Implement IDBVersionChangeEvent ctor w/ IDBVersionChangeEventInit dict

Source/bindings/v8/Nullable.h

+// Copyright 2014 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 Nullable_h
+#define Nullable_h
+
+#include "wtf/Assertions.h"
+
+namespace WebCore {

[jsbell, 2014/02/27 20:59:06]

Could be in wtf, but I started conservatively.

+
+template <typename T>
+class Nullable {
+public:
+    Nullable()

[jsbell, 2014/02/27 20:59:06]

I also considered Nullable<type>::null() as a static constructor, and/or
allowing Nullable<type>(nullptr) via wtf/NullPtr.h. I have no strong
preferences.

Unfortunately, callers can't write the concise:

Nullable<t> value(c ? a : nullptr);

... since the type of the ternary operator's branches are not consistent.

+        : m_value()
+        , m_isNull(true) { }
+
+    Nullable(const T& value)
+        : m_value(value)
+        , m_isNull(false) { }
+
+    Nullable(const Nullable& other)
+        : m_value(other.m_value)
+        , m_isNull(other.m_isNull) { }
+
+    Nullable& operator=(const Nullable& other)
+    {
+        m_value = other.m_value;
+        m_isNull = other.m_isNull;
+        return *this;
+    }
+
+    T get() const { ASSERT(!m_isNull); return m_value; }

[jsbell, 2014/02/27 20:59:06]

I did NOT expose an `operator T()` conversion, favoring an explicit get().

+    bool isNull() const { return m_isNull; }
+
+    operator bool() const { return !m_isNull && m_value; }

[jsbell, 2014/02/27 20:59:06]

This was required by the Dictionary. I'll admit I didn't dig too deeply into
why.

+
+    bool operator==(const Nullable& other) const
+    {
+        return (m_isNull && other.m_isNull) || (!m_isNull && !other.m_isNull && m_value == other.m_value);
+    }
+
+private:
+    T m_value;
+    bool m_isNull;
+};
+
+} // namespace WebCore
+
+#endif // Nullable_h