eigen/doc/examples/function_taking_eigenbase.cpp

#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());
}