Fixed Sized Eigen types as parameters

Question

I am trying to write a function that takes fixed size Eigen Types (but templated on Scalar type e.g. float/double). I have read http://eigen.tuxfamily.org/dox/TopicFunctionTakingEigenTypes.html but I am not able to make it work perfectly.

Here is the function definition:

template <typename T>
inline Matrix<T, 3, 3> makeSkewSymmetric(const Matrix<T, 3, 1>& v)
{
  Matrix<T, 3, 3> out;
  out <<     0, -v[2],  v[1],
          v[2],     0, -v[0],
         -v[1],  v[0],     0;

  return out;
}

Now I am using this as following:

Vector3d a(1,2,3);
Matrix3d ass = makeSkewSymmetric(a); // Compiles
Matrix3d ass = makeSkewSymmetric(a + a); // does NOT compile

I guess, I need to use some sort of MatrixBase<Derived>, but then how do I restrict the size, as the function only makes sense for vectors of length 3.

Edit: I redefined the function as following. It works, but is there a better way?

template <typename Derived>
inline Matrix<typename Derived::Scalar, 3, 3> makeSkewSymmetric(const MatrixBase<Derived>& v)
{
    BOOST_STATIC_ASSERT(Derived::RowsAtCompileTime == 3 && Derived::ColsAtCompileTime == 1);
  Matrix<typename Derived::Scalar, 3, 3> out;
  out <<     0, -v[2],  v[1],
          v[2],     0, -v[0],
         -v[1],  v[0],     0;

  return out;
}

Answer 1

I just thought of a good way of checking the way the Eigen developers would want you to solve this problem. Eigen comes with a cross function on MatrixBase, but this function, like yours, is only sensible for 3D vectors - so I dug up the relevant part from the Eigen3 source: (cf Eigen/src/Geometry/OrthoMethods.h)

...
inline typename MatrixBase<Derived>::template cross_product_return_type<OtherDerived>::type
MatrixBase<Derived>::cross(const MatrixBase<OtherDerived>& other) const
{
  EIGEN_STATIC_ASSERT_VECTOR_SPECIFIC_SIZE(Derived,3)
  EIGEN_STATIC_ASSERT_VECTOR_SPECIFIC_SIZE(OtherDerived,3)
  ...

and indeed, Eigen itself uses asserts (albeit its own flavor) to check for dimensions in generalized functions.