diff --git a/cmdopt.cpp b/cmdopt.cpp
index 04dc5246..c478534b 100644
--- a/cmdopt.cpp
+++ b/cmdopt.cpp
@@ -465,7 +465,7 @@ invalid_key::what (void) const noexcept
 
 ///////////////////////////////////////////////////////////////////////////////
 invalid_value::invalid_value (std::string _value):
-    m_value (_value)
+    m_value (std::move (_value))
 { ; }
 
 
diff --git a/json/except.cpp b/json/except.cpp
index 2eb0551f..bb85fd1a 100644
--- a/json/except.cpp
+++ b/json/except.cpp
@@ -38,5 +38,5 @@ json::parse_error::what (void) const noexcept
 ///////////////////////////////////////////////////////////////////////////////
 json::key_error::key_error (std::string _key):
     error (to_string (util::format::printf ("missing key '%s'", _key))),
-    key (_key)
+    key (std::move (_key))
 { ; }
diff --git a/stringid.cpp b/stringid.cpp
index fad528d5..f292ecfc 100644
--- a/stringid.cpp
+++ b/stringid.cpp
@@ -23,13 +23,13 @@ using util::stringid;
 
 //-----------------------------------------------------------------------------
 stringid::id_t
-stringid::add (const std::string key) {
+stringid::add (std::string key) {
     auto pos = m_map.find (key);
     if (pos != m_map.end ())
         throw std::invalid_argument ("duplicate stringid key");
 
     id_t id = m_map.size ();
-    m_map[key] = id;
+    m_map.insert ({ std::move (key), id });
     return id;
 }
 
diff --git a/time.cpp b/time.cpp
index 2be46660..f3599d0b 100644
--- a/time.cpp
+++ b/time.cpp
@@ -101,7 +101,7 @@ util::rate_limiter::poll (void)
 
 ///////////////////////////////////////////////////////////////////////////////
 util::polled_duration::polled_duration (std::string name, uint64_t interval):
-    m_name     (name),
+    m_name     (std::move (name)),
     m_interval (interval),
     m_next     (nanoseconds () + interval)
 { ; }