Use Eigen::fix<N> to pass compile-time sizes.

doc/examples/class_Reshaped.cpp

@@ -0,0 +1,23 @@
+#include <Eigen/Core>
+#include <iostream>
+using namespace std;
+using namespace Eigen;
+
+template<typename Derived>
+const Reshaped<const Derived>
+reshape_helper(const MatrixBase<Derived>& m, int rows, int cols)
+{
+  return Reshaped<const Derived>(m.derived(), rows, cols);
+}
+
+int main(int, char**)
+{
+  MatrixXd m(3, 4);
+  m << 1, 4, 7, 10,
+       2, 5, 8, 11,
+       3, 6, 9, 12;
+  cout << m << endl;
+  Ref<const MatrixXd> n = reshape_helper(m, 2, 6);
+  cout << "Matrix m is:" << endl << m << endl;
+  cout << "Matrix n is:" << endl << n << endl;
+}