Create "Prebind" a wrapper to tr1::bind.

base/prebind.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.
+
+#ifndef BASE_PREBIND_H
+#define BASE_PREBIND_H
+
+#include <tr1/functional>
+
+#include <base/ref_counted.h>
+
+namespace base {
+
+// These are needed to implement a wrapper that disables refcounting on
+// objects used with a Thunk.
+template <typename O>
+class UnretainedWrapper {
+ public:
+  explicit UnretainedWrapper(O* o) : obj_(o) {}
+  O* get() const { return obj_; }
+
+ private:
+  O* obj_;
+};
+
+template <typename O>
+UnretainedWrapper<O> Unretained(O* o) {
+  return UnretainedWrapper<O>(o);
+}
+
+// Invoker is adds a level of indirection into the function call using syntax
+// that will work with either a raw pointer to an object, or a scoped_refptr.
+//
+// This allows us to pass a scoped_refptr into bind for an object we wish to
+// retrain and maintain the same syntax for invoking its methods.
+//
+// O is either a scoped_refptr<T> or T* where T is the target object.
+template <typename O, typename Sig>
+struct Invoker;
+
+template <typename O, typename R, typename T>
+struct Invoker<O, R(T::*)(void)> {
+  static R invoke(R(T::*f)(void), const O& o) {
+    return (o->*f)();
+  }
+};
+
+template <typename O, typename R, typename T, typename A0>
+struct Invoker<O, R(T::*)(A0)> {
+  static R invoke(R(T::*f)(A0), const O& o, const A0& a0) {
+    return (o->*f)(a0);
+  }
+};
+
+template <typename O, typename R, typename T, typename A0, typename A1>
+struct Invoker<O, R(T::*)(A0, A1)> {
+  static R invoke(R(T::*f)(A0, A1), const O& o, const A0& a0, const A1& a1) {
+    return (o->*f)(a0, a1);
+  }
+};
+
+template <typename O, typename R, typename T, typename A0, typename A1,
+         typename A2>
+struct Invoker<O, R(T::*)(A0, A1, A2)> {
+  static R invoke(R(T::*f)(A0, A1, A2), const O& o, const A0& a0, const A1& a1,
+                  const A2& a2) {
+    return (o->*f)(a0, a1, a2);
+  }
+};
+
+template <typename O, typename R, typename T, typename A0, typename A1,
+         typename A2, typename A3>
+struct Invoker<O, R(T::*)(A0, A1, A2, A3)> {
+  static R invoke(R(T::*f)(A0, A1, A2, A3), const O& o, const A0& a0,
+                  const A1& a1, const A2& a2, const A3& a3) {
+    return (o->*f)(a0, a1, a2, a3);
+  }
+};
+
+template <typename O, typename R, typename T, typename A0, typename A1,
+         typename A2, typename A3, typename A4>
+struct Invoker<O, R(T::*)(A0, A1, A2, A3, A4)> {
+  static R invoke(R(T::*f)(A0, A1, A2, A3, A4), const O& o, const A0& a0,
+                  const A1& a1, const A2& a2, const A3& a3, const A4& a4) {
+    return (o->*f)(a0, a1, a2, a3, a4);
+  }
+};
+
+template <typename O>
+struct RetainTraits;
+
+template <typename O>
+struct RetainTraits<O*> {
+  // TODO(ajwong): Do we need the "type" typedef? Can't we just use O* in
+  // prebind since O != T?
+  typedef O type;
+  typedef scoped_refptr<O> prebind_type;
+
+  static O* unwrap(O* o) { return o; }
+  static prebind_type MaybeScopedRefptr(O* o) { return o; }
+};
+
+template <typename O>
+struct RetainTraits<UnretainedWrapper<O> > {
+  typedef O type;
+  typedef O* prebind_type;
+
+  static O* unwrap(const UnretainedWrapper<O>& o) { return o.get(); }
+  static prebind_type MaybeScopedRefptr(O* o) { return o; }
+};
+
+template <typename Sig>
+class Thunk;
+
+// TODO(ajwong): Thunk should have a pointer that can be set to refer to a
+// "tracked" object. This should be set/unset by messageloop when it creates
