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

Add Pass(), which implements move semantics, to scoped_ptr, scoped_array, and scoped_ptr_malloc.

This modification to the scopers implements the "moveable but not copyable" semantics that were introduced in C++11's unique_ptr<>.

With this, is now possible to use scopers as an argument type or a return type.  This signifies, in the type system, transfer of ownership into a function or out of a function respectively.  Calling, or returning such a function MUST use the temporary resultin

This distinction makes it possible to avoid the implicit ownership transfer issues of auto_ptr, but still allow us to have compiler enforced ownership transfer.

Also adds a Passed() helper that allows using a scoper with Bind().

BUG=96118
TEST=new unittests

Original review URL: http://codereview.chromium.org/8774032

Review URL: http://codereview.chromium.org/8897005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@113922 0039d316-1c4b-4281-b951-d872f2087c98

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_

base/bind_internal.h.pump

@@ -82,7 +82,6 @@ namespace internal {
 //                 into the Bind() system, doing most of the type resolution.
 //                 There are ARITY BindState types.
 
-
 // RunnableAdapter<>
 //
 // The RunnableAdapter<> templates provide a uniform interface for invoking
@@ -121,7 +120,7 @@ class RunnableAdapter<R(*)($for ARG , [[A$(ARG)]])> {
   }
 
   R Run($for ARG , [[typename CallbackParamTraits<A$(ARG)>::ForwardType a$(ARG)]]) {
-    return function_($for ARG , [[a$(ARG)]]);
+    return function_($for ARG , [[CallbackForward(a$(ARG))]]);
   }
 
  private:
@@ -143,7 +142,7 @@ $if ARITY > 0[[, ]] $for ARG , [[A$(ARG)]]);
 
   R Run(T* object[[]]
 $if ARITY > 0[[, ]]  $for ARG, [[typename CallbackParamTraits<A$(ARG)>::ForwardType a$(ARG)]]) {
-    return (object->*method_)($for ARG , [[a$(ARG)]]);
+    return (object->*method_)($for ARG , [[CallbackForward(a$(ARG))]]);
   }
 
  private:
@@ -165,7 +164,7 @@ $if ARITY > 0[[, ]] $for ARG , [[A$(ARG)]]);
 
   R Run(const T* object[[]]
 $if ARITY > 0[[, ]]  $for ARG, [[typename CallbackParamTraits<A$(ARG)>::ForwardType a$(ARG)]]) {
-    return (object->*method_)($for ARG , [[a$(ARG)]]);
+    return (object->*method_)($for ARG , [[CallbackForward(a$(ARG))]]);
   }
 
  private:
@@ -291,7 +290,7 @@ struct InvokeHelper<false, ReturnType, Runnable,
     void($for ARG , [[A$(ARG)]])>  {
   static ReturnType MakeItSo(Runnable runnable[[]]
 $if ARITY > 0[[, ]] $for ARG , [[A$(ARG) a$(ARG)]]) {
-    return runnable.Run($for ARG , [[a$(ARG)]]);
+    return runnable.Run($for ARG , [[CallbackForward(a$(ARG))]]);
   }
 };
 
@@ -301,7 +300,7 @@ struct InvokeHelper<false, void, Runnable,
     void($for ARG , [[A$(ARG)]])>  {
   static void MakeItSo(Runnable runnable[[]]
 $if ARITY > 0[[, ]] $for ARG , [[A$(ARG) a$(ARG)]]) {
-    runnable.Run($for ARG , [[a$(ARG)]]);
+    runnable.Run($for ARG , [[CallbackForward(a$(ARG))]]);
   }
 };
 
@@ -316,7 +315,7 @@ $if ARITY > 0[[, ]] $for ARG , [[A$(ARG) a$(ARG)]]) {
       return;
     }
 
-    runnable.Run($for ARG , [[a$(ARG)]]);
+    runnable.Run($for ARG , [[CallbackForward(a$(ARG))]]);
   }
 };
 
