The Tor Browser: comparison security/sandbox/chromium/base/memory/weak

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+:061aafb2ad5a
+// Copyright (c) 2012 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.
+// Weak pointers are pointers to an object that do not affect its lifetime,
+// and which may be invalidated (i.e. reset to NULL) by the object, or its
+// owner, at any time, most commonly when the object is about to be deleted.
+// Weak pointers are useful when an object needs to be accessed safely by one
+// or more objects other than its owner, and those callers can cope with the
+// object vanishing and e.g. tasks posted to it being silently dropped.
+// Reference-counting such an object would complicate the ownership graph and
+// make it harder to reason about the object's lifetime.
+// EXAMPLE:
+//
+//  class Controller {
+//   public:
+//    void SpawnWorker() { Worker::StartNew(weak_factory_.GetWeakPtr()); }
+//    void WorkComplete(const Result& result) { ... }
+//   private:
+//    // Member variables should appear before the WeakPtrFactory, to ensure
+//    // that any WeakPtrs to Controller are invalidated before its members
+//    // variable's destructors are executed, rendering them invalid.
+//    WeakPtrFactory<Controller> weak_factory_;
+//  };
+//
+//  class Worker {
+//   public:
+//    static void StartNew(const WeakPtr<Controller>& controller) {
+//      Worker* worker = new Worker(controller);
+//      // Kick off asynchronous processing...
+//    }
+//   private:
+//    Worker(const WeakPtr<Controller>& controller)
+//        : controller_(controller) {}
+//    void DidCompleteAsynchronousProcessing(const Result& result) {
+//      if (controller_)
+//        controller_->WorkComplete(result);
+//    }
+//    WeakPtr<Controller> controller_;
+//  };
+//
+// With this implementation a caller may use SpawnWorker() to dispatch multiple
+// Workers and subsequently delete the Controller, without waiting for all
+// Workers to have completed.
+// ------------------------- IMPORTANT: Thread-safety -------------------------
+// Weak pointers may be passed safely between threads, but must always be
+// dereferenced and invalidated on the same thread otherwise checking the
+// pointer would be racey.
+//
+// To ensure correct use, the first time a WeakPtr issued by a WeakPtrFactory
+// is dereferenced, the factory and its WeakPtrs become bound to the calling
+// thread, and cannot be dereferenced or invalidated on any other thread. Bound
+// WeakPtrs can still be handed off to other threads, e.g. to use to post tasks
+// back to object on the bound thread.
+//
+// Invalidating the factory's WeakPtrs un-binds it from the thread, allowing it
+// to be passed for a different thread to use or delete it.
+#ifndef BASE_MEMORY_WEAK_PTR_H_
+#define BASE_MEMORY_WEAK_PTR_H_
+#include "base/basictypes.h"
+#include "base/base_export.h"
+#include "base/logging.h"
+#include "base/memory/ref_counted.h"
+#include "base/sequence_checker.h"
+#include "base/template_util.h"
+namespace base {
+template <typename T> class SupportsWeakPtr;
+template <typename T> class WeakPtr;
+namespace internal {
+// These classes are part of the WeakPtr implementation.
+// DO NOT USE THESE CLASSES DIRECTLY YOURSELF.
+class BASE_EXPORT WeakReference {
+public:
+// Although Flag is bound to a specific thread, it may be deleted from another
+// via base::WeakPtr::~WeakPtr().
+class Flag : public RefCountedThreadSafe<Flag> {
+public:
+Flag();
+void Invalidate();
+bool IsValid() const;
+private:
+friend class base::RefCountedThreadSafe<Flag>;
+~Flag();
+SequenceChecker sequence_checker_;
+bool is_valid_;
+};
+WeakReference();
+explicit WeakReference(const Flag* flag);
+~WeakReference();
+bool is_valid() const;
+private:
+scoped_refptr<const Flag> flag_;
+};
+class BASE_EXPORT WeakReferenceOwner {
+public:
+WeakReferenceOwner();
+~WeakReferenceOwner();
+WeakReference GetRef() const;
+bool HasRefs() const {
+return flag_.get() && !flag_->HasOneRef();
+}
+void Invalidate();
+private:
+mutable scoped_refptr<WeakReference::Flag> flag_;
+};
+// This class simplifies the implementation of WeakPtr's type conversion
+// constructor by avoiding the need for a public accessor for ref_.  A
+// WeakPtr<T> cannot access the private members of WeakPtr<U>, so this
+// base class gives us a way to access ref_ in a protected fashion.
