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

base/closure.h

+// Copyright (c) 2009 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_CLOSURE_H_
+#define BASE_CLOSURE_H_
+#pragma once
+
+#include <tr1/functional>
+
+#include "base/ref_counted.h"
+#include "base/scoped_ptr.h"
+#include "base/tracked.h"
+
+namespace base {
+
+extern void DoNothing();  // Defined in base/task.h.
+
+// We are trying to mimic value-object semantics of tr1::function in the
+// Closure API.  Thus, we need to detach the tracked object, and some other
+// state as a shared state.
+//
+// As is, there are some threadsafety issues, but the general idea should be
+// correct.
+class ClosureState : public RefCountedThreadSafe<ClosureState> {
+ public:
+  // TODO(ajwong): Put these behind an accessor.
+  ::std::tr1::function<void()> release_thunk_;
+
+  explicit ClosureState(tracked_objects::Tracked* tracked)
+      : release_thunk_(&DoNothing),
+        tracked_(tracked) {
+  }
+
+  ~ClosureState() {
+    release_thunk_();
+  }
+
+  tracked_objects::Tracked* tracked() const {
+    return tracked_.get();
+  }
+
+ private:
+  scoped_ptr<tracked_objects::Tracked> tracked_;
+};
+
+
+// These are needed to implement a wrapper that disables refcounting on
+// objects used with a Closure.
+//
+// TODO(ajwong): If we implement this similar to std::tr1::reference_wrapper we
+// can probably simplify the ObjectTraits below and remove one specialization.
+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);
+}
+
+// Note, Closure does not support binding a member function to a reference
+// object.  That is, this will not compile:
+//
+//   Foo f;
+//   Closure c = Closure(&Foo::func, f);
+//
+// You have to pass in a pointer to it.
+//
+//   Foo f;
+//   Closure c = Closure(&Foo::func, &f);
+//
+// We can make the syntax work out, but it's extra complex and given that we
+// expect Closure's to refcount the member functions they abstract unless
+// explicitly told not to, allowing pass by reference doesn't make much sense.
+class Closure {
+ public:
+
+// --- Traits object defintions --
+//  This handle changing the syntax, and refcounting for the various types of
+//  parameters that can be passed in.
+
+  template <typename O, bool is_member_function_pointer>
+  struct ObjectTraits;
+
+  // Handle regular pointers.
+  template <typename O>
+  struct ObjectTraits<O*, true> {
+    static O* Unwrap(O* o) {
+      return o;
+    }
+
+    static ::std::tr1::function<void()> RefThunk(O* o) {

[willchan no longer on Chromium, 2011/01/05 00:30:17]

Do we really need to use Thunks here?  Is the flexibility worth the extra heap
allocation here?  Can't we have a bool parameter in the Closure object that
indicates whether or not to AddRef()/Release()?  Note that to do this, we'd have
to define a helper function with a boolean template parameter.  Specialize one
boolean value to call AddRef()/Release() and the other to be a no-op.  This is
necessary since otherwise the code may not compile since O may not define
AddRef()/Release().

[awong, 2011/01/05 03:17:42]

Looking at this more, I don't think it's quite that simple.  We probably don't
need a fully generalized thunk.  However, if we specialize the Thunk object
based on a boolean refcount, then the caller will have to always declare that in
the type.  So, the API would look like:

  Thunk<void(void), true> t = base::Prebind(&O::blah, &o);

Also, if t held the reference, to o, then all copies of t would ref/deref o. 
We'd be trading a heap allocation at creation for an extra ref/deref per copy.

I think a nicer solution would be to bind a scoped_refptr into the argument list
when we're refcounting.  See the "Invoker" think I just added.

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

I never said template parameter.  I said parameter.  A template parameter would
cause the bad API as you pointed out.
Yes, that'd be bad, and that's not what I'm suggesting :)
This approach adds unnecessary AddRef()/Release() pairs.  I'll point them out in
the new code.