@@ -404,7 +403,7 @@ typename CallbackParamTraits<X$(UNBOUND_ARG)>::ForwardType x$(UNBOUND_ARG)
 ]]
 )>
                ::MakeItSo(storage->runnable_
-$if ARITY > 0[[, ]] $for ARG , [[x$(ARG)]]);
+$if ARITY > 0[[, ]] $for ARG , [[CallbackForward(x$(ARG))]]);
   }
 };
 

base/bind_unittest.cc

@@ -5,6 +5,9 @@
 #include "base/bind.h"
 
 #include "base/callback.h"
+#include "base/memory/ref_counted.h"
+#include "base/memory/scoped_ptr.h"
+#include "base/memory/weak_ptr.h"
 #include "testing/gmock/include/gmock/gmock.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
@@ -102,7 +105,7 @@ class CopyCounter {
     (*copies_)++;
   }
 
-  // Probing for copies from coerscion.
+  // Probing for copies from coercion.
   CopyCounter(const DerivedCopyCounter& other)
       : copies_(other.copies_),
         assigns_(other.assigns_) {
@@ -149,6 +152,11 @@ class DeleteCounter {
   int* deletes_;
 };
 
+template <typename T>
+T PassThru(T scoper) {
+  return scoper.Pass();
+}
+
 // Some test functions that we can Bind to.
 template <typename T>
 T PolymorphicIdentity(T t) {
@@ -641,8 +649,8 @@ TEST_F(BindTest, Owned) {
   // return the same value.
   Callback<DeleteCounter*(void)> no_capture_cb =
       Bind(&PolymorphicIdentity<DeleteCounter*>, Owned(counter));
-  EXPECT_EQ(counter, no_capture_cb.Run());
-  EXPECT_EQ(counter, no_capture_cb.Run());
+  ASSERT_EQ(counter, no_capture_cb.Run());
+  ASSERT_EQ(counter, no_capture_cb.Run());
   EXPECT_EQ(0, deletes);
   no_capture_cb.Reset();  // This should trigger a delete.
   EXPECT_EQ(1, deletes);
@@ -657,11 +665,60 @@ TEST_F(BindTest, Owned) {
   EXPECT_EQ(1, deletes);
 }
 
+// Passed() wrapper support.
+//   - Passed() can be constructed from a pointer to scoper.
+//   - Passed() can be constructed from a scoper rvalue.
+//   - Using Passed() gives Callback Ownership.
+//   - Ownership is transferred from Callback to callee on the first Run().
+//   - Callback supports unbound arguments.
+TEST_F(BindTest, ScopedPtr) {
+  int deletes = 0;
+
+  // Tests the Passed() function's support for pointers.
+  scoped_ptr<DeleteCounter> ptr(new DeleteCounter(&deletes));
+  Callback<scoped_ptr<DeleteCounter>(void)> unused_callback =
+      Bind(&PassThru<scoped_ptr<DeleteCounter> >, Passed(&ptr));
+  EXPECT_FALSE(ptr.get());
+  EXPECT_EQ(0, deletes);
+
+  // If we never invoke the Callback, it retains ownership and deletes.
+  unused_callback.Reset();
+  EXPECT_EQ(1, deletes);
+
+  // Tests the Passed() function's support for rvalues.
+  deletes = 0;
+  DeleteCounter* counter = new DeleteCounter(&deletes);
+  Callback<scoped_ptr<DeleteCounter>(void)> callback =
+      Bind(&PassThru<scoped_ptr<DeleteCounter> >,
+           Passed(scoped_ptr<DeleteCounter>(counter)));
+  EXPECT_FALSE(ptr.get());
+  EXPECT_EQ(0, deletes);
+
+  // Check that ownership can be transferred back out.
+  scoped_ptr<DeleteCounter> result = callback.Run();
+  ASSERT_EQ(counter, result.get());
+  EXPECT_EQ(0, deletes);
+
+  // Resetting does not delete since ownership was transferred.
+  callback.Reset();
+  EXPECT_EQ(0, deletes);
+
+  // Ensure that we actually did get ownership.
+  result.reset();
+  EXPECT_EQ(1, deletes);
+
+  // Test unbound argument forwarding.
+  Callback<scoped_ptr<DeleteCounter>(scoped_ptr<DeleteCounter>)> cb_unbound =
+      Bind(&PassThru<scoped_ptr<DeleteCounter> >);
+  ptr.reset(new DeleteCounter(&deletes));
+  cb_unbound.Run(ptr.Pass());
+}
+
 // Argument Copy-constructor usage for non-reference parameters.
 //   - Bound arguments are only copied once.
 //   - Forwarded arguments are only copied once.
-//   - Forwarded arguments with coerscions are only copied twice (once for the
-//     coerscion, and one for the final dispatch).
+//   - Forwarded arguments with coercions are only copied twice (once for the
+//     coercion, and one for the final dispatch).
 TEST_F(BindTest, ArgumentCopies) {
   int copies = 0;
   int assigns = 0;

