Add nullptr support to scoped_ptr.

This adds support to use nullptr to construct, assign, or return a
scoped_ptr<T> and scoped_ptr<T[]>. Support for this requires the use
of a move-only constructor.

The changes are:

- Add a constructor that takes decltype(nullptr) as a parameter. This
allows behaviour such as scoped_ptr<T>(nullptr), but also allows a
function with return type scoped_ptr<T> to "return nullptr;" instead
of "return scoped_ptr<T>();".

- Add an operator=(decltype(nullptr)) that resets the scoped_ptr to
empty and deletes anything it held.

- Add/Modify a constructor to take a scoped_ptr<U,E>&& parameter for
constructing a scoped_ptr from another using move-only semantics. This
piece is critical for allowing the function returning nullptr to be
assigned to some other scoped_ptr at the callsite. In particular, take
the following code:
  scoped_ptr<T> Function() { return nullptr; }
  scoped_ptr<T> var = Function();
In this case the constructor which takes a nullptr allows Function() to
be written, but not to be used. The move-only constructor allows the
assignment from Function() to var. See "C++11 feature proposal:
Move-only constructors" on chromium-dev for more explanation why.

The scoped_ptr<T> class already had a constructor which took
scoped_ptr<U,E> as an argument, so this was changed to be
scoped_ptr<U,E>&& instead. The scoped_ptr<T[]> class had no such
constructor, so a scoped_ptr&& constructor was added. These match
the constructors found on the unique_ptr class.

- Remove the RValue type and the contructor that constructs a
scoped_ptr from an RValue. Change Pass() to return a scoped_ptr&&
instead of a scoped_ptr::RValue, to avoid the type conversion and
remove some complexity. This is done with a new emulation macro that
still provides Pass() and makes the type go down the MoveOnlyType
path in base::Callback code.

This adds base_unittests to demonstrate and use these changes.

The use of Pass() remains unchanged until std::move() is written
or allowed. At that time std::move() could be used instead of Pass.

R=brettw@chromium.org, jamesr@chromium.org

Review URL: https://codereview.chromium.org/599313003

Cr-Commit-Position: refs/heads/master@{#297072}

2 files changed, 122 insertions, 39 deletions

diff --git a/base/memory/scoped_ptr.h b/base/memory/scoped_ptr.h
index bf2e0b6..81b4a62 100644
--- a/base/memory/scoped_ptr.h
+++ b/base/memory/scoped_ptr.h
@@ -58,7 +58,7 @@
 //     TakesOwnership(ptr.Pass());           // ptr no longer owns Foo("yay").
 //     scoped_ptr<Foo> ptr2 = CreateFoo();   // ptr2 owns the return Foo.
 //     scoped_ptr<Foo> ptr3 =                // ptr3 now owns what was in ptr2.
-//         PassThru(ptr2.Pass());            // ptr2 is correspondingly NULL.
+//         PassThru(ptr2.Pass());            // ptr2 is correspondingly nullptr.
 //   }
 //
 // Notice that if you do not call Pass() when returning from PassThru(), or
@@ -189,7 +189,7 @@ template <typename T> struct IsNotRefCounted {
 template <class T, class D>
 class scoped_ptr_impl {
  public:
-  explicit scoped_ptr_impl(T* p) : data_(p) { }
+  explicit scoped_ptr_impl(T* p) : data_(p) {}
 
   // Initializer for deleters that have data parameters.
   scoped_ptr_impl(T* p, const D& d) : data_(p, d) {}
@@ -214,7 +214,7 @@ class scoped_ptr_impl {
   }
 
   ~scoped_ptr_impl() {
-    if (data_.ptr != NULL) {
+    if (data_.ptr != nullptr) {
       // Not using get_deleter() saves one function call in non-optimized
       // builds.
       static_cast<D&>(data_)(data_.ptr);
@@ -223,7 +223,7 @@ class scoped_ptr_impl {
 
   void reset(T* p) {
     // This is a self-reset, which is no longer allowed: http://crbug.com/162971
-    if (p != NULL && p == data_.ptr)
+    if (p != nullptr && p == data_.ptr)
       abort();
 