+// the pending task. That should give nearly equivalent functionality to
+// Task inheriting from Tracked.
+template <typename R>
+class Thunk<R(void)> {
+ public:
+  typedef std::tr1::function<R(void)> ThunkType;
+  ThunkType f_;
+
+  explicit Thunk(ThunkType f) : f_(f) {}
+
+  R operator()(void) {
+    return f_();
+  }
+};
+
+template <typename R, typename A0>
+class Thunk<R(A0)> {
+ public:
+  typedef std::tr1::function<R(A0)> ThunkType;
+  ThunkType f_;
+
+  explicit Thunk(ThunkType f) : f_(f) {}
+
+  R operator()(A0& a0) {
+    return f_(a0);
+  }
+};
+
+template <typename R, typename A0, typename A1>
+class Thunk<R(A0, A1)> {
+ public:
+  typedef std::tr1::function<R(A0,A1)> ThunkType;
+  ThunkType f_;
+  std::tr1::function<void(void)> cleanup_;
+
+  explicit Thunk(ThunkType f) : f_(f) {}
+
+  R operator()(const A0& a0, const A1& a1) {
+    return f_(a0, a1);
+  }
+};
+
+// Note that when declaring these template parameters, the types used in the
+// function signature MUST not be shared with the types used in the arguments.
+// If they are shared, then automatic conversions break. For example, this
+// should work:
+//
+//   void foo(double d);
+//   function<void(void)> f = Prebind(&foo, 2);
+//
+// However, if you declare the template for prebind as follows:
+//
+//   template <typename R, typename P0>
+//   function<R(void)> Prebind(R(*)(P0), P0 p0);
+//
+// Then the line invoking Prebind will fail because 2 is an integer, and P0 is
+// locked to be a double.  If instead, you declare the template to not tie the
+// function signature directly to the parameters, the compiler will have the
+// flexibility to do the right conversion.  Thus, the correct declaration will
+// look like this:
+//
+//   template <typename R, typename X0, typename P0>
+//   function<R(void)> Prebind(R(*)(X0), P0 p0);
+//
+// The signature uses the type X0, and the argument uses the type P0.  There is
+// no directly relationship enforced by the template declaration. Instead, we
+// rely on the compiler to output in an error of P0 is not converatble to X0.
+//
+
+
+// 1 -> 0
+template <typename R, typename X0, typename P0>
+Thunk<R(void)>
+Prebind(R(*f)(X0), P0 p0) {
+  return Thunk<R(void)>(std::tr1::bind(f, p0));
+}
+
+// 2 -> 0
+template <typename R, typename X0, typename X1, typename P0, typename P1>
+Thunk<R(void)>
+Prebind(R(*f)(X0, X1), P0 p0, P1 p1) {
+  return Thunk<R(void)>(std::tr1::bind(f, p0, p1));
+}
+
+// 2 -> 1
+template <typename R, typename X0, typename P0, typename A0>
+Thunk<R(A0)>
+Prebind(R(*f)(X0, A0), P0 p0) {
+  return Thunk<R(A0)>(std::tr1::bind(f, p0, std::tr1::placeholders::_1));
+}
+
+// (curry) 1 -> 0
+template <typename R, typename X0, typename P0>
+Thunk<R(void)>
+Prebind(Thunk<R(X0)> f, P0 p0) {
+  return Thunk<R(void)>(std::tr1::bind(f, p0));
+}
+
+// (curry) 2 -> 0
+template <typename R, typename X0, typename X1, typename P0, typename P1>
+Thunk<R(void)>
+Prebind(Thunk<R(X0, X1)> f, P0 p0, P1 p1) {
+  return Thunk<R(void)>(std::tr1::bind(f, p0, p1));
+}
+
+// (curry) 2 -> 1
+template <typename R, typename X0, typename P0, typename A0>
+Thunk<R(A0)>
+Prebind(Thunk<R(X0, A0)> f, P0 p0) {
+  return Thunk<R(A0)>(std::tr1::bind(f, p0, std::tr1::placeholders::_1));
+}
+
+// Method 0 -> 0
+template <typename R, typename O, typename T>
+Thunk<R(void)>
+Prebind(R(O::*f)(), T t) {
+  typename RetainTraits<T>::type* object_(RetainTraits<T>::unwrap(t));

