mojom: Add associative arrays to the mojom language.

mojo/public/cpp/bindings/map.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 MOJO_PUBLIC_CPP_BINDINGS_MAP_H_
+#define MOJO_PUBLIC_CPP_BINDINGS_MAP_H_
+
+#include <map>
+
+#include "mojo/public/cpp/bindings/lib/map_internal.h"
+
+namespace mojo {
+
+template <typename Key, typename Value>
+class Map {
+  MOJO_MOVE_ONLY_TYPE_FOR_CPP_03(Map, RValue)
+ public:
+  // Map keys can not be move only classes.
+  static_assert(!internal::IsMoveOnlyType<Key>::value,
+                "Map keys can not be move only types.");
+
+  typedef internal::MapTraits<Key,
+                              Value,
+                              internal::IsMoveOnlyType<Value>::value> Traits;
+  typedef typename Traits::KeyStorageType KeyStorageType;
+  typedef typename Traits::KeyRefType KeyRefType;
+  typedef typename Traits::KeyConstRefType KeyConstRefType;
+  typedef typename Traits::KeyForwardType KeyForwardType;
+
+  typedef typename Traits::ValueStorageType ValueStorageType;
+  typedef typename Traits::ValueRefType ValueRefType;
+  typedef typename Traits::ValueConstRefType ValueConstRefType;
+  typedef typename Traits::ValueForwardType ValueForwardType;
+
+  typedef internal::Map_Data<typename internal::WrapperTraits<Key>::DataType,
+                             typename internal::WrapperTraits<Value>::DataType>
+      Data_;
+
+  Map() : is_null_(true) {}
+
+  Map(mojo::Array<Key> keys, mojo::Array<Value> values) : is_null_(false) {
+    MOJO_DCHECK(keys.size() == values.size());
+    Traits::InitializeFrom(&map_, keys.Pass(), values.Pass());
+  }
+
+  ~Map() { Traits::Finalize(&map_); }
+
+  Map(RValue other) : is_null_(true) { Take(other.object); }
+  Map& operator=(RValue other) {
+    Take(other.object);
+    return *this;
+  }
+
+  template <typename U>
+  static Map From(const U& other) {
+    return TypeConverter<Map, U>::Convert(other);
+  }
+
+  template <typename U>
+  U To() const {
+    return TypeConverter<U, Map>::Convert(*this);
+  }
+
+  void reset() {
+    if (!map_.empty()) {
+      Traits::Finalize(&map_);
+      map_.clear();
+    }
+    is_null_ = true;
+  }
+
+  bool is_null() const { return is_null_; }
+
+  size_t size() const { return map_.size(); }
+
+  // Inserts a key-value pair into the mpa. Like std::map, this does not insert

[yzshen1, 2014/10/08 21:58:33]

mpa -> map

+  // |value| if |key| is already a member of the map.
+  void insert(KeyForwardType key, ValueForwardType value) {

[yzshen1, 2014/10/08 21:58:33]

Do we also want a remove ?

[Elliot Glaysher, 2014/10/08 23:40:49]

I'm not sure. It was unused in this patch, and I'm fine adding it once there's a
need for it, so I'd like to punt on that.

[yzshen1, 2014/10/09 17:41:58]

That is fine. I am pretty sure people will want it, but we can add it later.

+    is_null_ = false;
+    Traits::Insert(&map_, key, value);
+  }
+
+  ValueRefType at(KeyForwardType key) { return Traits::at(&map_, key); }
+  ValueConstRefType at(KeyForwardType key) const {
+    return Traits::at(&map_, key);
+  }
+
+  void Swap(Map<Key, Value>* other) {
+    std::swap(is_null_, other->is_null_);
+    map_.swap(other->map_);
+  }
+  void Swap(std::map<Key, Value>* other) {
+    is_null_ = false;
+    map_.swap(*other);
+  }
+
+  // This moves all values in the map to a set of parallel arrays. This action
+  // is destructive because we can have move-only objects as values; therefore
+  // we can't have copy semantics here.
+  void DecomposeMapTo(mojo::Array<Key>* keys, mojo::Array<Value>* values) {
+    Traits::Decompose(&map_, keys, values);
+    Traits::Finalize(&map_);
+    map_.clear();
+    is_null_ = true;
+  }
+
+  // Please note that calling this method will fail compilation if the element

