Add ListContainer::swap

This patch implements a simple swap function on ListContainer. Users
can use this function to swap a list container with... wait for it...
another list container!

BUG=484943
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1199343004

Cr-Commit-Position: refs/heads/master@{#335977}

2 files changed, 49 insertions, 2 deletions

diff --git a/cc/base/list_container.h b/cc/base/list_container.h
index 5463ca2..a9444cd 100644
--- a/cc/base/list_container.h
+++ b/cc/base/list_container.h
@@ -288,6 +288,11 @@ class ListContainer : public ListContainerBase {
     return new (*at) DerivedElementType();
   }
 
+  template <typename DerivedElementType>
+  void swap(ListContainer<DerivedElementType>& other) {
+    data_.swap(other.data_);
+  }
+
   // Appends a new item without copying. The original item will not be
   // destructed and will be replaced with a new DerivedElementType. The
   // DerivedElementType does not have to match the moved type as a full block
diff --git a/cc/base/list_container_unittest.cc b/cc/base/list_container_unittest.cc
index 2c56e85..536a55e 100644
--- a/cc/base/list_container_unittest.cc
+++ b/cc/base/list_container_unittest.cc
@@ -107,6 +107,14 @@ class SimpleDerivedElementConstructMagicNumberTwo
   }
 };
 
+class SimpleDerivedElementConstructMagicNumberThree
+    : public SimpleDerivedElement {
+ public:
+  SimpleDerivedElementConstructMagicNumberThree() {
+    set_value(kMagicNumberToUseForSimpleDerivedElementThree);
+  }
+};
+
 class MockDerivedElement : public SimpleDerivedElementConstructMagicNumberOne {
  public:
   ~MockDerivedElement() override { Destruct(); }
@@ -881,8 +889,7 @@ TEST(ListContainerTest, AppendByMovingReplacesSourceWithNewDerivedElement) {
 
   // AppendByMoving replaces the source element with a new derived element so
   // we do not expect sizes to shrink after AppendByMoving is called.
-  EXPECT_EQ(2u,
-            list_1.size());  // One direct allocation and one AppendByMoving.
+  EXPECT_EQ(2u, list_1.size());  // One direct allocation, one AppendByMoving.
   EXPECT_EQ(1u, list_2.size());  // One AppendByMoving.
 }
 
@@ -954,5 +961,40 @@ TEST(ListContainerTest, AppendByMovingLongAndSimpleDerivedElements) {
             list.back()->get_value());
 }
 
+TEST(ListContainerTest, Swap) {
+  ListContainer<SimpleDerivedElement> list_1(kCurrentLargestDerivedElementSize);
+  list_1.AllocateAndConstruct<SimpleDerivedElementConstructMagicNumberOne>();
+  ListContainer<SimpleDerivedElement> list_2(kCurrentLargestDerivedElementSize);
+  list_2.AllocateAndConstruct<SimpleDerivedElementConstructMagicNumberTwo>();
+  list_2.AllocateAndConstruct<SimpleDerivedElementConstructMagicNumberThree>();
+
+  SimpleDerivedElement* pre_swap_list_1_front = list_1.front();
+
+  EXPECT_EQ(kMagicNumberToUseForSimpleDerivedElementOne,
+            list_1.front()->get_value());
+  EXPECT_EQ(1u, list_1.size());
+
+  EXPECT_EQ(kMagicNumberToUseForSimpleDerivedElementTwo,
+            list_2.front()->get_value());
+  EXPECT_EQ(kMagicNumberToUseForSimpleDerivedElementThree,
+            list_2.back()->get_value());
+  EXPECT_EQ(2u, list_2.size());
+
+  list_2.swap(list_1);
+
+  EXPECT_EQ(kMagicNumberToUseForSimpleDerivedElementTwo,
+            list_1.front()->get_value());
+  EXPECT_EQ(kMagicNumberToUseForSimpleDerivedElementThree,
+            list_1.back()->get_value());
+  EXPECT_EQ(2u, list_1.size());
+
+  EXPECT_EQ(kMagicNumberToUseForSimpleDerivedElementOne,
+            list_2.front()->get_value());
+  EXPECT_EQ(1u, list_2.size());
+
+  // Ensure pointers are still valid after swapping.
+  EXPECT_EQ(pre_swap_list_1_front, list_2.front());
+}
+
 }  // namespace
 }  // namespace cc