Replace WTF::Optional the base::Optional implementation.

third_party/WebKit/Source/wtf/Optional.h

 // Copyright 2015 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.
 
 #ifndef Optional_h
 #define Optional_h
 
-#include "wtf/Alignment.h"
-#include "wtf/Allocator.h"
-#include "wtf/Assertions.h"
-#include "wtf/Noncopyable.h"
-#include "wtf/StdLibExtras.h"
-
-#include <new>
-#include <utility>
+#include "base/optional.h"
+#include "wtf/TypeTraits.h"
 
 namespace WTF {
 
-// This is a lightweight template similar to std::experimental::optional.
-// It currently does not support assignment, swapping, comparison, etc.
-//
-// Use this instead of OwnPtr for cases where you only want to conditionally
-// construct a "scope" object.
-//
-// Example:
-//   Optional<DrawingRecorder> recorder;
-//   if (shouldDraw)
-//       recorder.emplace(constructor, args, here);
-//   // recorder destroyed at end of scope
-//
-// It can be used in WTF::Vector.
-//
-// Note in particular that unlike a pointer, though, dereferencing a const
-// optional yields a const reference.
+// See base/optional.h for documentation.
+template <typename T>
+class Optional : public base::Optional<T> {
+    static_assert(!IsGarbageCollectedType<T>::value, "Garbage collected types should not be stack-allocated.");

[jbroman, 2016/04/20 21:13:24]

FYI, doing things this way will cause most of the constructors to go away. I
think "using base::Optional::Optional;" fixes it, but I'm not 100% sure this is
complete. There are a few other awkward hazards, like operator= returning the
base type instead of the derived type.

Another approach might be something like this (warning: untested code):

template <typename T, typename =
std::enable_if<!IsGarbageCollectedType<T>::value>::type>
using Optional = base::Optional<T>;

This should have a similar effect, but should make the type alias, washing a
number of these issues away.

[danakj, 2016/04/20 21:33:22]

Yes! That's what I really want, thank you for the great idea. Confirmed that it
works.

+};
 
-template <typename T>
-class Optional final {
-    DISALLOW_NEW_EXCEPT_PLACEMENT_NEW();
-    WTF_MAKE_NONCOPYABLE(Optional);
-public:
-    Optional() : m_ptr(nullptr) { }
-    ~Optional()
-    {
-        if (m_ptr)
-            m_ptr->~T();
-    }
-
-    typedef T* Optional::*UnspecifiedBoolType;
-    operator UnspecifiedBoolType() const { return m_ptr ? &Optional::m_ptr : nullptr; }
-
-    T& operator*() { ASSERT_WITH_SECURITY_IMPLICATION(m_ptr); return *m_ptr; }
-    const T& operator*() const { ASSERT_WITH_SECURITY_IMPLICATION(m_ptr); return *m_ptr; }
-    T* operator->() { ASSERT_WITH_SECURITY_IMPLICATION(m_ptr); return m_ptr; }
-    const T* operator->() const { ASSERT_WITH_SECURITY_IMPLICATION(m_ptr); return m_ptr; }
-    T* get() { return m_ptr; }
-    const T* get() const { return m_ptr; }
-
-    template <typename... Args>
-    void emplace(Args&&... args)
-    {
-        RELEASE_ASSERT(!m_ptr);
-        m_ptr = reinterpret_cast_ptr<T*>(&m_storage.buffer);
-        new (m_ptr) T(std::forward<Args>(args)...);
-    }
-
-private:
-    T* m_ptr;
-    AlignedBuffer<sizeof(T), WTF_ALIGN_OF(T)> m_storage;
-};
+constexpr base::nullopt_t nullopt = base::nullopt;
+constexpr base::in_place_t in_place = base::in_place;
 
 } // namespace WTF
 
 using WTF::Optional;
 
 #endif // Optional_h