weak_ptr.h

// 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 nullptr) 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:
//    Controller() : weak_factory_(this) {}
//    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 SequencedTaskRunner 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 or current SequencedWorkerPool token, and cannot be dereferenced or
// invalidated on any other task runner. Bound WeakPtrs can still be handed
// off to other task runners, e.g. to use to post tasks back to object on the
// bound sequence.
//
// If all WeakPtr objects are destroyed or invalidated then the factory is
// unbound from the SequencedTaskRunner/Thread. The WeakPtrFactory may then be
// destroyed, or new WeakPtr objects may be used, from a different sequence.
//
// Thus, at least one WeakPtr object must exist and have been dereferenced on
// the correct thread to enforce that other WeakPtr objects will enforce they
// are used on the desired thread.

#ifndef BASE_MEMORY_WEAK_PTR_H_
#define BASE_MEMORY_WEAK_PTR_H_

#include <cstddef>
#include <type_traits>

#include "base/base_export.h"
#include "base/logging.h"
#include "base/macros.h"
#include "base/memory/ref_counted.h"
#include "base/sequence_checker.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 SequencedTaskRunner, it may be
  // deleted from another via base::WeakPtr::~WeakPtr().
  class BASE_EXPORT 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();
  WeakReference(const WeakReference& other);
  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) {
    static_assert(
        std::is_base_of<internal::SupportsWeakPtrBase, Derived>::value,
        "AsWeakPtr argument must inherit 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_(nullptr) {}

  WeakPtr(std::nullptr_t) : ptr_(nullptr) {}

  // 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_ : nullptr; }

  T& operator*() const {
    DCHECK(get() != nullptr);
    return *get();
  }
  T* operator->() const {
    DCHECK(get() != nullptr);
    return get();
  }

  void reset() {
    ref_ = internal::WeakReference();
    ptr_ = nullptr;
  }

  // Implement "Safe Bool Idiom"
  // https://en.wikibooks.org/wiki/More_C%2B%2B_Idioms/Safe_bool
  //
  // Allow WeakPtr<element_type> to be used in boolean expressions such as
  //   if (weak_ptr_instance)
  // But do not become convertible to a real bool (which is dangerous).
  //   Implementation requires:
  //     typedef Testable
  //     operator Testable() const
  //     operator==
  //     operator!=
  //
  // == and != operators must be declared explicitly or dissallowed, as
  // otherwise "ptr1 == ptr2" will compile but do the wrong thing (i.e., convert
  // to Testable and then do the comparison).
  //
  // C++11 provides for "explicit operator bool()", however it is currently
  // banned due to MSVS2013. https://chromium-cpp.appspot.com/#core-blacklist
 private:
  typedef T* WeakPtr::*Testable;

 public:
  operator Testable() const { return get() ? &WeakPtr::ptr_ : nullptr; }

 private:
  // Explicitly declare comparison operators as required by the "Safe Bool
  // Idiom", 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 nullptr).
  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_ = nullptr; }

  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_