Unified callback system.

base/uber_callback_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 file contains utility functions and classes that help the implemenation,
+// and management of the Callback objects.
+
+#ifndef BASE_UBER_CALLBACK_HELPERS_H_
+#define BASE_UBER_CALLBACK_HELPERS_H_
+#pragma once
+
+#include "base/ref_counted.h"
+
+namespace base {
+namespace internal {
+
+// InvokerStorageBase 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
+// PolymorphicInvoke function to perform the function execution.  This allows
+// us to shield the Callback class from the types of the bound argument via
+// "type erasure."
+class InvokerStorageBase : public RefCountedThreadSafe<InvokerStorageBase> {
+ protected:
+  friend class RefCountedThreadSafe<InvokerStorageBase>;
+  virtual ~InvokerStorageBase() {}
+};
+
+template <typename T>
+struct InvokerStorageHolder {
+  explicit InvokerStorageHolder(T* invoker_storage)
+      : invoker_storage_(invoker_storage) {
+  }

[willchan no longer on Chromium, 2011/02/07 23:25:15]

Perhaps the destructor should DCHECK that invoker_storage_ is NULL, since you're
only ever using this to pass the concrete type to the function traits, during
which you are taking ownership of the |invoker_storage_|.

[awong, 2011/02/08 18:52:26]

We could...but then we'd be adding a non-trivial destructor.  Not sure if it's
worth it.

+  mutable scoped_refptr<InvokerStorageBase> invoker_storage_;

[willchan no longer on Chromium, 2011/02/06 10:26:50]

Why is this mutable?

[awong, 2011/02/08 18:52:26]

Added comment explaining.

+};
+
+template <typename T>
+InvokerStorageHolder<T> MakeInvokerStorageHolder(T* o) {
+  return InvokerStorageHolder<T>(o);
+}
+
+}  // namespace internal
+}  // namespace base
+
+#endif  // BASE_UBER_CALLBACK_HELPERS_H_