michael@0: // Copyright (c) 2012 The Chromium Authors. All rights reserved.
michael@0: // Use of this source code is governed by a BSD-style license that can be
michael@0: // found in the LICENSE file.
michael@0: 
michael@0: // Weak pointers are pointers to an object that do not affect its lifetime,
michael@0: // and which may be invalidated (i.e. reset to NULL) by the object, or its
michael@0: // owner, at any time, most commonly when the object is about to be deleted.
michael@0: 
michael@0: // Weak pointers are useful when an object needs to be accessed safely by one
michael@0: // or more objects other than its owner, and those callers can cope with the
michael@0: // object vanishing and e.g. tasks posted to it being silently dropped.
michael@0: // Reference-counting such an object would complicate the ownership graph and
michael@0: // make it harder to reason about the object's lifetime.
michael@0: 
michael@0: // EXAMPLE:
michael@0: //
michael@0: //  class Controller {
michael@0: //   public:
michael@0: //    void SpawnWorker() { Worker::StartNew(weak_factory_.GetWeakPtr()); }
michael@0: //    void WorkComplete(const Result& result) { ... }
michael@0: //   private:
michael@0: //    // Member variables should appear before the WeakPtrFactory, to ensure
michael@0: //    // that any WeakPtrs to Controller are invalidated before its members
michael@0: //    // variable's destructors are executed, rendering them invalid.
michael@0: //    WeakPtrFactory<Controller> weak_factory_;
michael@0: //  };
michael@0: //
michael@0: //  class Worker {
michael@0: //   public:
michael@0: //    static void StartNew(const WeakPtr<Controller>& controller) {
michael@0: //      Worker* worker = new Worker(controller);
michael@0: //      // Kick off asynchronous processing...
michael@0: //    }
michael@0: //   private:
michael@0: //    Worker(const WeakPtr<Controller>& controller)
michael@0: //        : controller_(controller) {}
michael@0: //    void DidCompleteAsynchronousProcessing(const Result& result) {
michael@0: //      if (controller_)
michael@0: //        controller_->WorkComplete(result);
michael@0: //    }
michael@0: //    WeakPtr<Controller> controller_;
michael@0: //  };
michael@0: //
michael@0: // With this implementation a caller may use SpawnWorker() to dispatch multiple
michael@0: // Workers and subsequently delete the Controller, without waiting for all
michael@0: // Workers to have completed.
michael@0: 
michael@0: // ------------------------- IMPORTANT: Thread-safety -------------------------
michael@0: 
michael@0: // Weak pointers may be passed safely between threads, but must always be
michael@0: // dereferenced and invalidated on the same thread otherwise checking the
michael@0: // pointer would be racey.
michael@0: //
michael@0: // To ensure correct use, the first time a WeakPtr issued by a WeakPtrFactory
michael@0: // is dereferenced, the factory and its WeakPtrs become bound to the calling
michael@0: // thread, and cannot be dereferenced or invalidated on any other thread. Bound
michael@0: // WeakPtrs can still be handed off to other threads, e.g. to use to post tasks
michael@0: // back to object on the bound thread.
michael@0: //
michael@0: // Invalidating the factory's WeakPtrs un-binds it from the thread, allowing it
michael@0: // to be passed for a different thread to use or delete it.
michael@0: 
michael@0: #ifndef BASE_MEMORY_WEAK_PTR_H_
michael@0: #define BASE_MEMORY_WEAK_PTR_H_
michael@0: 
michael@0: #include "base/basictypes.h"
michael@0: #include "base/base_export.h"
michael@0: #include "base/logging.h"
michael@0: #include "base/memory/ref_counted.h"
michael@0: #include "base/sequence_checker.h"
michael@0: #include "base/template_util.h"
michael@0: 
michael@0: namespace base {
michael@0: 
michael@0: template <typename T> class SupportsWeakPtr;
michael@0: template <typename T> class WeakPtr;
michael@0: 
michael@0: namespace internal {
michael@0: // These classes are part of the WeakPtr implementation.
