eigen/doc/snippets/Tutorial_solve_triangular.cpp

Matrix3f A;
Vector3f b;
A << 1,2,3,  0,5,6,  0,0,10;
b << 3, 3, 4;
cout << "Here is the matrix A:" << endl << A << endl;
cout << "Here is the vector b:" << endl << b << endl;
Vector3f x = A.triangularView<Upper>().solve(b);
cout << "The solution is:" << endl << x << endl;