[SharedArrayBuffer] Add "AllowShared" extended attribute, used for WebGL

third_party/WebKit/Source/core/dom/MaybeShared.h

+// Copyright 2017 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 MaybeShared_h
+#define MaybeShared_h
+
+// A wrapper template type that specifies that a TypedArray may be backed by a
+// SharedArrayBuffer.
+//
+// Typically this is used as an annotation on C++ functions that are called by
+// the bindings layer, e.g.:
+//
+//   void Foo(MaybeShared<DOMUint32Array> param) {
+//     DOMUint32Array* array = param.View();
+//     ...
+//   }
+
+#include "platform/heap/Handle.h"
+
+namespace blink {
+
+template <typename T>
+class MaybeShared {

[haraken, 2017/04/12 04:37:13]

A couple of suggestions (which you can address in follow-up CLs):

- NotShared<> and MaybeShared<> sound a bit too generic. I'm fine with the class
names but would it be possible to put them in ArrayBufferViewHelper.h to clarify
that they are helper classes for ArrayBuffers?

- Would it be possible to add some DCHECK or static_assert to check that
NotShared<> and MaybeShared<> are used only for ArrayBufferViews?

+  STACK_ALLOCATED();
+
+ public:
+  using TypedArrayType = T;
+
+  MaybeShared() {}
+
+  explicit MaybeShared(T* typedArray) : typed_array_(typedArray) {
+    DCHECK(!typedArray);
+  }
+  MaybeShared(const MaybeShared& other) = default;
+  template <typename U>
+  MaybeShared(const MaybeShared<U>& other) : typed_array_(other.View()) {}
+  template <typename U>
+  MaybeShared(const Member<U>& other) {
+    typed_array_ = other.Get();
+  }
+
+  MaybeShared& operator=(const MaybeShared& other) = default;
+  template <typename U>
+  MaybeShared& operator=(const MaybeShared<U>& other) {
+    typed_array_ = other.View();
+    return *this;
+  }
+
+  T* View() const { return typed_array_.Get(); }
+
+  bool operator!() const { return !typed_array_; }
+  explicit operator bool() const { return !!typed_array_; }
+
+ private:
+  Member<T> typed_array_;
+};
+
+}  // namespace blink
+
+#endif  // MaybeShared_h