Add template aliases to support both reference counting and garbage collection for a transition per…

Source/wtf/Ptr.h

+/*
+ * Copyright (C) 2014 Google, Inc. All Rights Reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY

[haraken, 2014/01/16 05:21:22]

Not APPLE :) Please use the copyright in heap/Heap.h.

[Mads Ager (chromium), 2014/01/16 07:04:15]

Heh! Without thinking I copied this from RefPtr.h. Fixed!

+ * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+ * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL APPLE INC. OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
+ * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+ * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
+ * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
+ * OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#ifndef WTF_Ptr_h
+#define WTF_Ptr_h
+
+// Ptr is a simple wrapper for a raw pointer that provides the
+// interface (get, clear) of other pointer types such as RefPtr,

[haraken, 2014/01/16 05:21:22]

Are compilers smart enough to optimize Ptr operations just like raw pointer
operations? Maybe we want to make sure of it before starting a lot of edits.

[Mads Ager (chromium), 2014/01/16 07:04:15]

Yes, they should be. The issue that we saw when we used Result was that passing
and returning one of these by value was not properly optimized by some
compilers. However, we will mostly be using these as members of classes and
extract the pointer and pass it around directly. We will also use them where a
similar pointer class such as PassRefPtr was already used but then the compiler
will be able to do as well as it did before. I would not expect performance
regressions based on this.

[haraken, 2014/01/16 07:06:40]

Makes sense. Thanks for the clarification.

+// Persistent and Member. This is used for the Blink garbage
+// collection work in order to be able to write shared code that will
+// use reference counting or garbage collection based on a
+// compile-time flag.
+
+namespace WTF {
+
+template<typename T>
+class Ptr {
+public:
+    Ptr(T* ptr) : m_ptr(ptr) { }
+    Ptr(std::nullptr_t) : m_ptr(0) { }
+
+    template<typename U>
+    Ptr(const Ptr<U>& other)
+        : m_ptr(other.get())
+    {
+    }
+
+    T* get() const { return m_ptr; }
+    void clear() { m_ptr = 0; }
+
+    Ptr& operator=(T* ptr)
+    {
+        m_ptr = ptr;
+        return *this;
+    }
+
+    Ptr& operator=(std::nullptr_t)
+    {
+        m_ptr = 0;
+        return *this;
+    }
+
+    operator T*() const { return m_ptr; }
+    T& operator*() const { return *m_ptr; }

[haraken, 2014/01/16 05:21:22]

Don't we need:

  const T& operator*() const;

?

[Mads Ager (chromium), 2014/01/16 07:04:15]

I don't think so, no. T itself can be something like 'const Node' here and you
can always use a 'Node' where a 'const Node' is expected (but not the other way
around).

+    T* operator->() const { return m_ptr; }
+    bool operator!() const { return !m_ptr; }
+
+    // This conversion operator allows implicit conversion to bool but
+    // not to other integer types.
+    typedef T* Ptr::*UnspecifiedBoolType;
+    operator UnspecifiedBoolType() const { return m_ptr ? &m_ptr : 0; }
+
+private:
+    T* m_ptr;
+};
+
+} // namespace WTF
+
+using WTF::Ptr;
+
+#endif