rtc::Bind: Capture method objects as scoped_refptr if they are ref counted

webrtc/base/bind.h.pump

 /*
  *  Copyright 2012 The WebRTC Project Authors. All rights reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
 // To generate bind.h from bind.h.pump, execute:
 // /home/build/google3/third_party/gtest/scripts/pump.py bind.h.pump
 
 // Bind() is an overloaded function that converts method calls into function
-// objects (aka functors). It captures any arguments to the method by value
-// when Bind is called, producing a stateful, nullary function object. Care
-// should be taken about the lifetime of objects captured by Bind(); the
-// returned functor knows nothing about the lifetime of the method's object or
-// any arguments passed by pointer, and calling the functor with a destroyed
-// object will surely do bad things.
+// objects (aka functors). The method object is captured as a scoped_refptr<> if
+// possible, and as a raw pointer otherwise. Any arguments to the method are
+// captured by value. The return value of Bind is a stateful, nullary function
+// object. Care should be taken about the lifetime of objects captured by
+// Bind(); the returned functor knows nothing about the lifetime of a non
+// ref-counted method object or any arguments passed by pointer, and calling the
+// functor with a destroyed object will surely do bad things.
 //
 // Example usage:
 //   struct Foo {
 //     int Test1() { return 42; }
 //     int Test2() const { return 52; }
 //     int Test3(int x) { return x*x; }
 //     float Test4(int x, float y) { return x + y; }
 //   };
 //
 //   int main() {
 //     Foo foo;
 //     cout << rtc::Bind(&Foo::Test1, &foo)() << endl;
 //     cout << rtc::Bind(&Foo::Test2, &foo)() << endl;
 //     cout << rtc::Bind(&Foo::Test3, &foo, 3)() << endl;
 //     cout << rtc::Bind(&Foo::Test4, &foo, 7, 8.5f)() << endl;
 //   }
+//
+// Example usage of ref counted objects:
+//   struct Bar {
+//     int AddRef();
+//     int Release();
+//
+//     void Test() {}
+//     void BindThis() {
+//       // The functor passed to AsyncInvoke() will keep this object alive.
+//       invoker.AsyncInvoke(rtc::Bind(&Bar::Test, this));
+//     }
+//   };
+//
+//   int main() {
+//     rtc::scoped_refptr<Bar> bar = new rtc::RefCountedObject<Bar>();
+//     auto functor = rtc::Bind(&Bar::Test, bar);
+//     bar = nullptr;
+//     // The functor stores an internal scoped_refptr<Bar>, so this is safe.
+//     functor();
+//   }
+//
 
