Mojo C++ bindings: add a new mode to generator to use native STL/WTF types

mojo/public/cpp/bindings/map_traits_wtf_hash_map.h

+// Copyright 2016 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 MOJO_PUBLIC_CPP_BINDINGS_MAP_TRAITS_WTF_HASH_MAP_H_
+#define MOJO_PUBLIC_CPP_BINDINGS_MAP_TRAITS_WTF_HASH_MAP_H_
+
+#include "base/logging.h"
+#include "mojo/public/cpp/bindings/map_traits.h"
+#include "third_party/WebKit/Source/wtf/HashMap.h"
+
+namespace mojo {
+
+template <typename K, typename V>
+struct MapTraits<WTF::HashMap<K, V>> {
+  using Key = K;
+  using Value = V;
+  using Iterator = typename WTF::HashMap<K, V>::iterator;
+  using ConstIterator = typename WTF::HashMap<K, V>::const_iterator;
+
+  static bool IsNull(const WTF::HashMap<K, V>& input) {
+    // WTF::HashMap<> is always converted to non-null mojom map.
+    return false;
+  }
+
+  static void SetToNull(WTF::HashMap<K, V>* output) {
+    // WTF::HashMap<> doesn't support null state. Set it to empty instead.

[dcheng, 2016/07/12 04:19:42]

I don't know if anyone depends on this behavior, but it seems like it'd be
better to static assert instead.

[yzshen1, 2016/07/12 16:29:20]

The bindings use WTF::Optional<WTF::HashMap<>> to represent mojom nullable map,
so IsNull/SetToNull is not used by the auto-generated code. If a user doesn't
care about null state, however, in StructTraits::Read() he could read a nullable
map field directly into a WTF::HashMap<> and convert null to empty; or he could
return HashMap<> from a StructTraits getter which serializes to nullable map. So
it seems useful to me to keep IsNull()/SetToNull() here.

Besides, it is not possible to static assert here because the "target mojom
type" is not part of this MapTraits.

WDYT? Thanks!

[dcheng, 2016/07/13 14:04:43]

It just seems weird that it's not symmetric: we declare that this thing will
never be null when serializing, and yet we have to handle it when deserializing.

I don't think it should block this CL, just something to think about.

+    output->clear();
+  }
+
+  static size_t GetSize(const WTF::HashMap<K, V>& input) {
+    return input.size();
+  }
+
+  static ConstIterator GetBegin(const WTF::HashMap<K, V>& input) {
+    return input.begin();
+  }
+  static Iterator GetBegin(WTF::HashMap<K, V>& input) { return input.begin(); }
+
+  static void AdvanceIterator(ConstIterator& iterator) { ++iterator; }
+  static void AdvanceIterator(Iterator& iterator) { ++iterator; }
+
+  static const K& GetKey(Iterator& iterator) { return iterator->key; }
+  static const K& GetKey(ConstIterator& iterator) { return iterator->key; }
+
+  static V& GetValue(Iterator& iterator) { return iterator->value; }
+  static const V& GetValue(ConstIterator& iterator) { return iterator->value; }
+
+  static bool Insert(WTF::HashMap<K, V>& input, const K& key, V&& value) {
+    if (!WTF::HashMap<K, V>::isValidKey(key)) {
+      LOG(ERROR) << "The key value is disallowed by WTF::HashMap: " << key;
+      return false;
+    }
+    input.add(key, std::forward<V>(value));
+    return true;
+  }
+  static bool Insert(WTF::HashMap<K, V>& input, const K& key, const V& value) {
+    if (!WTF::HashMap<K, V>::isValidKey(key)) {
+      LOG(ERROR) << "The key value is disallowed by WTF::HashMap: " << key;
+      return false;
+    }
+    input.add(key, value);
+    return true;
+  }
+
+  static void SetToEmpty(WTF::HashMap<K, V>* output) { output->clear(); }
+};
+
+}  // namespace mojo
+
+#endif  // MOJO_PUBLIC_CPP_BINDINGS_MAP_TRAITS_WTF_HASH_MAP_H_