diff --git a/examples/bal_problem.h b/examples/bal_problem.h
index 95e8d8f..64672cc 100644
--- a/examples/bal_problem.h
+++ b/examples/bal_problem.h
@@ -44,9 +44,11 @@
 
 class BALProblem {
  public:
-  explicit BALProblem(const std::string filename, bool use_quaternions);
+  explicit BALProblem(const std::string& filename, bool use_quaternions);
   ~BALProblem();
 
+  void WriteToFile(const std::string& filename) const;
+
   // Perturb the camera pose and the geometry with random normal
   // numbers with corresponding standard deviations.
   void Perturb(const double rotation_sigma,