRef(T* o) { o->AddRef(); }
+  static void Release(T* o) { o->Release(); }
 };
 
 // No need to additionally AddRef() and Release() since we are storing a
@@ -379,19 +462,13 @@ struct MaybeRefcount<true, const T*> {
   static void Release(const T* o) { o->Release(); }
 };
 
-template <typename T>
-struct MaybeRefcount<true, WeakPtr<T> > {
-  static void AddRef(const WeakPtr<T>&) {}
-  static void Release(const WeakPtr<T>&) {}
-};
-
 template <typename R>
 void VoidReturnAdapter(Callback<R(void)> callback) {
   callback.Run();
 }
 
 // IsWeakMethod is a helper that determine if we are binding a WeakPtr<> to a
-// method.  It is unsed internally by Bind() to select the correct
+// method.  It is used internally by Bind() to select the correct
 // InvokeHelper that will no-op itself in the event the WeakPtr<> for
 // the target object is invalidated.
 //
@@ -422,6 +499,20 @@ static inline internal::OwnedWrapper<T> Owned(T* o) {
   return internal::OwnedWrapper<T>(o);
 }
 
+// We offer 2 syntaxes for calling Passed().  The first takes a temporary and
+// is best suited for use with the return value of a function. The second
+// takes a pointer to the scoper and is just syntactic sugar to avoid having
+// to write Passed(scoper.Pass()).
+template <typename T>
+static inline internal::PassedWrapper<T> Passed(T scoper) {
+  return internal::PassedWrapper<T>(scoper.Pass());
+}
+template <typename T>
+static inline internal::PassedWrapper<T> Passed(T* scoper) {
+  return internal::PassedWrapper<T>(scoper->Pass());
+}
+
+// -- DEPRECATED -- Use IgnoreResult instead.
 template <typename R>
 static inline Closure IgnoreReturn(Callback<R(void)> callback) {
   return Bind(&internal::VoidReturnAdapter<R>, callback);
@@ -438,7 +529,6 @@ IgnoreResult(const Callback<T>& data) {
   return internal::IgnoreResultHelper<Callback<T> >(data);
 }
 
-
 }  // namespace base
 
 #endif  // BASE_BIND_HELPERS_H_