Unified callback system.

base/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
+// implementation, and management of the Callback objects.
+
+#ifndef BASE_CALLBACK_HELPERS_H_
+#define BASE_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
+// DoInvoke 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() {}
+};
+
+// This structure exists purely to pass the returned |invoker_storage_| from
+// Bind() to Callback while avoiding an extra AddRef/Release() pair.
+//
+// To do this, the constructor of Callback<> must take a const-ref.  The
+// reference must be to a const object otherwise the compiler will emit a
+// warning about taking a reference to a temporary.
+//
+// Unfortunately, this means that the internal |invoker_storage_| field must
+// be made mutable.
+template <typename T>
+struct InvokerStorageHolder {
+  explicit InvokerStorageHolder(T* invoker_storage)
+      : invoker_storage_(invoker_storage) {
+  }
+
+  mutable scoped_refptr<InvokerStorageBase> invoker_storage_;
+};
+
+template <typename T>
+InvokerStorageHolder<T> MakeInvokerStorageHolder(T* o) {
+  return InvokerStorageHolder<T>(o);
+}
+
+}  // namespace internal
+}  // namespace base
+
+#endif  // BASE_CALLBACK_HELPERS_H_