Add Maplike<> utility mixin class for implementing maplike interfaces

Source/core/dom/Maplike.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 Maplike_h
+#define Maplike_h
+
+#include "bindings/core/v8/ScriptValue.h"
+#include "bindings/core/v8/ToV8.h"
+#include "core/dom/Iterable.h"

[haraken, 2015/02/12 13:41:36]

Nit: Would core/dom/ be a right place for Iterable, Maplike etc? Another
candidate would be bindings/core/v8/. (I don't have a strong opinion.)

[Jens Widell, 2015/02/12 13:50:47]

I don't know. Maybe bindings/core/v8/ makes more sense. (I have no strong
opinion either.)

I put it here because this is where Iterator.h is, but they don't have to be in
the same place. (And maybe it should move too...)

[haraken, 2015/02/12 13:56:50]

Either way let's consider in a separate CL.

+
+namespace blink {
+
+template <typename KeyType, typename ValueType>
+class Maplike : public PairIterable<KeyType, ValueType> {
+public:
+    bool hasForBinding(ScriptState* scriptState, const KeyType& key, ExceptionState& exceptionState)
+    {
+        ValueType value;
+        return getMapEntry(scriptState, key, value, exceptionState);

[haraken, 2015/02/12 13:39:39]

It's a bit inefficient to call getMapEntry() to implement has(). For
performance, get() should be implemented using has() (just like the current
getForBinding is doing). What do you think?

[Jens Widell, 2015/02/12 13:50:48]

It's not a lot inefficient though in most cases. In the current uses, I'm not
worried at all about it. I did it this way to reduce the amount of
implementation needed in a sub-class.

If extracting the value is especially expensive in some case, it's always
possible to explicitly declare has() (and/or get()) in the IDL and implement it
manually, or to simply override this hasForBinding() manually in the sub-class,
and, of course, to not use this class at all.

[haraken, 2015/02/12 13:56:50]

Sounds reasonable.

+    }
+
+    ScriptValue getForBinding(ScriptState* scriptState, const KeyType& key, ExceptionState& exceptionState)
+    {
+        ValueType value;
+        if (getMapEntry(scriptState, key, value, exceptionState))
+            return ScriptValue(scriptState, toV8(value, scriptState->context()->Global(), scriptState->isolate()));
+        return ScriptValue(scriptState, v8::Undefined(scriptState->isolate()));
+    }
+
+private:
+    virtual bool getMapEntry(ScriptState*, const KeyType&, ValueType&, ExceptionState&) = 0;
+};
+
+} // namespace blink
+
+#endif // Maplike_h