     // Note that running data_.ptr = p can lead to undefined behavior if
@@ -236,13 +236,13 @@ class scoped_ptr_impl {
     // then it will incorrectly dispatch calls to |p| rather than the original
     // value of |data_.ptr|.
     //
-    // During the transition period, set the stored pointer to NULL while
+    // During the transition period, set the stored pointer to nullptr while
     // deleting the object. Eventually, this safety check will be removed to
     // prevent the scenario initially described from occuring and
     // http://crbug.com/176091 can be closed.
     T* old = data_.ptr;
-    data_.ptr = NULL;
-    if (old != NULL)
+    data_.ptr = nullptr;
+    if (old != nullptr)
       static_cast<D&>(data_)(old);
     data_.ptr = p;
   }
@@ -263,7 +263,7 @@ class scoped_ptr_impl {
 
   T* release() {
     T* old_ptr = data_.ptr;
-    data_.ptr = NULL;
+    data_.ptr = nullptr;
     return old_ptr;
   }
 
@@ -293,8 +293,8 @@ class scoped_ptr_impl {
 // A scoped_ptr<T> is like a T*, except that the destructor of scoped_ptr<T>
 // automatically deletes the pointer it holds (if any).
 // That is, scoped_ptr<T> owns the T object that it points to.
-// Like a T*, a scoped_ptr<T> may hold either NULL or a pointer to a T object.
-// Also like T*, scoped_ptr<T> is thread-compatible, and once you
+// Like a T*, a scoped_ptr<T> may hold either nullptr or a pointer to a T
+// object. Also like T*, scoped_ptr<T> is thread-compatible, and once you
 // dereference it, you get the thread safety guarantees of T.
 //
 // The size of scoped_ptr is small. On most compilers, when using the
@@ -318,14 +318,17 @@ class scoped_ptr {
   typedef T element_type;
   typedef D deleter_type;
 
-  // Constructor.  Defaults to initializing with NULL.
-  scoped_ptr() : impl_(NULL) { }
+  // Constructor.  Defaults to initializing with nullptr.
+  scoped_ptr() : impl_(nullptr) {}
 
   // Constructor.  Takes ownership of p.
-  explicit scoped_ptr(element_type* p) : impl_(p) { }
+  explicit scoped_ptr(element_type* p) : impl_(p) {}
 
   // Constructor.  Allows initialization of a stateful deleter.
-  scoped_ptr(element_type* p, const D& d) : impl_(p, d) { }
+  scoped_ptr(element_type* p, const D& d) : impl_(p, d) {}
+
+  // Constructor.  Allows construction from a nullptr.
+  scoped_ptr(decltype(nullptr)) : impl_(nullptr) {}
 
   // Constructor.  Allows construction from a scoped_ptr rvalue for a
   // convertible type and deleter.
@@ -338,12 +341,13 @@ class scoped_ptr {
   // use of SFINAE. You only need to care about this if you modify the
   // implementation of scoped_ptr.
   template <typename U, typename V>
-  scoped_ptr(scoped_ptr<U, V> other) : impl_(&other.impl_) {
+  scoped_ptr(scoped_ptr<U, V>&& other)
+      : impl_(&other.impl_) {
     COMPILE_ASSERT(!base::is_array<U>::value, U_cannot_be_an_array);
   }
 
   // Constructor.  Move constructor for C++03 move emulation of this type.
-  scoped_ptr(RValue rvalue) : impl_(&rvalue.object->impl_) { }
+  scoped_ptr(RValue rvalue) : impl_(&rvalue.object->impl_) {}
 
   // operator=.  Allows assignment from a scoped_ptr rvalue for a convertible
   // type and deleter.
@@ -356,24 +360,31 @@ class scoped_ptr {
   // You only need to care about this if you modify the implementation of
   // scoped_ptr.
   template <typename U, typename V>
-  scoped_ptr& operator=(scoped_ptr<U, V> rhs) {
+  scoped_ptr& operator=(scoped_ptr<U, V>&& rhs) {
     COMPILE_ASSERT(!base::is_array<U>::value, U_cannot_be_an_array);
     impl_.TakeState(&rhs.impl_);
     return *this;
   }
 
+  // operator=.  Allows assignment from a nullptr. Deletes the currently owned
+  // object, if any.
+  scoped_ptr& operator=(decltype(nullptr)) {
+    reset();
+    return *this;
+  }
+
   // Reset.  Deletes the currently owned object, if any.
   // Then takes ownership of a new object, if given.
-  void reset(element_type* p = NULL) { impl_.reset(p); }
+  void reset(element_type* p = nullptr) { impl_.reset(p); }
 
