#include <iostream>
#include <Eigen/Core>
using namespace Eigen;
template <typename Derived>
void print_size(const EigenBase<Derived>& b)
{
std::cout << "size (rows, cols): " << b.size() << " (" << b.rows()
<< ", " << b.cols() << ")" << std::endl;
}
int main()
{
Vector3f v;
print_size(v);
// v.asDiagonal() returns a 3x3 diagonal matrix pseudo-expression
print_size(v.asDiagonal());
}