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

doc/examples/class_FixedReshaped.cpp

@@ -0,0 +1,22 @@
+#include <Eigen/Core>
+#include <iostream>
+using namespace Eigen;
+using namespace std;
+
+template<typename Derived>
+Eigen::Reshaped<Derived, 4, 2>
+reshape_helper(MatrixBase<Derived>& m)
+{
+  return Eigen::Reshaped<Derived, 4, 2>(m.derived());
+}
+
+int main(int, char**)
+{
+  MatrixXd m(2, 4);
+  m << 1, 2, 3, 4,
+       5, 6, 7, 8;
+  MatrixXd n = reshape_helper(m);
+  cout << "matrix m is:" << endl << m << endl;
+  cout << "matrix n is:" << endl << n << endl;
+  return 0;
+}