base/callback.h

@@ -312,7 +312,7 @@ class Callback<R(A1)> : public internal::CallbackBase {
     PolymorphicInvoke f =
         reinterpret_cast<PolymorphicInvoke>(polymorphic_invoke_);
 
-    return f(bind_state_.get(), a1);
+    return f(bind_state_.get(), internal::CallbackForward(a1));
   }
 
  private:
@@ -355,8 +355,8 @@ class Callback<R(A1, A2)> : public internal::CallbackBase {
     PolymorphicInvoke f =
         reinterpret_cast<PolymorphicInvoke>(polymorphic_invoke_);
 
-    return f(bind_state_.get(), a1,
-             a2);
+    return f(bind_state_.get(), internal::CallbackForward(a1),
+             internal::CallbackForward(a2));
   }
 
  private:
@@ -401,9 +401,9 @@ class Callback<R(A1, A2, A3)> : public internal::CallbackBase {
     PolymorphicInvoke f =
         reinterpret_cast<PolymorphicInvoke>(polymorphic_invoke_);
 
-    return f(bind_state_.get(), a1,
-             a2,
-             a3);
+    return f(bind_state_.get(), internal::CallbackForward(a1),
+             internal::CallbackForward(a2),
+             internal::CallbackForward(a3));
   }
 
  private:
@@ -450,10 +450,10 @@ class Callback<R(A1, A2, A3, A4)> : public internal::CallbackBase {
     PolymorphicInvoke f =
         reinterpret_cast<PolymorphicInvoke>(polymorphic_invoke_);
 
-    return f(bind_state_.get(), a1,
-             a2,
-             a3,
-             a4);
+    return f(bind_state_.get(), internal::CallbackForward(a1),
+             internal::CallbackForward(a2),
+             internal::CallbackForward(a3),
+             internal::CallbackForward(a4));
   }
 
  private:
@@ -503,11 +503,11 @@ class Callback<R(A1, A2, A3, A4, A5)> : public internal::CallbackBase {
     PolymorphicInvoke f =
         reinterpret_cast<PolymorphicInvoke>(polymorphic_invoke_);
 
-    return f(bind_state_.get(), a1,
-             a2,
-             a3,
-             a4,
-             a5);
+    return f(bind_state_.get(), internal::CallbackForward(a1),
+             internal::CallbackForward(a2),
+             internal::CallbackForward(a3),
+             internal::CallbackForward(a4),
+             internal::CallbackForward(a5));
   }
 
  private:
@@ -559,12 +559,12 @@ class Callback<R(A1, A2, A3, A4, A5, A6)> : public internal::CallbackBase {
     PolymorphicInvoke f =
         reinterpret_cast<PolymorphicInvoke>(polymorphic_invoke_);
 
