build: switch to C++17

Change-Id: Ie68020a04b7e20b74778b6d0370544ded55c5e26
diff --git a/daemon/rib/readvertise/host-to-gateway-readvertise-policy.cpp b/daemon/rib/readvertise/host-to-gateway-readvertise-policy.cpp
index 9ee7985..89f11fd 100644
--- a/daemon/rib/readvertise/host-to-gateway-readvertise-policy.cpp
+++ b/daemon/rib/readvertise/host-to-gateway-readvertise-policy.cpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /*
- * Copyright (c) 2014-2019,  Regents of the University of California,
+ * Copyright (c) 2014-2022,  Regents of the University of California,
  *                           Arizona Board of Regents,
  *                           Colorado State University,
  *                           University Pierre & Marie Curie, Sorbonne University,
@@ -44,13 +44,13 @@
   m_refreshInterval = interval ? time::seconds(*interval) : DEFAULT_REFRESH_INTERVAL;
 }
 
-optional<ReadvertiseAction>
+std::optional<ReadvertiseAction>
 HostToGatewayReadvertisePolicy::handleNewRoute(const RibRouteRef& ribRoute) const
 {
   auto ribEntryName = ribRoute.entry->getName();
   if (scope_prefix::LOCALHOST.isPrefixOf(ribEntryName) ||
       ribEntryName == RibManager::LOCALHOP_TOP_PREFIX) {
-    return nullopt;
+    return std::nullopt;
   }
 
   // find out the shortest identity whose name is a prefix of the RIB entry name
@@ -73,12 +73,10 @@
     }
   }
 
-  if (isFound) {
-    return ReadvertiseAction{prefixToAdvertise, ndn::security::signingByIdentity(signingIdentity)};
+  if (!isFound) {
+    return std::nullopt;
   }
-  else {
-    return nullopt;
-  }
+  return ReadvertiseAction{prefixToAdvertise, ndn::security::signingByIdentity(signingIdentity)};
 }
 
 time::milliseconds