Initialize armadillo matrix with std vector in c++

Armadillo's documentation page shows us how to initialize a arma::mat from a c array, here is how to do it with a c++ std vector:

// [[Rcpp::export]]
void testvec1() {
    std::vector y;
    y.push_back(1);
    y.push_back(2);
    y.push_back(3);

    // notice that &y.front() is the address of the first data element
    arma::mat x(&y.front(), 3, 1);
    x.print();
}

// [[Rcpp::export]]
void testvec2() {
    std::vector y;
    vector y1 {1, 2};
    vector y2 {1, 3};
    vector y3 {1, 4};
    y.reserve(y1.size() * 3);
    y.insert(y.end(), y1.begin(), y1.end());
    y.insert(y.end(), y2.begin(), y2.end());
    y.insert(y.end(), y3.begin(), y3.end());

    arma::mat x(&y.front(), 6, 1);
    x.print();
}

If you want to sourceCpp this into R, you need to enable c++11 features:

# R code

Sys.setenv("PKG_CXXFLAGS"="-std=c++11")
require(Rcpp)