Diagonal matrix properly in armadillo

Question

My code works but I'm just curious to see if someone knows how to do this but properly using Armadillo library. Thanks for your time :)

arma::mat W = arma::mat(4, 4, arma::fill::ones);
arma::mat D = arma::mat(4, 4, arma::fill::zeros);

for (size_t i = 0; i < W.n_rows; i++)
{
    for (size_t j = 0; j < W.n_cols; j++)
    {
        D(i, i) += W(i, j);
    }
}
std::cout<< "W = \n"<< W <<std::endl;
std::cout<< "D = \n"<< D <<std::endl;