+class BASE_EXPORT WeakPtrBase {
+public:
+WeakPtrBase();
+~WeakPtrBase();
+protected:
+explicit WeakPtrBase(const WeakReference& ref);
+WeakReference ref_;
+};
+// This class provides a common implementation of common functions that would
+// otherwise get instantiated separately for each distinct instantiation of
+// SupportsWeakPtr<>.
+class SupportsWeakPtrBase {
+public:
+// A safe static downcast of a WeakPtr<Base> to WeakPtr<Derived>. This
+// conversion will only compile if there is exists a Base which inherits
+// from SupportsWeakPtr<Base>. See base::AsWeakPtr() below for a helper
+// function that makes calling this easier.
+template<typename Derived>
+static WeakPtr<Derived> StaticAsWeakPtr(Derived* t) {
+typedef
+is_convertible<Derived, internal::SupportsWeakPtrBase&> convertible;
+COMPILE_ASSERT(convertible::value,
+AsWeakPtr_argument_inherits_from_SupportsWeakPtr);
+return AsWeakPtrImpl<Derived>(t, *t);
+}
+private:
+// This template function uses type inference to find a Base of Derived
+// which is an instance of SupportsWeakPtr<Base>. We can then safely
+// static_cast the Base* to a Derived*.
+template <typename Derived, typename Base>
+static WeakPtr<Derived> AsWeakPtrImpl(
+Derived* t, const SupportsWeakPtr<Base>&) {
+WeakPtr<Base> ptr = t->Base::AsWeakPtr();
+return WeakPtr<Derived>(ptr.ref_, static_cast<Derived*>(ptr.ptr_));
+}
+};
+}  // namespace internal
+template <typename T> class WeakPtrFactory;
+// The WeakPtr class holds a weak reference to |T*|.
+//
+// This class is designed to be used like a normal pointer.  You should always
+// null-test an object of this class before using it or invoking a method that
+// may result in the underlying object being destroyed.
+//
+// EXAMPLE:
+//
+//   class Foo { ... };
+//   WeakPtr<Foo> foo;
+//   if (foo)
+//     foo->method();
+//
+template <typename T>
+class WeakPtr : public internal::WeakPtrBase {
+public:
+WeakPtr() : ptr_(NULL) {
+}
+// Allow conversion from U to T provided U "is a" T. Note that this
+// is separate from the (implicit) copy constructor.
+template <typename U>
+WeakPtr(const WeakPtr<U>& other) : WeakPtrBase(other), ptr_(other.ptr_) {
+}
+T* get() const { return ref_.is_valid() ? ptr_ : NULL; }
+T& operator*() const {
+DCHECK(get() != NULL);
+return *get();
+}
+T* operator->() const {
+DCHECK(get() != NULL);
+return get();
+}
+// Allow WeakPtr<element_type> to be used in boolean expressions, but not
+// implicitly convertible to a real bool (which is dangerous).
+//
+// Note that this trick is only safe when the == and != operators
+// are declared explicitly, as otherwise "weak_ptr1 == weak_ptr2"
+// will compile but do the wrong thing (i.e., convert to Testable
+// and then do the comparison).
+private:
+typedef T* WeakPtr::*Testable;
+public:
+operator Testable() const { return get() ? &WeakPtr::ptr_ : NULL; }
+void reset() {
+ref_ = internal::WeakReference();
+ptr_ = NULL;
+}
+private:
+// Explicitly declare comparison operators as required by the bool
+// trick, but keep them private.