[willchan no longer on Chromium, 2011/01/05 19:44:09]

Here you take T, which may be scoped_refptr<O>, and withdraw the raw pointer. 
In the next line, you use MaybeScopedRefptr to wrap it within a scoped_refptr<O>
again.  This then gets copied by value as a parameter to bind().  bind() then
will copy construct it into its member variable to store the parameter.  Then
when the Thunk is returned, it will be copy constructed to the local variable. 
Every time the Thunk is copied, the scoped_refptr will be copied.  So each time
we pass around the Thunk, we get extra AddRef()/Release() pairs.  This leads to
a lot of unnecessary refcounting.

Note that this is not a problem for C++0x, because of move semantics with rvalue
references.  But this implementation as is will multiply the number of
AddRef()/Release() pairs in Chromium for all Callbacks/Tasks (which would now be
Thunks).

Note that the current implementation avoids this problem because Task objects
are heap allocated and generally not copied, whereas Thunk is intended to be
copied on the stack.

+  return Thunk<R(void)>(
+      std::tr1::bind(
+          &Invoker<typename RetainTraits<T>::prebind_type,
+                   R(O::*)(void)>::invoke,
+          f,
+          RetainTraits<T>::MaybeScopedRefptr(object_)));

[willchan no longer on Chromium, 2011/01/05 19:44:09]

MaybeScopedRefptr() basically always forces using a scoped_refptr for a raw
pointer, unless Unretained() is used.  This causes the problem for
DISABLE_RUNNABLE_METHOD_REFCOUNT() which basically says an object shouldn't be
refcounted.  As I proposed in my fix to DISABLE_RUNNABLE_METHOD_REFCOUNT, a
base::integral_type value should be added to the UnretainedWrapper class, so
that MaybeScopedRefptr adds the scoped_refptr if T::is_refcounted::value.

