diff --git a/lout/container.cc b/lout/container.cc
index 10400ae..292a790 100644
--- a/lout/container.cc
+++ b/lout/container.cc
@@ -121,7 +121,7 @@ Vector::~Vector()
    DBG_OBJ_DELETE ();
 }
 
-int Vector::size ()
+int Vector::size () const
 {
    return numElements;
 }
@@ -481,7 +481,7 @@ HashSet::~HashSet()
    delete[] table;
 }
 
-int HashSet::size ()
+int HashSet::size () const
 {
    return numElements;
 }
diff --git a/lout/container.hh b/lout/container.hh
index 076757e..9963ccd 100644
--- a/lout/container.hh
+++ b/lout/container.hh
@@ -137,7 +137,7 @@ class Collection: public Collection0
 public:
    void intoStringBuffer(misc::StringBuffer *sb) const override;
    inline Iterator iterator() { Iterator it(createIterator()); return it; }
-   virtual int size() = 0;
+   virtual int size() const = 0;
 };
 
 
@@ -173,7 +173,7 @@ public:
    Vector(int initSize, bool ownerOfObjects);
    ~Vector();
 
-   int size();
+   int size() const override;
 
    void put(object::Object *newElement, int newPos = -1);
    void insert(object::Object *newElement, int pos);
@@ -315,7 +315,7 @@ public:
    HashSet(bool ownerOfObjects, int tableSize = 251);
    ~HashSet();
 
-   int size ();
+   int size () const override;
 
    void put (object::Object *object);
    bool contains (object::Object *key) const;