michael@0: // DO NOT USE THESE CLASSES DIRECTLY YOURSELF.
michael@0: 
michael@0: class BASE_EXPORT WeakReference {
michael@0:  public:
michael@0:   // Although Flag is bound to a specific thread, it may be deleted from another
michael@0:   // via base::WeakPtr::~WeakPtr().
michael@0:   class Flag : public RefCountedThreadSafe<Flag> {
michael@0:    public:
michael@0:     Flag();
michael@0: 
michael@0:     void Invalidate();
michael@0:     bool IsValid() const;
michael@0: 
michael@0:    private:
michael@0:     friend class base::RefCountedThreadSafe<Flag>;
michael@0: 
michael@0:     ~Flag();
michael@0: 
michael@0:     SequenceChecker sequence_checker_;
michael@0:     bool is_valid_;
michael@0:   };
michael@0: 
michael@0:   WeakReference();
michael@0:   explicit WeakReference(const Flag* flag);
michael@0:   ~WeakReference();
michael@0: 
michael@0:   bool is_valid() const;
michael@0: 
michael@0:  private:
michael@0:   scoped_refptr<const Flag> flag_;
michael@0: };
michael@0: 
michael@0: class BASE_EXPORT WeakReferenceOwner {
michael@0:  public:
michael@0:   WeakReferenceOwner();
michael@0:   ~WeakReferenceOwner();
michael@0: 
michael@0:   WeakReference GetRef() const;
michael@0: 
michael@0:   bool HasRefs() const {
michael@0:     return flag_.get() && !flag_->HasOneRef();
michael@0:   }
michael@0: 
michael@0:   void Invalidate();
michael@0: 
michael@0:  private:
michael@0:   mutable scoped_refptr<WeakReference::Flag> flag_;
michael@0: };
michael@0: 
michael@0: // This class simplifies the implementation of WeakPtr's type conversion
michael@0: // constructor by avoiding the need for a public accessor for ref_.  A
michael@0: // WeakPtr<T> cannot access the private members of WeakPtr<U>, so this
michael@0: // base class gives us a way to access ref_ in a protected fashion.
michael@0: class BASE_EXPORT WeakPtrBase {
michael@0:  public:
michael@0:   WeakPtrBase();
michael@0:   ~WeakPtrBase();
michael@0: 
michael@0:  protected:
michael@0:   explicit WeakPtrBase(const WeakReference& ref);
michael@0: 
michael@0:   WeakReference ref_;
michael@0: };
michael@0: 
michael@0: // This class provides a common implementation of common functions that would
michael@0: // otherwise get instantiated separately for each distinct instantiation of
michael@0: // SupportsWeakPtr<>.
michael@0: class SupportsWeakPtrBase {
michael@0:  public:
michael@0:   // A safe static downcast of a WeakPtr<Base> to WeakPtr<Derived>. This
michael@0:   // conversion will only compile if there is exists a Base which inherits
michael@0:   // from SupportsWeakPtr<Base>. See base::AsWeakPtr() below for a helper
michael@0:   // function that makes calling this easier.
michael@0:   template<typename Derived>
michael@0:   static WeakPtr<Derived> StaticAsWeakPtr(Derived* t) {
michael@0:     typedef
michael@0:         is_convertible<Derived, internal::SupportsWeakPtrBase&> convertible;
michael@0:     COMPILE_ASSERT(convertible::value,
michael@0:                    AsWeakPtr_argument_inherits_from_SupportsWeakPtr);
michael@0:     return AsWeakPtrImpl<Derived>(t, *t);
michael@0:   }
michael@0: 
michael@0:  private:
michael@0:   // This template function uses type inference to find a Base of Derived
michael@0:   // which is an instance of SupportsWeakPtr<Base>. We can then safely
michael@0:   // static_cast the Base* to a Derived*.
