Callback API Change: Add in an Owned() wrapper to base::Bind().

base/bind_helpers.h

 // Copyright (c) 2011 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 defines a set of argument wrappers and related factory methods that
 // 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::ConstRef(), and
 // base::IgnoreReturn().
@@ skipping 181 matching lines @@
 
 template <typename T>
 struct UnsafeBindtoRefCountedArg<T*>
     : UnsafeBindtoRefCountedArgHelper<is_class<T>::value, T> {
 };
 
 
 template <typename T>
 class UnretainedWrapper {
  public:
-  explicit UnretainedWrapper(T* o) : obj_(o) {}
-  T* get() { return obj_; }
+  explicit UnretainedWrapper(T* o) : ptr_(o) {}
+  T* get() const { return ptr_; }
  private:
-  T* obj_;
+  T* ptr_;
 };
 
 template <typename T>
 class ConstRefWrapper {
  public:
   explicit ConstRefWrapper(const T& o) : ptr_(&o) {}
-  const T& get() { return *ptr_; }
+  const T& get() const { return *ptr_; }
  private:
   const T* ptr_;
 };
 
+// An alternate implementation is to avoid the destructive copy, and instead
+// specialize ParamTraits<> for OwnedWrapper<> to change the StorageType to
+// a class that is essentially a scoped_ptr<>.
+//
+// This has the benefit though of leaving ParamTrais<> fully in
+// callback_internal.h as well as avoiding type conversions during storage.
+template <typename T>
+class OwnedWrapper {
+ public:
+  explicit OwnedWrapper(T* o) : ptr_(o) {}
+  ~OwnedWrapper() { delete ptr_; }
+  T* get() const { return ptr_; }
+  OwnedWrapper(const OwnedWrapper& other) {
+    ptr_ = other.ptr_;
+    other.ptr_ = NULL;
+  }
+
+ private:
+  mutable T* ptr_;
+};
+
 
 // Unwrap the stored parameters for the wrappers above.
 template <typename T>
 T Unwrap(T o) { return o; }
 
 template <typename T>
 T* Unwrap(UnretainedWrapper<T> unretained) { return unretained.get(); }
 
 template <typename T>
 const T& Unwrap(ConstRefWrapper<T> const_ref) {
   return const_ref.get();
 }
 
 template <typename T>
 T* Unwrap(const scoped_refptr<T>& o) { return o.get(); }
 
 template <typename T>
 const WeakPtr<T>& Unwrap(const WeakPtr<T>& o) { return o; }
 
+template <typename T>
+T* Unwrap(const OwnedWrapper<T>& o) {
+  return o.get();
+}
+
 // Utility for handling different refcounting semantics in the Bind()
 // function.
 template <typename IsMethod, typename T>
 struct MaybeRefcount;
 
 template <typename T>
 struct MaybeRefcount<base::false_type, T> {
   static void AddRef(const T&) {}
   static void Release(const T&) {}
 };
 
 template <typename T, size_t n>
 struct MaybeRefcount<base::false_type, T[n]> {
   static void AddRef(const T*) {}
   static void Release(const T*) {}
 };
 
 template <typename T>
+struct MaybeRefcount<base::true_type, T*> {
+  static void AddRef(T* o) { o->AddRef(); }
+  static void Release(T* o) { o->Release(); }
+};
+
+template <typename T>
 struct MaybeRefcount<base::true_type, UnretainedWrapper<T> > {
   static void AddRef(const UnretainedWrapper<T>&) {}
   static void Release(const UnretainedWrapper<T>&) {}
 };
 
 template <typename T>
-struct MaybeRefcount<base::true_type, T*> {
-  static void AddRef(T* o) { o->AddRef(); }
-  static void Release(T* o) { o->Release(); }
+struct MaybeRefcount<base::true_type, OwnedWrapper<T> > {
+  static void AddRef(const OwnedWrapper<T>&) {}
+  static void Release(const OwnedWrapper<T>&) {}
 };
 
 // No need to additionally AddRef() and Release() since we are storing a
 // scoped_refptr<> inside the storage object already.
 template <typename T>
 struct MaybeRefcount<base::true_type, scoped_refptr<T> > {
   static void AddRef(const scoped_refptr<T>& o) {}
   static void Release(const scoped_refptr<T>& o) {}
 };
 
@@ skipping 19 matching lines @@
 template <typename T>
 inline internal::UnretainedWrapper<T> Unretained(T* o) {
   return internal::UnretainedWrapper<T>(o);
 }
 
 template <typename T>
 inline internal::ConstRefWrapper<T> ConstRef(const T& o) {
   return internal::ConstRefWrapper<T>(o);
 }
 
+template <typename T>
+inline internal::OwnedWrapper<T> Owned(T* o) {

[darin (slow to review), 2011/10/11 05:13:50]

how about some documentation?  i'm concerned that this system is growing
increasingly complicated.

i also wish there was some consistency to how these coercion helpers were named
:-)

[awong, 2011/10/11 17:24:27]

This is the start of concern?  Or are you concerned that it is already too
complicated?

I think the endpoint of features will be:
  1) Solving ownership (probably just Owned())
  2) Solving cancellation.

The second one, I expect, to be some sort of wrapper on Callback.

Both these use cases I've been seeing concrete needs for in the CLs I've been
reviewing.

Does that sound too complicated?  I do agree with the general worry though.
 
Hmm...you mean that no one has a way to know that Unretained(), ConstRef(),
IgnoreResult(), and Owned() have anything to do with one another?

I supposed we could add a prefix, but I shy away from a making the names longer
cause they are already a bit unwieldy.  We could also dump them into a
namespace, but that will pragmatically cause a similar problem since people
don't often use using statements...

-Albert

+  return internal::OwnedWrapper<T>(o);
+}
+
 template <typename R>
 Closure IgnoreReturn(Callback<R(void)> callback) {
   return Bind(&internal::VoidReturnAdapter<R>, callback);
 }
 
 }  // namespace base
 
 #endif  // BASE_BIND_HELPERS_H_