diff --git a/libsrc/core/archive.hpp b/libsrc/core/archive.hpp
index 76cb3567..dc381a4a 100644
--- a/libsrc/core/archive.hpp
+++ b/libsrc/core/archive.hpp
@@ -936,6 +936,11 @@ namespace ngcore
     Archive & operator & (char *& str) override
     { char* s = str; while(*s != '\0') ApplyHash(*(s++)); return *this; }
 
+    // HashArchive can be used in const context
+    template<typename T>
+      Archive & operator& (const T& val) const
+    { return (*this) & const_cast<T&>(val); }
+
     size_t GetHash() const { return hash_value; }
 
   private:
diff --git a/libsrc/core/bitarray.hpp b/libsrc/core/bitarray.hpp
index dff55daf..4005fc81 100644
--- a/libsrc/core/bitarray.hpp
+++ b/libsrc/core/bitarray.hpp
@@ -131,7 +131,7 @@ public:
     return Test(i);
   }
 
-  bool operator==(const BitArray& other) const;
+  NGCORE_API bool operator==(const BitArray& other) const;
 
   /// invert all bits
   NGCORE_API BitArray & Invert ();
@@ -147,7 +147,7 @@ public:
 
   NGCORE_API size_t NumSet () const;
 
-  void DoArchive(Archive& archive);
+  NGCORE_API void DoArchive(Archive& archive);
 
 private:
   ///