michael@0:   template <typename Derived, typename Base>
michael@0:   static WeakPtr<Derived> AsWeakPtrImpl(
michael@0:       Derived* t, const SupportsWeakPtr<Base>&) {
michael@0:     WeakPtr<Base> ptr = t->Base::AsWeakPtr();
michael@0:     return WeakPtr<Derived>(ptr.ref_, static_cast<Derived*>(ptr.ptr_));
michael@0:   }
michael@0: };
michael@0: 
michael@0: }  // namespace internal
michael@0: 
michael@0: template <typename T> class WeakPtrFactory;
michael@0: 
michael@0: // The WeakPtr class holds a weak reference to |T*|.
michael@0: //
michael@0: // This class is designed to be used like a normal pointer.  You should always
michael@0: // null-test an object of this class before using it or invoking a method that
michael@0: // may result in the underlying object being destroyed.
michael@0: //
michael@0: // EXAMPLE:
michael@0: //
michael@0: //   class Foo { ... };
michael@0: //   WeakPtr<Foo> foo;
michael@0: //   if (foo)
michael@0: //     foo->method();
michael@0: //
michael@0: template <typename T>
michael@0: class WeakPtr : public internal::WeakPtrBase {
michael@0:  public:
michael@0:   WeakPtr() : ptr_(NULL) {
michael@0:   }
michael@0: 
michael@0:   // Allow conversion from U to T provided U "is a" T. Note that this
michael@0:   // is separate from the (implicit) copy constructor.
michael@0:   template <typename U>
michael@0:   WeakPtr(const WeakPtr<U>& other) : WeakPtrBase(other), ptr_(other.ptr_) {
michael@0:   }
michael@0: 
michael@0:   T* get() const { return ref_.is_valid() ? ptr_ : NULL; }
michael@0: 
michael@0:   T& operator*() const {
michael@0:     DCHECK(get() != NULL);
michael@0:     return *get();
michael@0:   }
michael@0:   T* operator->() const {
michael@0:     DCHECK(get() != NULL);
michael@0:     return get();
michael@0:   }
michael@0: 
michael@0:   // Allow WeakPtr<element_type> to be used in boolean expressions, but not
michael@0:   // implicitly convertible to a real bool (which is dangerous).
michael@0:   //
michael@0:   // Note that this trick is only safe when the == and != operators
michael@0:   // are declared explicitly, as otherwise "weak_ptr1 == weak_ptr2"
michael@0:   // will compile but do the wrong thing (i.e., convert to Testable
michael@0:   // and then do the comparison).
michael@0:  private:
michael@0:   typedef T* WeakPtr::*Testable;
michael@0: 
michael@0:  public:
michael@0:   operator Testable() const { return get() ? &WeakPtr::ptr_ : NULL; }
michael@0: 
michael@0:   void reset() {
michael@0:     ref_ = internal::WeakReference();
michael@0:     ptr_ = NULL;
michael@0:   }
michael@0: 
michael@0:  private:
michael@0:   // Explicitly declare comparison operators as required by the bool
michael@0:   // trick, but keep them private.
michael@0:   template <class U> bool operator==(WeakPtr<U> const&) const;
michael@0:   template <class U> bool operator!=(WeakPtr<U> const&) const;
michael@0: 
michael@0:   friend class internal::SupportsWeakPtrBase;
michael@0:   template <typename U> friend class WeakPtr;
michael@0:   friend class SupportsWeakPtr<T>;
michael@0:   friend class WeakPtrFactory<T>;
michael@0: 
michael@0:   WeakPtr(const internal::WeakReference& ref, T* ptr)
michael@0:       : WeakPtrBase(ref),
michael@0:         ptr_(ptr) {
michael@0:   }
michael@0: 
michael@0:   // This pointer is only valid when ref_.is_valid() is true.  Otherwise, its
michael@0:   // value is undefined (as opposed to NULL).
michael@0:   T* ptr_;
michael@0: };
michael@0: 
michael@0: // A class may be composed of a WeakPtrFactory and thereby
michael@0: // control how it exposes weak pointers to itself.  This is helpful if you only
michael@0: // need weak pointers within the implementation of a class.  This class is also
michael@0: // useful when working with primitive types.  For example, you could have a
michael@0: // WeakPtrFactory<bool> that is used to pass around a weak reference to a bool.
