Extend the "functions taking Eigen type" doc page to present the Ref<> option.

doc/examples/function_taking_ref.cpp

@@ -0,0 +1,19 @@
+#include <iostream>
+#include <Eigen/SVD>
+using namespace Eigen;
+using namespace std;
+
+float inv_cond(const Ref<const MatrixXf>& a)
+{
+  const VectorXf sing_vals = a.jacobiSvd().singularValues();
+  return sing_vals(sing_vals.size()-1) / sing_vals(0);
+}
+
+int main()
+{
+  Matrix4f m = Matrix4f::Random();
+  cout << "matrix m:" << endl << m << endl << endl;
+  cout << "inv_cond(m):          " << inv_cond(m)                      << endl;
+  cout << "inv_cond(m(1:3,1:3)): " << inv_cond(m.topLeftCorner(3,3))   << endl;
+  cout << "inv_cond(m+I):        " << inv_cond(m+Matrix4f::Identity()) << endl;
+}