-    return f(bind_state_.get(), a1,
-             a2,
-             a3,
-             a4,
-             a5,
-             a6);
+    return f(bind_state_.get(), internal::CallbackForward(a1),
+             internal::CallbackForward(a2),
+             internal::CallbackForward(a3),
+             internal::CallbackForward(a4),
+             internal::CallbackForward(a5),
+             internal::CallbackForward(a6));
   }
 
  private:
@@ -618,13 +618,13 @@ class Callback<R(A1, A2, A3, A4, A5, A6, A7)> : public internal::CallbackBase {
     PolymorphicInvoke f =
         reinterpret_cast<PolymorphicInvoke>(polymorphic_invoke_);
 
-    return f(bind_state_.get(), a1,
-             a2,
-             a3,
-             a4,
-             a5,
-             a6,
-             a7);
+    return f(bind_state_.get(), internal::CallbackForward(a1),
+             internal::CallbackForward(a2),
+             internal::CallbackForward(a3),
+             internal::CallbackForward(a4),
+             internal::CallbackForward(a5),
+             internal::CallbackForward(a6),
+             internal::CallbackForward(a7));
   }
 
  private:

base/callback.h.pump

@@ -291,7 +291,7 @@ class Callback<R($for ARG , [[A$(ARG)]])> : public internal::CallbackBase {
     return f(bind_state_.get()[[]]
 $if ARITY != 0 [[, ]]
 $for ARG ,
-             [[a$(ARG)]]);
+             [[internal::CallbackForward(a$(ARG))]]);
   }
 
  private:

base/callback_internal.h

@@ -13,6 +13,7 @@
 
 #include "base/base_export.h"
 #include "base/memory/ref_counted.h"
+#include "base/memory/scoped_ptr.h"
 
 namespace base {
 namespace internal {
@@ -130,6 +131,64 @@ struct CallbackParamTraits<T[]> {
   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().
+//
+// TODO(ajwong): We might be able to use SFINAE to search for the existence of
+// a Pass() function in the type and avoid the whitelist in CallbackParamTraits
+// and CallbackForward.
+template <typename T>
+struct CallbackParamTraits<scoped_ptr<T> > {
+  typedef scoped_ptr<T> ForwardType;
+  typedef scoped_ptr<T> StorageType;
+};
+
+template <typename T>
+struct CallbackParamTraits<scoped_array<T> > {
+  typedef scoped_array<T> ForwardType;
+  typedef scoped_array<T> StorageType;
+};
+
+template <typename T>
+struct CallbackParamTraits<scoped_ptr_malloc<T> > {
+  typedef scoped_ptr_malloc<T> ForwardType;
+  typedef scoped_ptr_malloc<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
+// default template compiles out to be a no-op.
+//
+// In C++11, std::forward would replace all uses of this function.  However, it
+// is impossible to implement a general std::forward with C++11 due to a lack
+// of rvalue references.
+template <typename T>
+T& CallbackForward(T& t) { return t; }
+
+template <typename T>
+scoped_ptr<T> CallbackForward(scoped_ptr<T>& p) { return p.Pass(); }
+
+template <typename T>
+scoped_ptr<T> CallbackForward(scoped_array<T>& p) { return p.Pass(); }
+
+template <typename T>
+scoped_ptr<T> CallbackForward(scoped_ptr_malloc<T>& p) { return p.Pass(); }
+
 }  // namespace internal
 }  // namespace base
 

