Unified callback system.

base/prebind_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 for
+// changing the refcounting, and reference semantics of arguments that are bound
+// by the Prebind() function in base/prebind.h.
+//
+// The public functions are base::Unretained() and base::ConstRef().
+// Unretained() allows Prebind() to bind a non-refcounted class.  ConstRef()
+// allows binding a constant reference to an argument rather than a copy.
+//
+//
+// EXAMPLE OF Unretained():
+//
+//   class Foo {
+//    public:
+//     void func() { cout << "Foo:f" << endl;
+//   };
+//
+//   // In some function somewhere.
+//   Foo foo;
+//   Callback<void(void)> foo_callback = Prebind(&Foo::func, Unretained(&foo));
+//   foo_callback.Run();  // Prints "Foo:f".
+//
+// Without the Unretained() wrapper on |&foo|, the above call would fail to
+// compile because Foo does not support the AddRef() and Release() methods.
+//
+//
+// EXAMPLE OF ConstRef();
+//   void foo(int arg) { cout << arg << endl }
+//
+//   int n = 1;
+//   Callback<void(void)> no_ref = Prebind(&foo, n);
+//   Callback<void(void)> has_ref = Prebind(&foo, ConstRef(n));
+//
+//   no_ref.Run();  // Prints "1"
+//   has_ref.Run();  // Prints "1"
+//
+//   n = 2;
+//   no_ref.Run();  // Prints "1"
+//   has_ref.Run();  // Prints "2"
+//
+// Note that because ConstRef() takes a reference on |n|, |n| must outlive all
+// its bound callbacks.
+//
+
+#ifndef BASE_PREBIND_HELPERS_H_
+#define BASE_PREBIND_HELPERS_H_
+#pragma once
+
+namespace base {
+
+namespace internal {
+template <typename T>
+class UnretainedWrapper {
+ public:
+  explicit UnretainedWrapper(T* o) : obj_(o) {}
+  T* get() { return obj_; }
+ private:
+  T* obj_;
+};
+
+template <typename T>
+class ConstRefWrapper {
+ public:
+  explicit ConstRefWrapper(const T& o) : ptr_(&o) {}
+  const T& get() { return *ptr_; }
+ private:
+  const 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(); }
+
+
+// Utility for handling different refcounting semantics in the Prebind()
+// function.
+template <bool ref, typename O>
+struct MaybeRefcount;
+
+template <typename O>
+struct MaybeRefcount<false, O> {
+  static void AddRef(const O&) {}
+  static void Release(const O&) {}
+};
+
+template <typename O>
+struct MaybeRefcount<true, UnretainedWrapper<O> > {
+  static void AddRef(const UnretainedWrapper<O>&) {}
+  static void Release(const UnretainedWrapper<O>&) {}
+};
+
+template <typename O>
+struct MaybeRefcount<true, O* > {
+  static void AddRef(O* o) { o->AddRef(); }
+  static void Release(O* o) { o->Release(); }
+};
+
+template <typename O>
+struct MaybeRefcount<true, const O* > {
+  static void AddRef(const O* o) { o->AddRef(); }
+  static void Release(const O* o) { o->Release(); }
+};
+
+}  // namespace internal
+
+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);
+}
+
+}  // namespace base
+
+#endif  // BASE_PREBIND_HELPERS_H_