+}
+
+// Method 1 -> 0
+template <typename R, typename O, typename T, typename X0, typename P0>
+Thunk<R(void)>
+Prebind(R(O::*f)(X0), T t, P0 p0) {
+  typename RetainTraits<T>::type* object_(RetainTraits<T>::unwrap(t));
+
+  return Thunk<R(void)>(
+      std::tr1::bind(
+          &Invoker<typename RetainTraits<T>::prebind_type,
+                   R(O::*)(X0)>::invoke,
+          f,
+          RetainTraits<T>::MaybeScopedRefptr(object_),
+          p0));
+}
+
+// Method 1 -> 1
+template <typename R, typename O, typename T, typename A0>
+Thunk<R(A0)>
+Prebind(R(O::*f)(A0), T t) {
+  typename RetainTraits<T>::type* object_(RetainTraits<T>::unwrap(t));
+
+  return Thunk<R(void)>(
+      std::tr1::bind(
+          &Invoker<typename RetainTraits<T>::prebind_type,
+                   R(O::*)(A0)>::invoke,
+          f,
+          RetainTraits<T>::MaybeScopedRefptr(object_),
+          std::tr1::placeholders::_1));
+}
+
+// Method 2 -> 0
+template <typename R, typename O, typename T, typename X0, typename X1,
+          typename P0, typename P1>
+Thunk<R(void)>
+Prebind(R(O::*f)(X0, X1), T t, P0 p0, P1 p1) {
+  typename RetainTraits<T>::type* object_(RetainTraits<T>::unwrap(t));
+  return Thunk<R(void)>(
+      std::tr1::bind(
+          &Invoker<typename RetainTraits<T>::prebind_type,
+                   R(O::*)(X0,X1)>::invoke,
+          f,
+          RetainTraits<T>::MaybeScopedRefptr(object_),
+          p0,
+          p1));
+}
+
+// Method 2 -> 1
+template <typename R, typename O, typename T, typename X0, typename P0,
+         typename A0>
+Thunk<R(A0)>
+Prebind(R(O::*f)(X0, A0), T t, P0 p0) {
+  typename RetainTraits<T>::type* object_(RetainTraits<T>::unwrap(t));
+  return Thunk<R(A0)>(
+      std::tr1::bind(
+          &Invoker<typename RetainTraits<T>::prebind_type,
+                   R(O::*)(X0, A0)>::invoke,
+          f,
+          RetainTraits<T>::MaybeScopedRefptr(object_),
+          p0,
+          std::tr1::placeholders::_1));
+}
+
+// Method 2 -> 2
+template <typename R, typename O, typename T, typename A0, typename A1>
+Thunk<R(A0, A1)>
+Prebind(R(O::*f)(A0, A1), T t) {
+  typename RetainTraits<T>::type* object_(RetainTraits<T>::unwrap(t));
+
+  return Thunk<R(A0, A1)>(
+      std::tr1::bind(
+          &Invoker<typename RetainTraits<T>::prebind_type,
+                   R(O::*)(A0, A1)>::invoke,
+          f,
+          RetainTraits<T>::MaybeScopedRefptr(object_),
+          std::tr1::placeholders::_1,
+          std::tr1::placeholders::_2));
+}
+
+// Method 5 -> 2  ...yeah, I skipped a few...got bored of typing it.
+template <typename R, typename O, typename T,
+         typename X0, typename X1, typename X2,
+         typename P0, typename P1, typename P2,
+         typename A0, typename A1>
+Thunk<R(A0, A1)>
+Prebind(R(O::*f)(X0, X1, X2, A0, A1), T t, P0 p0, P1 p1, P2 p2) {
+  typename RetainTraits<T>::type* object_(RetainTraits<T>::unwrap(t));
+
+  return Thunk<R(A0, A1)>(
+      std::tr1::bind(
+          &Invoker<typename RetainTraits<T>::prebind_type,
+                   R(O::*)(X0, X1, X2, A0, A1)>::invoke,
+          f,
+          RetainTraits<T>::MaybeScopedRefptr(object_),
+          p0, p1, p2,
+          std::tr1::placeholders::_1,
+          std::tr1::placeholders::_2));
+}
+
+// Wraps a Thunk to automatically cancel a task when the ThunkCanceller is
+// deleted.  This allows a caller to "nop" all outstanding callbacks registered
+// with the ThunkCanceller.
+//
+// Note that if you're also looking at the Closure code, this is nearly
+// identical to ClosureCanceller.  In fact, it can be used interchangeably, but
+// I put a forked version here just for completeness.
+class ThunkCanceller {
+ public:
+  ThunkCanceller() : cancel_state_(new CancelState()) {}
+
+  ~ThunkCanceller() {
+    cancel_state_->is_canceled = true;
+  }
+
+  template <typename T>
+  Thunk<void(void)> Wrap(T c) {
+    using std::tr1::bind;
+    return base::Thunk<void(void)>(
+        Prebind(&ThunkCanceller::Run<T>, cancel_state_, c));
+  }
+
+  bool empty() const {
+    // The ThunkCanceller has the only reference, no tasks are outstanding.
+    return cancel_state_->HasOneRef();
+  }
+
+  void RevokeAll() {
+    // Cancel all outstanding, then create a new cancel state so this object may
+    // be reused.
+    cancel_state_->is_canceled = true;
+    cancel_state_ = new CancelState();
+  }
+
+ private:
+  // The ecopedRunnableMethodFactory uses a WeakPtr.  This is because it is
+  // actually reimplementing the storage for the task object, so a pointer is
+  // necessary and thus WeakPtr can be overloaded to serve as a flag.
+  //
+  // In this design, it seems overkill to use WeakPtr instead of a simple flag
+  // class (which WeakPtr eventually devolves into anyways).
+  class CancelState : public RefCounted<CancelState> {
+   public:
+    CancelState() : is_canceled(false) {}
+
+    bool is_canceled;
+  };
+
+  template <typename T>
+  static void Run(scoped_refptr<CancelState> cancel_state, T c) {
+    if (!cancel_state->is_canceled) {
+      c();
+    }
+  }
+
+  scoped_refptr<CancelState> cancel_state_;
+
+  DISALLOW_COPY_AND_ASSIGN(ThunkCanceller);
+};
+
+}  // namespace base
+
+#endif  // BASE_PREBIND_H