Add internal C++14 polyfill for std::exchange (#1498)

Author: eramongodb

src/mongocxx/lib/CMakeLists.txt

@@ -262,6 +262,7 @@ set_dist_list(src_mongocxx_lib_DIST
     mongocxx/private/scoped_bson_value.hh
     mongocxx/private/scoped_bson.hh
     mongocxx/private/ssl.hh
+    mongocxx/private/utility.hh
     mongocxx/v_noabi/mongocxx/append_aggregate_options.hh
     mongocxx/v_noabi/mongocxx/bulk_write.hh
     mongocxx/v_noabi/mongocxx/change_stream.hh

src/mongocxx/lib/mongocxx/private/utility.hh

@@ -0,0 +1,34 @@
+// Copyright 2009-present MongoDB, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#pragma once
+
+#include <utility> // IWYU pragma: export
+
+//
+
+namespace mongocxx {
+
+#if defined(__cpp_lib_exchange_function) && __cpp_lib_exchange_function >= 201304L
+using std::exchange;
+#else
+template <typename T, typename U = T>
+T exchange(T& obj, U&& new_val) {
+    T old_val = std::move(obj);
+    obj = std::forward<U>(new_val);
+    return old_val;
+}
+#endif
+
+} // namespace mongocxx