cc: Add RandomAccessListContainer, which has more restricted API.

cc/base/random_access_list_container.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 CC_BASE_RANDOM_ACCESS_LIST_CONTAINER_H_
+#define CC_BASE_RANDOM_ACCESS_LIST_CONTAINER_H_
+
+#include <vector>
+
+#include "base/logging.h"
+#include "cc/base/list_container_helper.h"
+
+namespace cc {
+
+// RandomAccessListContainer is a container similar to ListContainer (see
+// list_container.h), but it allows random access into its elements via
+// operator[]. In order to have efficient support for random access, some
+// functionality is not available for RandomAccessListContainers, such as
+// insert/deletes in the middle of the list.
+template <class BaseElementType>
+class RandomAccessListContainer {
+ public:
+  // BaseElementType is the type of raw pointers this class hands out; however,
+  // its derived classes might require different memory sizes.
+  // max_size_for_derived_class the largest memory size required for all the
+  // derived classes to use for allocation.
+  explicit RandomAccessListContainer(size_t max_size_for_derived_class)
+      : helper_(max_size_for_derived_class) {}
+
+  // This constructor reserves the requested memory up front so only a single
+  // allocation is needed. When num_of_elements_to_reserve_for is zero, use the
+  // default size.
+  RandomAccessListContainer(size_t max_size_for_derived_class,
+                            size_t num_of_elements_to_reserve_for)
+      : helper_(max_size_for_derived_class, num_of_elements_to_reserve_for) {
+    items_.reserve(num_of_elements_to_reserve_for);
+  }
+
+  ~RandomAccessListContainer() {
+    for (BaseElementType* item : items_)
+      item->~BaseElementType();
+  }
+
+  void clear() {
+    for (BaseElementType* item : items_)
+      item->~BaseElementType();
+    helper_.clear();
+    items_.clear();
+  }
+
+  bool empty() const { return helper_.empty(); }
+  size_t size() const { return helper_.size(); }
+  size_t GetCapacityInBytes() const { return helper_.GetCapacityInBytes(); }
+
+  template <typename DerivedElementType>
+  DerivedElementType* AllocateAndConstruct() {
+    auto* value =
+        new (helper_.Allocate(sizeof(DerivedElementType))) DerivedElementType;
+    items_.push_back(value);
+    return value;
+  }
+
+  void RemoveLast() {
+    items_.back()->~BaseElementType();
+    items_.pop_back();
+    helper_.RemoveLast();
+  }
+
+  const BaseElementType* operator[](size_t index) const {
+    DCHECK_GE(index, 0u);
+    DCHECK_LT(index, items_.size());
+    return items_[index];
+  }
+
+  BaseElementType* operator[](size_t index) {
+    DCHECK_GE(index, 0u);
+    DCHECK_LT(index, items_.size());
+    return items_[index];
+  }
+
+  // Note that although BaseElementType objects can change, the pointer itself
+  // (in the vector) cannot. So this class only supports a const iterator.
+  using ConstIterator = typename std::vector<BaseElementType*>::const_iterator;
+  ConstIterator begin() const { return items_.begin(); }
+  ConstIterator end() const { return items_.end(); }
+
+ private:
+  ListContainerHelper helper_;
+  std::vector<BaseElementType*> items_;
+};
+
+}  // namespace cc
+
+#endif  // CC_BASE_RANDOM_ACCESS_LIST_CONTAINER_H_