index_sequence_for_tuple

Summary: A simple helper to construct an index sequence from a given tuple type

Reviewed By: yfeldblum

Differential Revision: D8626776

fbshipit-source-id: a4ea412293981d58e91a921dcae8468849f5f0bf

folly/functional/ApplyTuple.h

@@ -46,6 +46,13 @@ constexpr decltype(auto) apply(F&& func, Tuple&& tuple) {
       folly::make_index_sequence<size>{});
 }
 
+/**
+ * Helper to generate an index sequence from a tuple like type
+ */
+template <typename Tuple>
+using index_sequence_for_tuple =
+    make_index_sequence<std::tuple_size<Tuple>::value>;
+
 namespace detail {
 namespace apply_tuple {
 

folly/functional/test/ApplyTupleTest.cpp

@@ -416,3 +416,24 @@ TEST(MakeFromTupleTest, make_from_tuple) {
   EXPECT_EQ(expected.tuple_, s3.tuple_);
   EXPECT_TRUE(str.empty());
 }
+
+TEST(MakeIndexSequenceFromTuple, Basic) {
+  using folly::index_sequence;
+  using folly::index_sequence_for_tuple;
+  using OneElementTuple = std::tuple<int>;
+  using TwoElementTuple = std::tuple<int>;
+
+  EXPECT_TRUE((std::is_same<
+               index_sequence_for_tuple<OneElementTuple>,
+               index_sequence<0>>::value));
+  EXPECT_TRUE((std::is_same<
+               index_sequence_for_tuple<const OneElementTuple>,
+               index_sequence<0>>::value));
+
+  EXPECT_TRUE((std::is_same<
+               index_sequence_for_tuple<TwoElementTuple>,
+               index_sequence<0>>::value));
+  EXPECT_TRUE((std::is_same<
+               index_sequence_for_tuple<const TwoElementTuple>,
+               index_sequence<0>>::value));
+}