+template <class U> bool operator==(WeakPtr<U> const&) const;
+template <class U> bool operator!=(WeakPtr<U> const&) const;
+friend class internal::SupportsWeakPtrBase;
+template <typename U> friend class WeakPtr;
+friend class SupportsWeakPtr<T>;
+friend class WeakPtrFactory<T>;
+WeakPtr(const internal::WeakReference& ref, T* ptr)
+: WeakPtrBase(ref),
+ptr_(ptr) {
+}
+// This pointer is only valid when ref_.is_valid() is true.  Otherwise, its
+// value is undefined (as opposed to NULL).
+T* ptr_;
+};
+// A class may be composed of a WeakPtrFactory and thereby
+// control how it exposes weak pointers to itself.  This is helpful if you only
+// need weak pointers within the implementation of a class.  This class is also
+// useful when working with primitive types.  For example, you could have a
+// WeakPtrFactory<bool> that is used to pass around a weak reference to a bool.
+template <class T>
+class WeakPtrFactory {
+public:
+explicit WeakPtrFactory(T* ptr) : ptr_(ptr) {
+}
+~WeakPtrFactory() {
+ptr_ = NULL;
+}
+WeakPtr<T> GetWeakPtr() {
+DCHECK(ptr_);
+return WeakPtr<T>(weak_reference_owner_.GetRef(), ptr_);
+}
+// Call this method to invalidate all existing weak pointers.
+void InvalidateWeakPtrs() {
+DCHECK(ptr_);
+weak_reference_owner_.Invalidate();
+}
+// Call this method to determine if any weak pointers exist.
+bool HasWeakPtrs() const {
+DCHECK(ptr_);
+return weak_reference_owner_.HasRefs();
+}
+private:
+internal::WeakReferenceOwner weak_reference_owner_;
+T* ptr_;
+DISALLOW_IMPLICIT_CONSTRUCTORS(WeakPtrFactory);
+};
+// A class may extend from SupportsWeakPtr to let others take weak pointers to
+// it. This avoids the class itself implementing boilerplate to dispense weak
+// pointers.  However, since SupportsWeakPtr's destructor won't invalidate
+// weak pointers to the class until after the derived class' members have been
+// destroyed, its use can lead to subtle use-after-destroy issues.
+template <class T>
+class SupportsWeakPtr : public internal::SupportsWeakPtrBase {
+public:
+SupportsWeakPtr() {}
+WeakPtr<T> AsWeakPtr() {
+return WeakPtr<T>(weak_reference_owner_.GetRef(), static_cast<T*>(this));
+}
+protected:
+~SupportsWeakPtr() {}
+private:
+internal::WeakReferenceOwner weak_reference_owner_;
+DISALLOW_COPY_AND_ASSIGN(SupportsWeakPtr);
+};
+// Helper function that uses type deduction to safely return a WeakPtr<Derived>
+// when Derived doesn't directly extend SupportsWeakPtr<Derived>, instead it
+// extends a Base that extends SupportsWeakPtr<Base>.
+//
+// EXAMPLE:
+//   class Base : public base::SupportsWeakPtr<Producer> {};
+//   class Derived : public Base {};
+//
+//   Derived derived;
+//   base::WeakPtr<Derived> ptr = base::AsWeakPtr(&derived);
+//
+// Note that the following doesn't work (invalid type conversion) since
+// Derived::AsWeakPtr() is WeakPtr<Base> SupportsWeakPtr<Base>::AsWeakPtr(),
+// and there's no way to safely cast WeakPtr<Base> to WeakPtr<Derived> at
+// the caller.
+//
+//   base::WeakPtr<Derived> ptr = derived.AsWeakPtr();  // Fails.
+template <typename Derived>
+WeakPtr<Derived> AsWeakPtr(Derived* t) {
+return internal::SupportsWeakPtrBase::StaticAsWeakPtr<Derived>(t);
+}
+}  // namespace base
+#endif  // BASE_MEMORY_WEAK_PTR_H_

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comparison: security/sandbox/chromium/base/memory/weak_ptr.h

security/sandbox/chromium/base/memory/weak_ptr.h