michael@0: template <class T>
michael@0: class WeakPtrFactory {
michael@0:  public:
michael@0:   explicit WeakPtrFactory(T* ptr) : ptr_(ptr) {
michael@0:   }
michael@0: 
michael@0:   ~WeakPtrFactory() {
michael@0:     ptr_ = NULL;
michael@0:   }
michael@0: 
michael@0:   WeakPtr<T> GetWeakPtr() {
michael@0:     DCHECK(ptr_);
michael@0:     return WeakPtr<T>(weak_reference_owner_.GetRef(), ptr_);
michael@0:   }
michael@0: 
michael@0:   // Call this method to invalidate all existing weak pointers.
michael@0:   void InvalidateWeakPtrs() {
michael@0:     DCHECK(ptr_);
michael@0:     weak_reference_owner_.Invalidate();
michael@0:   }
michael@0: 
michael@0:   // Call this method to determine if any weak pointers exist.
michael@0:   bool HasWeakPtrs() const {
michael@0:     DCHECK(ptr_);
michael@0:     return weak_reference_owner_.HasRefs();
michael@0:   }
michael@0: 
michael@0:  private:
michael@0:   internal::WeakReferenceOwner weak_reference_owner_;
michael@0:   T* ptr_;
michael@0:   DISALLOW_IMPLICIT_CONSTRUCTORS(WeakPtrFactory);
michael@0: };
michael@0: 
michael@0: // A class may extend from SupportsWeakPtr to let others take weak pointers to
michael@0: // it. This avoids the class itself implementing boilerplate to dispense weak
michael@0: // pointers.  However, since SupportsWeakPtr's destructor won't invalidate
michael@0: // weak pointers to the class until after the derived class' members have been
michael@0: // destroyed, its use can lead to subtle use-after-destroy issues.
michael@0: template <class T>
michael@0: class SupportsWeakPtr : public internal::SupportsWeakPtrBase {
michael@0:  public:
michael@0:   SupportsWeakPtr() {}
michael@0: 
michael@0:   WeakPtr<T> AsWeakPtr() {
michael@0:     return WeakPtr<T>(weak_reference_owner_.GetRef(), static_cast<T*>(this));
michael@0:   }
michael@0: 
michael@0:  protected:
michael@0:   ~SupportsWeakPtr() {}
michael@0: 
michael@0:  private:
michael@0:   internal::WeakReferenceOwner weak_reference_owner_;
michael@0:   DISALLOW_COPY_AND_ASSIGN(SupportsWeakPtr);
michael@0: };
michael@0: 
michael@0: // Helper function that uses type deduction to safely return a WeakPtr<Derived>
michael@0: // when Derived doesn't directly extend SupportsWeakPtr<Derived>, instead it
michael@0: // extends a Base that extends SupportsWeakPtr<Base>.
michael@0: //
michael@0: // EXAMPLE:
michael@0: //   class Base : public base::SupportsWeakPtr<Producer> {};
michael@0: //   class Derived : public Base {};
michael@0: //
michael@0: //   Derived derived;
michael@0: //   base::WeakPtr<Derived> ptr = base::AsWeakPtr(&derived);
michael@0: //
michael@0: // Note that the following doesn't work (invalid type conversion) since
michael@0: // Derived::AsWeakPtr() is WeakPtr<Base> SupportsWeakPtr<Base>::AsWeakPtr(),
michael@0: // and there's no way to safely cast WeakPtr<Base> to WeakPtr<Derived> at
michael@0: // the caller.
michael@0: //
michael@0: //   base::WeakPtr<Derived> ptr = derived.AsWeakPtr();  // Fails.
michael@0: 
michael@0: template <typename Derived>
michael@0: WeakPtr<Derived> AsWeakPtr(Derived* t) {
michael@0:   return internal::SupportsWeakPtrBase::StaticAsWeakPtr<Derived>(t);
michael@0: }
michael@0: 
michael@0: }  // namespace base
michael@0: 
michael@0: #endif  // BASE_MEMORY_WEAK_PTR_H_