diff --git a/lemon/capacity_scaling.h b/lemon/capacity_scaling.h
--- a/lemon/capacity_scaling.h
+++ b/lemon/capacity_scaling.h
@@ -301,6 +301,10 @@
 
     /// @}
 
+  protected:
+
+    CapacityScaling() {}
+
   public:
 
     /// \brief Constructor.
diff --git a/lemon/cost_scaling.h b/lemon/cost_scaling.h
--- a/lemon/cost_scaling.h
+++ b/lemon/cost_scaling.h
@@ -325,6 +325,10 @@
 
     /// @}
 
+  protected:
+
+    CostScaling() {}
+
   public:
 
     /// \brief Constructor.
diff --git a/lemon/hartmann_orlin.h b/lemon/hartmann_orlin.h
--- a/lemon/hartmann_orlin.h
+++ b/lemon/hartmann_orlin.h
@@ -241,6 +241,10 @@
 
     /// @}
 
+  protected:
+
+    HartmannOrlin() {}
+
   public:
 
     /// \brief Constructor.
diff --git a/lemon/howard.h b/lemon/howard.h
--- a/lemon/howard.h
+++ b/lemon/howard.h
@@ -231,6 +231,10 @@
     
     /// @}
 
+  protected:
+
+    Howard() {}
+
   public:
 
     /// \brief Constructor.
diff --git a/lemon/karp.h b/lemon/karp.h
--- a/lemon/karp.h
+++ b/lemon/karp.h
@@ -237,6 +237,10 @@
 
     /// @}
 
+  protected:
+
+    Karp() {}
+
   public:
 
     /// \brief Constructor.
diff --git a/lemon/suurballe.h b/lemon/suurballe.h
--- a/lemon/suurballe.h
+++ b/lemon/suurballe.h
@@ -403,6 +403,10 @@
     PredMap *_init_pred;
     bool _full_init;
 
+  protected:
+
+    Suurballe() {}
+
   public:
 
     /// \brief Constructor.