eigen/doc/snippets/Tutorial_AdvancedInitialization_Join.cpp

RowVectorXd vec1(3);
vec1 << 1, 2, 3;
std::cout << "vec1 = " << vec1 << std::endl;

RowVectorXd vec2(4);
vec2 << 1, 4, 9, 16;
std::cout << "vec2 = " << vec2 << std::endl;

RowVectorXd joined(7);
joined << vec1, vec2;
std::cout << "joined = " << joined << std::endl;