[DenseMap] Add a C++17-style try_emplace method.

This provides an elegant pattern to solve the "construct if not in map
already" problem we have many times in LLVM. Without try_emplace we
either have to rely on a sentinel value (nullptr) or do two lookups.

llvm-svn: 276277

1 files changed, 10 insertions, 0 deletions

diff --git a/llvm/unittests/ADT/DenseMapTest.cpp b/llvm/unittests/ADT/DenseMapTest.cpp
index db00f8c..cbf1a44 100644
--- a/llvm/unittests/ADT/DenseMapTest.cpp
+++ b/llvm/unittests/ADT/DenseMapTest.cpp
@@ -619,4 +619,14 @@ TEST(DenseMapCustomTest, SmallDenseMapGrowTest) {
   EXPECT_TRUE(map.find(32) == map.end());
 }
 
+TEST(DenseMapCustomTest, TryEmplaceTest) {
+  DenseMap<int, std::unique_ptr<int>> Map;
+  std::unique_ptr<int> P(new int(2));
+  auto Try1 = Map.try_emplace(0, new int(1));
+  EXPECT_TRUE(Try1.second);
+  auto Try2 = Map.try_emplace(0, std::move(P));
+  EXPECT_FALSE(Try2.second);
+  EXPECT_EQ(Try1.first, Try2.first);
+  EXPECT_NE(nullptr, P);
+}
 }