Move base to DEPS

base/memory/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 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 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.
-//
-// Invalidating the factory's WeakPtrs un-binds it from the sequence, allowing
-// it to be passed for a different sequence 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 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();
-  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_