+      return ::std::tr1::bind(&O::AddRef, o);
+    }
+    static ::std::tr1::function<void()> ReleaseThunk(O* o) {
+      return ::std::tr1::bind(&O::Release, o);
+    }
+  };
+
+  // If it's not a member function, there is no refcounting, and pass through
+  // the object.
+  template <typename O>
+  struct ObjectTraits<O, false> {
+    static O Unwrap(O o) {
+      return o;
+    }
+
+    static ::std::tr1::function<void()> RefThunk(O o) {
+      return ::std::tr1::bind(&DoNothing);
+    }
+    static ::std::tr1::function<void()> ReleaseThunk(O o) {
+      return ::std::tr1::bind(&DoNothing);
+    }
+  };
+
+  // Object wrapped in Unretained() should not refcount anything, and should
+  // unwrap to a pointer to their type.
+  template <typename O>
+  struct ObjectTraits<UnretainedWrapper<O>, true> {
+    static O* Unwrap(UnretainedWrapper<O> o) {
+      return o.get();
+    }
+
+    static ::std::tr1::function<void()> RefThunk(UnretainedWrapper<O> o) {
+      return ::std::tr1::bind(&DoNothing);
+    }
+
+    static ::std::tr1::function<void()> ReleaseThunk(UnretainedWrapper<O> o) {
+      return ::std::tr1::bind(&DoNothing);
+    }
+  };
+
+
+// ---- Constructors ----
+  template <typename Sig>
+  Closure(Sig f)
+    : state_(new ClosureState(new tracked_objects::Tracked())) {
+    func_ = ::std::tr1::bind(f);
+    state_->release_thunk_ = ::std::tr1::bind(&DoNothing);
+  }
+
+  template <typename Sig, typename A1>
+  Closure(Sig f, A1 a1)
+    : state_(new ClosureState(new tracked_objects::Tracked())) {
+    using std::tr1::is_member_function_pointer;
+    func_ = ::std::tr1::bind(
+        f,
+        ObjectTraits<A1, is_member_function_pointer<Sig>::value >::Unwrap(a1));
+
+    // Handle the refcounting.
+    ObjectTraits<A1, is_member_function_pointer<Sig>::value >::RefThunk(a1)();
+    state_->release_thunk_ =
+        ObjectTraits<A1, is_member_function_pointer<Sig>::value >::
+            ReleaseThunk(a1);
+  }
+
+  template <typename Sig, typename A1,  typename A2>
+  Closure(Sig f, A1 a1, A2 a2)
+    : state_(new ClosureState(new tracked_objects::Tracked())) {
+    using std::tr1::is_member_function_pointer;
+    func_ = ::std::tr1::bind(
+        f,
+        ObjectTraits<A1, is_member_function_pointer<Sig>::value >::Unwrap(a1),
+        a2);
+
+    // Handle the refcounting.
+    ObjectTraits<A1, is_member_function_pointer<Sig>::value >::RefThunk(a1)();
+    state_->release_thunk_ =
+        ObjectTraits<A1, is_member_function_pointer<Sig>::value >::
+            ReleaseThunk(a1);
+  }
+
+  template <typename Sig, typename A1,  typename A2,  typename A3>
+  Closure(Sig f, A1 a1, A2 a2, A3 a3)
+    : state_(new ClosureState(new tracked_objects::Tracked())) {
+    using std::tr1::is_member_function_pointer;
+    func_ = ::std::tr1::bind(
+        f,
+        ObjectTraits<A1, is_member_function_pointer<Sig>::value >::Unwrap(a1),
+        a2,
+        a3);
+
+    // Handle the refcounting.
+    ObjectTraits<A1, is_member_function_pointer<Sig>::value >::RefThunk(a1)();
+    state_->release_thunk_ =
+        ObjectTraits<A1, is_member_function_pointer<Sig>::value >::
+            ReleaseThunk(a1);
+  }
+
+  template <typename Sig, typename A1,  typename A2,  typename A3,
+           typename A4>
+  Closure(Sig f, A1 a1, A2 a2, A3 a3, A4 a4)
+    : state_(new ClosureState(new tracked_objects::Tracked())) {
+    using std::tr1::is_member_function_pointer;
+    func_ = ::std::tr1::bind(
+        f,
+        ObjectTraits<A1, is_member_function_pointer<Sig>::value >::Unwrap(a1),
+        a2,
+        a3,
+        a4);
+
+    // Handle the refcounting.
+    ObjectTraits<A1, is_member_function_pointer<Sig>::value >::RefThunk(a1)();
+    state_->release_thunk_ =
+        ObjectTraits<A1, is_member_function_pointer<Sig>::value >::
+            ReleaseThunk(a1);
+  }
+
+  template <typename Sig, typename A1,  typename A2,  typename A3,
+           typename A4,  typename A5>
+  Closure(Sig f, A1 a1, A2 a2, A3 a3, A4 a4, A5 a5)
+    : state_(new ClosureState(new tracked_objects::Tracked())) {
+    using std::tr1::is_member_function_pointer;
+    func_ = ::std::tr1::bind(
+        f,
+        ObjectTraits<A1, is_member_function_pointer<Sig>::value >::Unwrap(a1),
+        a2,
+        a3,
+        a4,
+        a5);
+
+    // Handle the refcounting.
+    ObjectTraits<A1, is_member_function_pointer<Sig>::value >::RefThunk(a1)();
+    state_->release_thunk_ =
+        ObjectTraits<A1, is_member_function_pointer<Sig>::value >::
+            ReleaseThunk(a1);
+  }
+
+  template <typename Sig, typename A1,  typename A2,  typename A3,
+           typename A4,  typename A5, typename A6>
+  Closure(Sig f, A1 a1, A2 a2, A3 a3, A4 a4, A5 a5, A6 a6)
+    : state_(new ClosureState(new tracked_objects::Tracked())) {
+    using std::tr1::is_member_function_pointer;
+    func_ = ::std::tr1::bind(
+        f,
+        ObjectTraits<A1, is_member_function_pointer<Sig>::value >::Unwrap(a1),
+        a2,
+        a3,
+        a4,
+        a5,
+        a6);
+
+    // Handle the refcounting.
+    ObjectTraits<A1, is_member_function_pointer<Sig>::value >::RefThunk(a1)();
+    state_->release_thunk_ =
+        ObjectTraits<A1, is_member_function_pointer<Sig>::value >::
+            ReleaseThunk(a1);
+  }
+
+  // This is to make Closure mimic tr1::function<void()>'s interface.  It could
+  // also just be void Run(), but mimicing the interface allows us to
+  // substitute Closure in whereever tr1::function<void()> could be used in the
+  // tr1 libraries.
+  void operator()() { func_(); }
+
+  tracked_objects::Tracked* tracked() {
+    return state_->tracked();
+  }
+
+  void Cancel() {
+    func_ = ::std::tr1::bind(&DoNothing);