[yzshen1, 2014/10/08 21:58:33]

nit: element -> value

+  // type cannot be cloned (which usually means that it is a Mojo handle type or
+  // a type contains Mojo handles).
+  Map Clone() const {
+    Map result;
+    result.is_null_ = is_null_;
+    Traits::Clone(map_, &result.map_);
+    return result.Pass();
+  }
+
+  class ConstMapIterator {
+   public:
+    ConstMapIterator(
+        const typename std::map<KeyStorageType,
+                                ValueStorageType>::const_iterator& it)
+        : it_(it) {}
+
+    KeyConstRefType GetKey() { return Traits::GetKey(it_); }
+    ValueConstRefType GetValue() { return Traits::GetValue(it_); }
+
+    ConstMapIterator& operator++() {
+      it_++;
+      return *this;
+    }
+    bool operator!=(const ConstMapIterator& rhs) const {
+      return it_ != rhs.it_;
+    }
+    bool operator==(const ConstMapIterator& rhs) const {
+      return it_ == rhs.it_;
+    }
+
+   private:
+    typename std::map<KeyStorageType, ValueStorageType>::const_iterator it_;
+  };
+
+  // Provide read-only iteration over map members.
+  ConstMapIterator begin() const { return ConstMapIterator(map_.begin()); }
+  ConstMapIterator end() const { return ConstMapIterator(map_.end()); }
+
+  ConstMapIterator find(KeyForwardType key) const {
+    return ConstMapIterator(map_.find(key));
+  }
+
+ private:
+  typedef std::map<KeyStorageType, ValueStorageType> Map::*Testable;
+
+ public:
+  operator Testable() const { return is_null_ ? 0 : &Map::map_; }
+
+ private:
+  void Take(Map* other) {
+    reset();
+    Swap(other);
+  }
+
+  std::map<KeyStorageType, ValueStorageType> map_;
+  bool is_null_;
+};
+
+template <typename MojoKey,
+          typename MojoValue,
+          typename STLKey,
+          typename STLValue>
+struct TypeConverter<Map<MojoKey, MojoValue>, std::map<STLKey, STLValue>> {
+  static Map<MojoKey, MojoValue> Convert(
+      const std::map<STLKey, STLValue>& input) {
+    Map<MojoKey, MojoValue> result;
+    for (auto& pair : input) {

[yzshen1, 2014/10/08 21:58:33]

In this case, an empty std::map is converted to a NULL Map.
I think this may cause confusion.
If I convert a std::map to a Map and then send it as a non-nullable field in
Mojo messages. It will break if the std::map is empty.

(We always convert empty std::vector to empty Array.)

[Elliot Glaysher, 2014/10/08 23:40:49]

I've added an accessor to change |is_null_|. Do you have a better idea than
that, or is that fine?

[yzshen1, 2014/10/09 17:41:58]

I am a little concerned that it may not be very obvious.

I am thinking about something like this:
Map<> a;  // Default to non-null.
Map<> b(nullptr);  // Set it to null.

a.clear();    // Clear to empty.
a = nullptr;  // Reset to null.

One issue with this proposal is that:
Array<Map<>> a(100);  // We are initializing 100 empty maps. This won't have a
performance difference locally, but if we send it over a message pipe, it is
significantly bigger.

I think this may be a rare case. And people can reset those maps to null if they
care, or directly push_back() null maps.

But if we choose this approach, we have to make changes to the Array
constructors (and current call sites) to be consistent. And we will need to ask
around to make sure people like this idea. I am fine if you think we should
handle this issue separately.

+      result.insert(TypeConverter<MojoKey, STLKey>::Convert(pair.first),
+                    TypeConverter<MojoValue, STLValue>::Convert(pair.second));
+    }
+    return result.Pass();
+  }
+};
+
+template <typename MojoKey,
+          typename MojoValue,
+          typename STLKey,
+          typename STLValue>
+struct TypeConverter<std::map<STLKey, STLValue>, Map<MojoKey, MojoValue>> {
+  static std::map<STLKey, STLValue> Convert(
+      const Map<MojoKey, MojoValue>& input) {
+    std::map<STLKey, STLValue> result;
+    if (!input.is_null()) {
+      for (auto it = input.begin(); it != input.end(); ++it) {
+        result.insert(std::make_pair(
+            TypeConverter<STLKey, MojoKey>::Convert(it.GetKey()),
+            TypeConverter<STLValue, MojoValue>::Convert(it.GetValue())));
+      }
+    }
+    return result;
+  }
+};
+
+}  // namespace mojo
+
+#endif  // MOJO_PUBLIC_CPP_BINDINGS_MAP_H_