diff --git a/src/factory/FieldCreateFactory.cpp b/src/factory/FieldCreateFactory.cpp
index 32df6cc..76eb7f1 100644
--- a/src/factory/FieldCreateFactory.cpp
+++ b/src/factory/FieldCreateFactory.cpp
@@ -399,11 +399,11 @@ void UnionArray::deserialize(ByteBuffer* /*buffer*/, DeserializableControl* /*co
     throw std::runtime_error("not valid operation, use FieldCreate::deserialize instead");
 }
 
-string Structure::DEFAULT_ID = Structure::defaultId();
+const string Structure::DEFAULT_ID = Structure::defaultId();
 
-string & Structure::defaultId()
+const string & Structure::defaultId()
 {
-    static string id = "structure";
+    static const string id = "structure";
     return id;
 }
 
@@ -537,19 +537,19 @@ void Structure::deserialize(ByteBuffer* /*buffer*/, DeserializableControl* /*con
     throw std::runtime_error("not valid operation, use FieldCreate::deserialize instead");
 }
 
-string Union::DEFAULT_ID = Union::defaultId();
+const string Union::DEFAULT_ID = Union::defaultId();
 
-string & Union::defaultId()
+const string & Union::defaultId()
 {
-    static string id = "union";
+    static const string id = "union";
     return id;
 }
 
-string Union::ANY_ID = Union::anyId();
+const string Union::ANY_ID = Union::anyId();
 
-string & Union::anyId()
+const string & Union::anyId()
 {
-    static string id = "any";
+    static const string id = "any";
     return id;
 }
 
diff --git a/src/pv/pvIntrospect.h b/src/pv/pvIntrospect.h
index 3166407..2ad3888 100644
--- a/src/pv/pvIntrospect.h
+++ b/src/pv/pvIntrospect.h
@@ -649,13 +649,13 @@ public:
     /**
      * Default structure ID.
      */
-    static std::string DEFAULT_ID;
+    static const std::string DEFAULT_ID;
 
     /**
      * Get the default structure ID.
      * @return The default structure ID.
      */
-    static std::string & defaultId();
+    static const std::string & defaultId();
 
     /**
      * Destructor.
@@ -759,24 +759,24 @@ public:
     /**
      * Default union ID.
      */
-    static std::string DEFAULT_ID;
+    static const std::string DEFAULT_ID;
 
     /**
      * Get the default union ID.
      * @return The default union ID.
      */
-    static std::string & defaultId();
+    static const std::string & defaultId();
 
     /**
      * Default variant union ID.
      */
-    static std::string ANY_ID;
+    static const std::string ANY_ID;
 
     /**
      * Get the default variant union ID.
      * @return The default variant union ID.
      */
-    static std::string & anyId();
+    static const std::string & anyId();
 
     /**
      * Destructor.