base/memory/scoped_ptr.h

@@ -32,6 +32,41 @@
 //     foo.get()->Method();  // Foo::Method on the 0th element.
 //     foo[10].Method();     // Foo::Method on the 10th element.
 //   }
+//
+// These scopers also implement part of the functionality of C++11 unique_ptr
+// in that they are "movable but not copyable."  You can use the scopers in
+// the parameter and return types of functions to signify ownership transfer
+// in to and out of a function.  When calling a function that has a scoper
+// as the argument type, it must be called with the result of an analogous
+// scoper's Pass() function or another function that generates a temporary;
+// passing by copy will NOT work.  Here is an example using scoped_ptr:
+//
+//   void TakesOwnership(scoped_ptr<Foo> arg) {
+//     // Do something with arg
+//   }
+//   scoped_ptr<Foo> CreateFoo() {
+//     // No need for calling Pass() because we are constructing a temporary
+//     // for the return value.
+//     return scoped_ptr<Foo>(new Foo("new"));
+//   }
+//   scoped_ptr<Foo> PassThru(scoped_ptr<Foo> arg) {
+//     return arg.Pass();
+//   }
+//
+//   {
+//     scoped_ptr<Foo> ptr(new Foo("yay"));  // ptr manages Foo("yay)"
+//     TakesOwnership(ptr.Pass());           // ptr no longer owns Foo("yay").
+//     scoped_ptr<Foo> ptr2 = CreateFoo();   // ptr2 owns the return Foo.
+//     scoped_ptr<Foo> ptr3 =                // ptr3 now owns what was in ptr2.
+//         PassThru(ptr2.Pass());            // ptr2 is correspondingly NULL.
+//   }
+//
+// Notice that if you do not call Pass() when returning from PassThru(), or
+// when invoking TakesOwnership(), the code will not compile because scopers
+// are not copyable; they only implement move semantics which require calling
+// the Pass() function to signify a destructive transfer of state. CreateFoo()
+// is different though because we are constructing a temporary on the return
+// line and thus can avoid needing to call Pass().
 
 #ifndef BASE_MEMORY_SCOPED_PTR_H_
 #define BASE_MEMORY_SCOPED_PTR_H_
@@ -47,12 +82,35 @@
 
 #include "base/compiler_specific.h"
 
+// Macro with the boilerplate C++03 move emulation for a class.
+//
+// In C++11, this is done via rvalue references.  Here, we use C++03 move
+// emulation to fake an rvalue reference.  For a more thorough explanation
+// of the technique, see:
+//
+//   http://en.wikibooks.org/wiki/More_C%2B%2B_Idioms/Move_Constructor
+//
+#define CPP_03_MOVE_EMULATION(scoper, field) \
+ private: \
+  struct RValue { \
+    explicit RValue(scoper& obj) : obj_(obj) {} \
+    scoper& obj_; \
+  }; \
+ public: \
+  operator RValue() { return RValue(*this); } \
+  scoper(RValue proxy) : field(proxy.obj_.release()) { } \
+  scoper& operator=(RValue proxy) { \
+    swap(proxy.obj_); \
+    return *this; \
+  } \
+  scoper Pass() { return scoper(RValue(*this)); }
+
 // A scoped_ptr<T> is like a T*, except that the destructor of scoped_ptr<T>
 // automatically deletes the pointer it holds (if any).
 // That is, scoped_ptr<T> owns the T object that it points to.
 // Like a T*, a scoped_ptr<T> may hold either NULL or a pointer to a T object.
 // Also like T*, scoped_ptr<T> is thread-compatible, and once you
-// dereference it, you get the threadsafety guarantees of T.
+// dereference it, you get the thread safety guarantees of T.
 //
 // The size of a scoped_ptr is small:
 // sizeof(scoped_ptr<C>) == sizeof(C*)
@@ -122,6 +180,8 @@ class scoped_ptr {
     return retVal;
   }
 
+  CPP_03_MOVE_EMULATION(scoped_ptr, ptr_);
+
  private:
   C* ptr_;
 
@@ -131,9 +191,10 @@ class scoped_ptr {
   template <class C2> bool operator==(scoped_ptr<C2> const& p2) const;
   template <class C2> bool operator!=(scoped_ptr<C2> const& p2) const;
 
-  // Disallow evil constructors
-  scoped_ptr(const scoped_ptr&);
-  void operator=(const scoped_ptr&);
+  // Disallow evil constructors.  Note that MUST NOT take a const& because we
+  // are implementing move semantics.  See the CPP_03_MOVE_EMULATION macro.
+  scoped_ptr(scoped_ptr&);
+  void operator=(scoped_ptr&);
 };
 
