diff --git a/src/samplefunctional.hh b/src/samplefunctional.hh
index 9b7a710047b7b932635da07e6956b89abfba1cb0..02fd5fc68065fcdd8cf6533092a46061df2fea43 100644
--- a/src/samplefunctional.hh
+++ b/src/samplefunctional.hh
@@ -22,9 +22,9 @@ template <int dimension> class SampleFunctional {
 
   typedef MyNonlinearity<dimension> NonlinearityType;
 
-  SampleFunctional(SmallMatrix _A, SmallVector _b,
-                   MyNonlinearity<dimension> _phi)
-      : A(_A), b(_b), phi(_phi) {}
+  SampleFunctional(SmallMatrix const &A, SmallVector const &b,
+                   MyNonlinearity<dimension> const &phi)
+      : A(A), b(b), phi(phi) {}
 
   double operator()(const SmallVector v) const {
     SmallVector y;
@@ -81,9 +81,9 @@ template <int dimension> class SampleFunctional {
     ret *= -1;
   }
 
-  SmallMatrix A;
-  SmallVector b;
-  NonlinearityType phi;
+  SmallMatrix const &A;
+  SmallVector const &b;
+  NonlinearityType const &phi;
 
 private:
   // Gradient of the smooth part