gtsam/doc/Code/LocalizationFactor.cpp

class UnaryFactor: public NoiseModelFactor1<Pose2> {
  double mx_, my_; ///< X and Y measurements

public:
  UnaryFactor(Key j, double x, double y, const SharedNoiseModel& model):
    NoiseModelFactor1<Pose2>(model, j), mx_(x), my_(y) {}

  Vector evaluateError(const Pose2& q,
                       boost::optional<Matrix&> H = boost::none) const
  {
    if (H) (*H) = (Matrix(2,3)<< 1.0,0.0,0.0, 0.0,1.0,0.0).finished();
    return (Vector(2) << q.x() - mx_, q.y() - my_).finished();
  }
};