eigen template doesn't work with matrices of fixed size

Question

I wrote the following template to check if a triangle is degenerate:

template<typename Derived>
bool nondegenerate(const Eigen::PlainObjectBase<Derived>& triangle) {

  typedef typename Derived::Scalar scalarType;

  if (triangle.cols() == 2) {
    Eigen::Matrix<scalarType, 1, 2> v1 = triangle.row(1) - triangle.row(0);
    Eigen::Matrix<scalarType, 1, 2> v2 = triangle.row(2) - triangle.row(0);
    return v1(0)*v2(1) - v1(1)*v2(0);
  } else if (triangle.cols() == 3) {
    Eigen::Matrix<scalarType, 1, 3> v1 = triangle.row(1) - triangle.row(0);
    Eigen::Matrix<scalarType, 1, 3> v2 = triangle.row(2) - triangle.row(0);
    return v1.cross(v2).norm();
  } else {
    std::cerr << "Undefined input." << std::endl;
    return false;
  }
}

The above code works fine if I call nondegenerate with a matrix of type Eigen::MatrixXi:

Eigen::MatrixXi triangle;
triangle.resize(3,2);
triangle << 1,2,3,4,5,6;
nondegenerate(triangle);

However, if I replace the above dynamic size matrix with a fixed size matrix:

Eigen::Matrix<int, 3, 2> triangle;
triangle << 1,2,3,4,5,6;
nondegenerate(triangle);

It reports compile errors:

/usr/local/include/eigen3/Eigen/src/Core/AssignEvaluator.h:833:3: error: static_assert failed due to requirement '(int(Eigen::internal::size_of_xpr_at_compile_time<Eigen::Matrix<int, 1, 3, 1, 1, 3> >::ret) == 0 && int(Eigen::internal::size_of_xpr_at_compile_time<Eigen::CwiseBinaryOp<Eigen::internal::scalar_difference_op<int, int>, const Eigen::Block<const Eigen::Matrix<int, 3, 2, 0, 3, 2>, 1, 2, false>, const Eigen::Block<const Eigen::Matrix<int, 3, 2, 0, 3, 2>, 1, 2, false> > >::ret) == 0) || ((int(Matrix<int, 1, 3, 1, 1, 3>::RowsAtCompileTime) == Eigen::Dynamic || int(CwiseBinaryOp<Eigen::internal::scalar_difference_op<int, int>, const Eigen::Block<const Eigen::Matrix<int, 3, 2, 0, 3, 2>, 1, 2, false>, const Eigen::Block<const Eigen::Matrix<int, 3, 2, 0, 3, 2>, 1, 2, false> >::RowsAtCompileTime) == Eigen::Dynamic || int(Matrix<int, 1, 3, 1, 1, 3>::RowsAtCompileTime) == int(CwiseBinaryOp<Eigen::internal::scalar_difference_op<int, int>, const Eigen::Block<const Eigen::Matrix<int, 3, 2, 0, 3, 2>, 1, 2, false>, const Eigen::Block<const Eigen::Matrix<int, 3, 2, 0, 3, 2>, 1, 2, false> >::RowsAtCompileTime)) && (int(Matrix<int, 1, 3, 1, 1, 3>::ColsAtCompileTime) == Eigen::Dynamic || int(CwiseBinaryOp<Eigen::internal::scalar_difference_op<int, int>, const Eigen::Block<const Eigen::Matrix<int, 3, 2, 0, 3, 2>, 1, 2, false>, const Eigen::Block<const Eigen::Matrix<int, 3, 2, 0, 3, 2>, 1, 2, false> >::ColsAtCompileTime) == Eigen::Dynamic || int(Matrix<int, 1, 3, 1, 1, 3>::ColsAtCompileTime) == int(CwiseBinaryOp<Eigen::internal::scalar_difference_op<int, int>, const Eigen::Block<const Eigen::Matrix<int, 3, 2, 0, 3, 2>, 1, 2, false>, const Eigen::Block<const Eigen::Matrix<int, 3, 2, 0, 3, 2>, 1, 2, false> >::ColsAtCompileTime)))' "YOU_MIXED_MATRICES_OF_DIFFERENT_SIZES" EIGEN_STATIC_ASSERT_SAME_MATRIX_SIZE(ActualDstTypeCleaned,Src) ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ /usr/local/include/eigen3/Eigen/src/Core/util/StaticAssert.h:190:3: note: expanded from macro 'EIGEN_STATIC_ASSERT_SAME_MATRIX_SIZE' EIGEN_STATIC_ASSERT(
^~~~~~~~~~~~~~~~~~~~~~ /usr/local/include/eigen3/Eigen/src/Core/util/StaticAssert.h:33:40: note: expanded from macro 'EIGEN_STATIC_ASSERT' #define EIGEN_STATIC_ASSERT(X,MSG) static_assert(X,#MSG); ^ ~ /usr/local/include/eigen3/Eigen/src/Core/PlainObjectBase.h:732:17: note: in instantiation of function template specialization 'Eigen::internal::call_assignment_no_alias<Eigen::Matrix<int, 1, 3, 1, 1, 3>, Eigen::CwiseBinaryOp<Eigen::internal::scalar_difference_op<int, int>, const Eigen::Block<const Eigen::Matrix<int, 3, 2, 0, 3, 2>, 1, 2, false>, const Eigen::Block<const Eigen::Matrix<int, 3, 2, 0, 3, 2>, 1, 2, false> >, Eigen::internal::assign_op<int, int> >' requested here internal::call_assignment_no_alias(this->derived(), other.derived(), internal::assign_op<Scalar,typename OtherDer... ^ /usr/local/include/eigen3/Eigen/src/Core/PlainObjectBase.h:537:7: note: in instantiation of function template specialization 'Eigen::PlainObjectBase<Eigen::Matrix<int, 1, 3, 1, 1, 3> >::_set_noalias<Eigen::CwiseBinaryOp<Eigen::internal::scalar_difference_op<int, int>, const Eigen::Block<const Eigen::Matrix<int, 3, 2, 0, 3, 2>, 1, 2, false>, const Eigen::Block<const Eigen::Matrix<int, 3, 2, 0, 3, 2>, 1, 2, false> > >' requested here _set_noalias(other); ^ /usr/local/include/eigen3/Eigen/src/Core/Matrix.h:377:9: note: in instantiation of function template specialization 'Eigen::PlainObjectBase<Eigen::Matrix<int, 1, 3, 1, 1, 3> >::PlainObjectBase<Eigen::CwiseBinaryOp<Eigen::internal::scalar_difference_op<int, int>, const Eigen::Block<const Eigen::Matrix<int, 3, 2, 0, 3, 2>, 1, 2, false>, const Eigen::Block<const Eigen::Matrix<int, 3, 2, 0, 3, 2>, 1, 2, false> > >' requested here : Base(other.derived()) ^ testEigenIgl.cpp:1052:42: note: in instantiation of function template specialization 'Eigen::Matrix<int, 1, 3, 1, 1, 3>::Matrix<Eigen::CwiseBinaryOp<Eigen::internal::scalar_difference_op<int, int>, const Eigen::Block<const Eigen::Matrix<int, 3, 2, 0, 3, 2>, 1, 2, false>, const Eigen::Block<const Eigen::Matrix<int, 3, 2, 0, 3, 2>, 1, 2, false> > >' requested here Eigen::Matrix<scalarType, 1, 3> v1 = triangle.row(1) - triangle.row(0); ^ testEigenIgl.cpp:1082:7: note: in instantiation of function template specialization 'nondegenerate<Eigen::Matrix<int, 3, 2, 0, 3, 2> >' requested here

How can I fix the template to accept both dynamic and fixed size matrices?

Thanks!

You should identify if sizes of the matrix are static and use if constexpr in such case. If you use C++14 or earlier versions then you ought to use SFINAE based techniques and overload the method for different cases. — ALX23z

Caleth Caleth · Accepted Answer · 2021-11-11T09:29:52

The issue you have is that both branches of your first if have to be valid, and in the case of a fixed size matrix, at least one branch isn't, because you specify exact dimensions for the result of your arithmetic.

template<typename Derived>
bool nondegenerate(const Eigen::PlainObjectBase<Derived>& triangle) {

  typedef typename Derived::Scalar scalarType;

  if (triangle.cols() == 2) {
    auto v1 = triangle.row(1) - triangle.row(0);
    auto v2 = triangle.row(2) - triangle.row(0);
    return v1(0)*v2(1) - v1(1)*v2(0);
  } else if (triangle.cols() == 3) {
    auto v1 = triangle.row(1) - triangle.row(0);
    auto v2 = triangle.row(2) - triangle.row(0);
    return v1.cross(v2).norm();
  } else {
    std::cerr << "Undefined input." << std::endl;
    return false;
  }
}

Aside: there are general formulae for finding the determinant of square matrixes of any size, why not use one of those methods instead?

eigen template doesn't work with matrices of fixed size

1 Answers