[willchan no longer on Chromium, 2011/01/05 00:30:17]

can't this just be std::tr1::function()?

[awong, 2011/01/05 03:17:42]

That actually asserted.

However, I can remove the bind call and just do func_ = &DoNothing. 

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

Oh really?  Ok, whatever.
Sounds good.

+  }
+
+ private:
+  // Don't let us convert from tr1 quickly.
+  Closure(::std::tr1::function<void(void)> f)
+    : state_(new ClosureState(new tracked_objects::Tracked())) {
+    func_ = ::std::tr1::bind(f);
+  }
+
+  // This is the tr1 function pointer.
+  ::std::tr1::function<void()> func_;
+
+  scoped_refptr<ClosureState> state_;
+};
+
+// Wraps a Closure or a tr1::function<void()) to automatically cancel a task
+// when the ClosureCanceller is deleted.  This allows a caller to "nop" all
+// outstanding callbacks registered with the ClosureCanceller.
+class ClosureCanceller {
+ public:
+  ClosureCanceller() : cancel_state_(new CancelState()) {}
+
+  ~ClosureCanceller() {
+    cancel_state_->is_canceled = true;
+  }
+
+  template <typename T>
+  Closure Wrap(T c) {
+    using ::std::tr1::bind;
+    return base::Closure(bind(&ClosureCanceller::Run<T>, cancel_state_, c));
+  }
+
+  bool empty() const {
+    // The ClosureCanceller 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 ScopedRunnableMethodFactory 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_;
+};
+
+}  // namespace base
+
+#endif // BASE_CLOSURE_H_