 // Free functions
@@ -158,7 +219,7 @@ bool operator!=(C* p1, const scoped_ptr<C>& p2) {
 // As with scoped_ptr<C>, a scoped_array<C> either points to an object
 // or is NULL.  A scoped_array<C> owns the object that it points to.
 // scoped_array<T> is thread-compatible, and once you index into it,
-// the returned objects have only the threadsafety guarantees of T.
+// the returned objects have only the thread safety guarantees of T.
 //
 // Size: sizeof(scoped_array<C>) == sizeof(C*)
 template <class C>
@@ -168,7 +229,7 @@ class scoped_array {
   // The element type
   typedef C element_type;
 
-  // Constructor.  Defaults to intializing with NULL.
+  // Constructor.  Defaults to initializing with NULL.
   // There is no way to create an uninitialized scoped_array.
   // The input parameter must be allocated with new [].
   explicit scoped_array(C* p = NULL) : array_(p) { }
@@ -229,6 +290,8 @@ class scoped_array {
     return retVal;
   }
 
+  CPP_03_MOVE_EMULATION(scoped_array, array_);
+
  private:
   C* array_;
 
@@ -236,9 +299,10 @@ class scoped_array {
   template <class C2> bool operator==(scoped_array<C2> const& p2) const;
   template <class C2> bool operator!=(scoped_array<C2> const& p2) const;
 
-  // Disallow evil constructors
-  scoped_array(const scoped_array&);
-  void operator=(const scoped_array&);
+  // Disallow evil constructors.  Note that MUST NOT take a const& because we
+  // are implementing move semantics.  See the CPP_03_MOVE_EMULATION macro.
+  scoped_array(scoped_array&);
+  void operator=(scoped_array&);
 };
 
 // Free functions
@@ -347,6 +411,8 @@ class scoped_ptr_malloc {
     return tmp;
   }
 
+  CPP_03_MOVE_EMULATION(scoped_ptr_malloc, ptr_);
+
  private:
   C* ptr_;
 
@@ -356,11 +422,14 @@ class scoped_ptr_malloc {
   template <class C2, class GP>
   bool operator!=(scoped_ptr_malloc<C2, GP> const& p) const;
 
-  // Disallow evil constructors
-  scoped_ptr_malloc(const scoped_ptr_malloc&);
-  void operator=(const scoped_ptr_malloc&);
+  // Disallow evil constructors.  Note that MUST NOT take a const& because we
+  // are implementing move semantics.  See the CPP_03_MOVE_EMULATION macro.
+  scoped_ptr_malloc(scoped_ptr_malloc&);
+  void operator=(scoped_ptr_malloc&);
 };
 
+#undef CPP_03_MOVE_EMULATION
+
 template<class C, class FP> inline
 void swap(scoped_ptr_malloc<C, FP>& a, scoped_ptr_malloc<C, FP>& b) {
   a.swap(b);

base/memory/scoped_ptr_unittest.cc

@@ -23,6 +23,19 @@ class ConDecLogger {
   DISALLOW_COPY_AND_ASSIGN(ConDecLogger);
 };
 
+scoped_ptr<ConDecLogger> PassThru(scoped_ptr<ConDecLogger> logger) {
+  return logger.Pass();
+}
+
+void GrabAndDrop(scoped_ptr<ConDecLogger> logger) {
+}
+
+// Do not delete this function!  It's existence is to test that you can
+// return a temporarily constructed version of the scoper.
+scoped_ptr<ConDecLogger> TestReturnOfType(int* constructed) {
+  return scoped_ptr<ConDecLogger>(new ConDecLogger(constructed));
+}
+
 }  // namespace
 
 TEST(ScopedPtrTest, ScopedPtr) {
@@ -166,4 +179,83 @@ TEST(ScopedPtrTest, ScopedArray) {
   EXPECT_EQ(0, constructed);
 }
 