   // Accessors to get the owned object.
   // operator* and operator-> will assert() if there is no current object.
   element_type& operator*() const {
-    assert(impl_.get() != NULL);
+    assert(impl_.get() != nullptr);
     return *impl_.get();
   }
   element_type* operator->() const  {
-    assert(impl_.get() != NULL);
+    assert(impl_.get() != nullptr);
     return impl_.get();
   }
   element_type* get() const { return impl_.get(); }
@@ -394,7 +405,9 @@ class scoped_ptr {
       scoped_ptr::*Testable;
 
  public:
-  operator Testable() const { return impl_.get() ? &scoped_ptr::impl_ : NULL; }
+  operator Testable() const {
+    return impl_.get() ? &scoped_ptr::impl_ : nullptr;
+  }
 
   // Comparison operators.
   // These return whether two scoped_ptr refer to the same object, not just to
@@ -408,10 +421,9 @@ class scoped_ptr {
   }
 
   // Release a pointer.
-  // The return value is the current pointer held by this object.
-  // If this object holds a NULL pointer, the return value is NULL.
-  // After this operation, this object will hold a NULL pointer,
-  // and will not own the object any more.
+  // The return value is the current pointer held by this object. If this object
+  // holds a nullptr, the return value is nullptr. After this operation, this
+  // object will hold a nullptr, and will not own the object any more.
   element_type* release() WARN_UNUSED_RESULT {
     return impl_.release();
   }
@@ -452,8 +464,8 @@ class scoped_ptr<T[], D> {
   typedef T element_type;
   typedef D deleter_type;
 
-  // Constructor.  Defaults to initializing with NULL.
-  scoped_ptr() : impl_(NULL) { }
+  // Constructor.  Defaults to initializing with nullptr.
+  scoped_ptr() : impl_(nullptr) {}
 
   // Constructor. Stores the given array. Note that the argument's type
   // must exactly match T*. In particular:
@@ -463,18 +475,27 @@ class scoped_ptr<T[], D> {
   //   T and the derived types had different sizes access would be
   //   incorrectly calculated). Deletion is also always undefined
   //   (C++98 [expr.delete]p3). If you're doing this, fix your code.
-  // - it cannot be NULL, because NULL is an integral expression, not a
-  //   pointer to T. Use the no-argument version instead of explicitly
-  //   passing NULL.
   // - it cannot be const-qualified differently from T per unique_ptr spec
   //   (http://cplusplus.github.com/LWG/lwg-active.html#2118). Users wanting
   //   to work around this may use implicit_cast<const T*>().
   //   However, because of the first bullet in this comment, users MUST
   //   NOT use implicit_cast<Base*>() to upcast the static type of the array.
-  explicit scoped_ptr(element_type* array) : impl_(array) { }
+  explicit scoped_ptr(element_type* array) : impl_(array) {}
+
+  // Constructor.  Allows construction from a nullptr.
+  scoped_ptr(decltype(nullptr)) : impl_(nullptr) {}
+
+  // Constructor.  Allows construction from a scoped_ptr rvalue.
+  scoped_ptr(scoped_ptr&& other) : impl_(&other.impl_) {}
 
   // Constructor.  Move constructor for C++03 move emulation of this type.
-  scoped_ptr(RValue rvalue) : impl_(&rvalue.object->impl_) { }
+  scoped_ptr(RValue rvalue) : impl_(&rvalue.object->impl_) {}
+
+  // operator=.  Allows assignment from a scoped_ptr rvalue.
+  scoped_ptr& operator=(scoped_ptr&& rhs) {
+    impl_.TakeState(&rhs.impl_);
+    return *this;
+  }
 
   // operator=.  Move operator= for C++03 move emulation of this type.
   scoped_ptr& operator=(RValue rhs) {
@@ -482,13 +503,20 @@ class scoped_ptr<T[], D> {
     return *this;
   }
 
+  // operator=.  Allows assignment from a nullptr. Deletes the currently owned
+  // array, if any.
+  scoped_ptr& operator=(decltype(nullptr)) {
+    reset();
+    return *this;
+  }
+
   // Reset.  Deletes the currently owned array, if any.
   // Then takes ownership of a new object, if given.
-  void reset(element_type* array = NULL) { impl_.reset(array); }
+  void reset(element_type* array = nullptr) { impl_.reset(array); }
 