 #ifndef WEBRTC_BASE_BIND_H_
 #define WEBRTC_BASE_BIND_H_
 
+#include "webrtc/base/scoped_ref_ptr.h"
+
 #define NONAME
 
 namespace rtc {
 namespace detail {
 // This is needed because the template parameters in Bind can't be resolved
 // if they're used both as parameters of the function pointer type and as
 // parameters to Bind itself: the function pointer parameters are exact
 // matches to the function prototype, but the parameters to bind have
 // references stripped. This trick allows the compiler to dictate the Bind
 // parameter types rather than deduce them.
 template <class T> struct identity { typedef T type; };
+
+// IsRefCounted<T>::value will be true for types that can be used in
+// rtc::scoped_refptr<T>, i.e. types that implements nullary functions AddRef()
+// and Release(), regardless of their return types. AddRef() and Release() can
+// be defined in T or any superclass of T.
+template <typename T>
+class IsRefCounted {
+  // Once you have figured out how the following code works, and realized what a
+  // terrible mistake that was, please increment the following counter as a
+  // warning to the next guy:
+  // total_hours_wasted_here = 5
+
+  // Define types such that sizeof(Yes) != sizeof(No).
+  struct Yes { char dummy[1]; };
+  struct No { char dummy[2]; };
+  // Define two overloaded template functions with return types of different
+  // size. This way, we can use sizeof() on the return type to determine which
+  // function the compiler would have chosen. One function will be preferred
+  // over the other if it is possible to create it without compiler errors,
+  // otherwise the compiler will simply remove it, and default to the less
+  // preferred function.
+  template <typename R>
+  static Yes test(R* r, decltype(r->AddRef(), r->Release(), 42));
+  template <typename C> static No test(...);
+
+public:
+  // Trick the compiler to tell if it's possible to call AddRef() and Release().
+  static const bool value = sizeof(test<T>((T*)nullptr, 42)) == sizeof(Yes);
+};
+
+// TernaryTypeOperator is a helper class to select a type based on a static bool
+// value.
+template <bool condition, typename IfTrueT, typename IfFalseT>
+struct TernaryTypeOperator {};
+
+template <typename IfTrueT, typename IfFalseT>
+struct TernaryTypeOperator<true, IfTrueT, IfFalseT> {
+  typedef IfTrueT type;
+};
+
+template <typename IfTrueT, typename IfFalseT>
+struct TernaryTypeOperator<false, IfTrueT, IfFalseT> {
+  typedef IfFalseT type;
+};
+
+// PointerType<T>::type will be scoped_refptr<T> for ref counted types, and T*
+// otherwise.
+template <class T>
+struct PointerType {
+  typedef typename TernaryTypeOperator<IsRefCounted<T>::value,
+                                       scoped_refptr<T>,
+                                       T*>::type type;
+};
+
 }  // namespace detail
 
 $var n = 6
 $range i 0..n
 $for i [[
 $range j 1..i
 
 template <class ObjectT, class MethodT, class R$for j [[,
           class P$j]]>
 class MethodFunctor$i {
  public:
   MethodFunctor$i(MethodT method, ObjectT* object$for j [[,
                  P$j p$j]])
       : method_(method), object_(object)$for j [[,
       p$(j)_(p$j)]] {}
   R operator()() const {
     return (object_->*method_)($for j , [[p$(j)_]]); }
  private:
   MethodT method_;
-  ObjectT* object_;$for j [[
+  typename detail::PointerType<ObjectT>::type object_;$for j [[
 
   P$j p$(j)_;]]
 
 };
 
 template <class FunctorT, class R$for j [[,
           class P$j]]>
 class Functor$i {
  public:
   $if i == 0 [[explicit ]]
@@ skipping 27 matching lines @@
 template <class ObjectT, class R$for j [[,
           class P$j]]>
 MethodFunctor$i<const ObjectT, FP_T(NONAME), R$for j [[, P$j]]>
 Bind(FP_T(method), const ObjectT* object$for j [[,
      typename detail::identity<P$j>::type p$j]]) {
   return MethodFunctor$i<const ObjectT, FP_T(NONAME), R$for j [[, P$j]]>(
       method, object$for j [[, p$j]]);
 }
 
 #undef FP_T
+#define FP_T(x) R (ObjectT::*x)($for j , [[P$j]])
+
+template <class ObjectT, class R$for j [[,
+          class P$j]]>
+MethodFunctor$i<ObjectT, FP_T(NONAME), R$for j [[, P$j]]>
+Bind(FP_T(method), const scoped_refptr<ObjectT>& object$for j [[,
+     typename detail::identity<P$j>::type p$j]]) {
+  return MethodFunctor$i<ObjectT, FP_T(NONAME), R$for j [[, P$j]]>(
+      method, object.get()$for j [[, p$j]]);
+}
+
+#undef FP_T
 #define FP_T(x) R (*x)($for j , [[P$j]])
 
 template <class R$for j [[,
           class P$j]]>
 Functor$i<FP_T(NONAME), R$for j [[, P$j]]>
 Bind(FP_T(function)$for j [[,
      typename detail::identity<P$j>::type p$j]]) {
   return Functor$i<FP_T(NONAME), R$for j [[, P$j]]>(
       function$for j [[, p$j]]);
 }
 
 #undef FP_T
 
 ]]
 
 }  // namespace rtc
 
 #undef NONAME
 
 #endif  // WEBRTC_BASE_BIND_H_