+TEST(ScopedPtrTest, PassBehavior) {
+  int constructed = 0;
+  {
+    ConDecLogger* logger = new ConDecLogger(&constructed);
+    scoped_ptr<ConDecLogger> scoper(logger);
+    EXPECT_EQ(1, constructed);
+
+    // Test Pass() with constructor;
+    scoped_ptr<ConDecLogger> scoper2(scoper.Pass());
+    EXPECT_EQ(1, constructed);
+
+    // Test Pass() with assignment;
+    scoped_ptr<ConDecLogger> scoper3;
+    scoper3 = scoper2.Pass();
+    EXPECT_EQ(1, constructed);
+    EXPECT_FALSE(scoper.get());
+    EXPECT_FALSE(scoper2.get());
+    EXPECT_TRUE(scoper3.get());
+  }
+
+  // Test uncaught Pass() does not leak.
+  {
+    ConDecLogger* logger = new ConDecLogger(&constructed);
+    scoped_ptr<ConDecLogger> scoper(logger);
+    EXPECT_EQ(1, constructed);
+
+    // Should auto-destruct logger by end of scope.
+    scoper.Pass();
+    EXPECT_FALSE(scoper.get());
+  }
+  EXPECT_EQ(0, constructed);
+
+  // Test that passing to function which does nothing does not leak.
+  {
+    ConDecLogger* logger = new ConDecLogger(&constructed);
+    scoped_ptr<ConDecLogger> scoper(logger);
+    EXPECT_EQ(1, constructed);
+
+    // Should auto-destruct logger by end of scope.
+    GrabAndDrop(scoper.Pass());
+    EXPECT_FALSE(scoper.get());
+  }
+  EXPECT_EQ(0, constructed);
+}
+
+TEST(ScopedPtrTest, ReturnTypeBehavior) {
+  int constructed = 0;
+
+  // Test that we can return a scoped_ptr.
+  {
+    ConDecLogger* logger = new ConDecLogger(&constructed);
+    scoped_ptr<ConDecLogger> scoper(logger);
+    EXPECT_EQ(1, constructed);
+
+    PassThru(scoper.Pass());
+    EXPECT_FALSE(scoper.get());
+  }
+  EXPECT_EQ(0, constructed);
+
+  // Test uncaught return type not leak.
+  {
+    ConDecLogger* logger = new ConDecLogger(&constructed);
+    scoped_ptr<ConDecLogger> scoper(logger);
+    EXPECT_EQ(1, constructed);
+
+    // Should auto-destruct logger by end of scope.
+    PassThru(scoper.Pass());
+    EXPECT_FALSE(scoper.get());
+  }
+  EXPECT_EQ(0, constructed);
+
+  // Call TestReturnOfType() so the compiler doesn't warn for an unused
+  // function.
+  {
+    TestReturnOfType(&constructed);
+  }
+  EXPECT_EQ(0, constructed);
+}
+
 // TODO scoped_ptr_malloc

chrome/common/extensions/extension_manifests_unittest.cc

@@ -74,6 +74,19 @@ class ExtensionManifestTest : public testing::Test {
     Manifest(DictionaryValue* manifest, const char* name)
         : name_(name), manifest_(manifest) {
     }
+    Manifest(const Manifest& m) {
+      // C++98 requires the copy constructor for a type to be visiable if you
+      // take a const-ref of a temporary for that type.  Since Manifest
+      // contains a scoped_ptr, its implicit copy constructor is declared
+      // Manifest(Manifest&) according to spec 12.8.5.  This breaks the first
+      // requirement and thus you cannot use it with LoadAndExpectError() or
+      // LoadAndExpectSuccess() easily.
+      //
+      // To get around this spec pedantry, we declare the copy constructor
+      // explicitly.  It will never get invoked.
+      NOTREACHED();
+    }
+
 
     const std::string& name() const { return name_; }