Bug 1595750 - Add MakeBackInserter function to create a back-inserting output iterator for nsTArray. r=froydnj

Differential Revision: https://phabricator.services.mozilla.com/D52685

--HG--
extra : moz-landing-system : lando

xpcom/ds/nsTArray.h

@@ -2730,6 +2730,30 @@ Span<const ElementType> MakeSpan(
   return aTArray;
 }
 
+template <typename T>
+class nsTArrayBackInserter
+    : public std::iterator<std::output_iterator_tag, void, void, void, void> {
+  nsTArray<T>* mArray;
+
+ public:
+  explicit nsTArrayBackInserter(nsTArray<T>& aArray) : mArray{&aArray} {}
+
+  template <typename O>
+  nsTArrayBackInserter& operator=(O&& aValue) {
+    mArray->EmplaceBack(std::forward<O>(aValue));
+    return *this;
+  }
+
+  nsTArrayBackInserter& operator*() { return *this; }
+
+  void operator++() {}
+};
+
+template <typename T>
+auto MakeBackInserter(nsTArray<T>& aArray) {
+  return nsTArrayBackInserter<T>{aArray};
+}
+
 }  // namespace mozilla
 
 // MOZ_DBG support

xpcom/tests/gtest/TestTArray.cpp

@@ -443,4 +443,22 @@ TEST(TArray, RemoveElementsAt_ByIterator)
   ASSERT_EQ(expected, array);
 }
 
+static_assert(std::is_copy_assignable<decltype(
+                  MakeBackInserter(std::declval<nsTArray<int>&>()))>::value,
+              "output iteraror must be copy-assignable");
+static_assert(std::is_copy_constructible<decltype(
+                  MakeBackInserter(std::declval<nsTArray<int>&>()))>::value,
+              "output iterator must be copy-constructible");
+
+TEST(TArray, MakeBackInserter)
+{
+  const std::vector<int> src{1, 2, 3, 4};
+  nsTArray<int> dst;
+
+  std::copy(src.begin(), src.end(), MakeBackInserter(dst));
+
+  const nsTArray<int> expected{1, 2, 3, 4};
+  ASSERT_EQ(expected, dst);
+}
+
 }  // namespace TestTArray