   // Accessors to get the owned array.
   element_type& operator[](size_t i) const {
-    assert(impl_.get() != NULL);
+    assert(impl_.get() != nullptr);
     return impl_.get()[i];
   }
   element_type* get() const { return impl_.get(); }
@@ -504,7 +532,9 @@ class scoped_ptr<T[], D> {
       scoped_ptr::*Testable;
 
  public:
-  operator Testable() const { return impl_.get() ? &scoped_ptr::impl_ : NULL; }
+  operator Testable() const {
+    return impl_.get() ? &scoped_ptr::impl_ : nullptr;
+  }
 
   // Comparison operators.
   // These return whether two scoped_ptr refer to the same object, not just to
@@ -518,10 +548,9 @@ class scoped_ptr<T[], D> {
   }
 
   // Release a pointer.
-  // The return value is the current pointer held by this object.
-  // If this object holds a NULL pointer, the return value is NULL.
-  // After this operation, this object will hold a NULL pointer,
-  // and will not own the object any more.
+  // The return value is the current pointer held by this object. If this object
+  // holds a nullptr, the return value is nullptr. After this operation, this
+  // object will hold a nullptr, and will not own the object any more.
   element_type* release() WARN_UNUSED_RESULT {
     return impl_.release();
   }
diff --git a/base/memory/scoped_ptr_unittest.cc b/base/memory/scoped_ptr_unittest.cc
index e0c1548..7cec70e 100644
--- a/base/memory/scoped_ptr_unittest.cc
+++ b/base/memory/scoped_ptr_unittest.cc
@@ -406,6 +406,8 @@ TEST(ScopedPtrTest, PassBehavior) {
 
     // Should auto-destruct logger by end of scope.
     scoper.Pass();
+    // This differs from unique_ptr, as Pass() has side effects but std::move()
+    // does not.
     EXPECT_FALSE(scoper.get());
   }
   EXPECT_EQ(0, constructed);
@@ -602,3 +604,55 @@ TEST(ScopedPtrTest, OverloadedNewAndDelete) {
   EXPECT_EQ(1, OverloadedNewAndDelete::delete_count());
   EXPECT_EQ(1, OverloadedNewAndDelete::new_count());
 }
+
+scoped_ptr<int> NullIntReturn() {
+  return nullptr;
+}
+
+TEST(ScopedPtrTest, Nullptr) {
+  scoped_ptr<int> scoper1(nullptr);
+  scoped_ptr<int> scoper2(new int);
+  scoper2 = nullptr;
+  scoped_ptr<int> scoper3(NullIntReturn());
+  scoped_ptr<int> scoper4 = NullIntReturn();
+  EXPECT_EQ(nullptr, scoper1.get());
+  EXPECT_EQ(nullptr, scoper2.get());
+  EXPECT_EQ(nullptr, scoper3.get());
+  EXPECT_EQ(nullptr, scoper4.get());
+}
+
+scoped_ptr<int[]> NullIntArrayReturn() {
+  return nullptr;
+}
+
+TEST(ScopedPtrTest, NullptrArray) {
+  scoped_ptr<int[]> scoper1(nullptr);
+  scoped_ptr<int[]> scoper2(new int);
+  scoper2 = nullptr;
+  scoped_ptr<int[]> scoper3(NullIntArrayReturn());
+  scoped_ptr<int[]> scoper4 = NullIntArrayReturn();
+  EXPECT_EQ(nullptr, scoper1.get());
+  EXPECT_EQ(nullptr, scoper2.get());
+  EXPECT_EQ(nullptr, scoper3.get());
+  EXPECT_EQ(nullptr, scoper4.get());
+}
+
+class Super {};
+class Sub : public Super {};
+
+scoped_ptr<Sub> SubClassReturn() {
+  return make_scoped_ptr(new Sub);
+}
+
+TEST(ScopedPtrTest, Conversion) {
+  scoped_ptr<Sub> sub1(new Sub);
+  scoped_ptr<Sub> sub2(new Sub);
+
+  // Upcast with Pass() works.
+  scoped_ptr<Super> super1 = sub1.Pass();
+  super1 = sub2.Pass();
+
+  // Upcast with an rvalue works.
+  scoped_ptr<Super> super2 = SubClassReturn();
+  super2 = SubClassReturn();
+}