Eigen, function of RowXpr - c++

Question

I am using Eigen, and am currently trying to write a function to operate on rows of a matrix. I've followed the guidelines in the docs but nothing I try compiles (with either clang or g++); I'm at my wits end. How should one actually write functions which will take a RowXpr?
For reference, here is what I have tried so far:
#include <iostream>
#include <Eigen/Core>
using namespace std;
using namespace Eigen;
constexpr Eigen::StorageOptions Order = ColMajor;
using vect_t = Matrix<double, 1, 3, Order>;
using matr_t = Matrix<double, Dynamic, 3, Order>;
#define FUNC 3
#if FUNC == 1
vect_t func(const vect_t& f)
{
return f;
}
#elif FUNC == 2
vect_t func(const Ref<vect_t>& f)
{
return f;
}
#elif FUNC == 3
template<class D> vect_t func(const MatrixBase<D>& f)
{
return f;
}
#endif
int main()
{
matr_t M = matr_t::Random(5,3);
cout << M << endl;
cout << func( M.row(2) ) << endl;
return 0;
}
Thanks!
Edit:
With clang (version 3.8.0-2ubuntu4) the error I get is as below. The error is comparable with g++.
dan#dan-laptop:~/workspace/scratch$ clang++ eigen_func_test.cpp -I /home/dan/Downloads/eigen_3.3.3/ --std=c++11 && ./a.out
In file included from eigen_func_test.cpp:2:
In file included from /home/dan/Downloads/eigen_3.3.3/Eigen/Core:436:
/home/dan/Downloads/eigen_3.3.3/Eigen/src/Core/PlainObjectBase.h:899:7: error: static_assert failed
"INVALID_MATRIX_TEMPLATE_PARAMETERS"
...EIGEN_STATIC_ASSERT((EIGEN_IMPLIES(MaxRowsAtCompileTime==1 && MaxColsAtCompileTime!=1, (Options&RowMajor)==RowMajor)
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/dan/Downloads/eigen_3.3.3/Eigen/src/Core/util/StaticAssert.h:32:40: note: expanded from macro
'EIGEN_STATIC_ASSERT'
#define EIGEN_STATIC_ASSERT(X,MSG) static_assert(X,#MSG);
^ ~
/home/dan/Downloads/eigen_3.3.3/Eigen/src/Core/PlainObjectBase.h:535:7: note: in instantiation of member function
'Eigen::PlainObjectBase<Eigen::Matrix<double, 1, 3, 0, 1, 3> >::_check_template_params' requested here
_check_template_params();
^
/home/dan/Downloads/eigen_3.3.3/Eigen/src/Core/Matrix.h:379:9: note: in instantiation of function template
specialization 'Eigen::PlainObjectBase<Eigen::Matrix<double, 1, 3, 0, 1, 3>
>::PlainObjectBase<Eigen::Block<Eigen::Matrix<double, -1, 3, 0, -1, 3>, 1, 3, false> >' requested here
: Base(other.derived())
^
eigen_func_test.cpp:32:9: note: in instantiation of function template specialization 'Eigen::Matrix<double, 1, 3, 0,
1, 3>::Matrix<Eigen::Block<Eigen::Matrix<double, -1, 3, 0, -1, 3>, 1, 3, false> >' requested here
return f;
^
eigen_func_test.cpp:41:10: note: in instantiation of function template specialization
'func<Eigen::Block<Eigen::Matrix<double, -1, 3, 0, -1, 3>, 1, 3, false> >' requested here
cout << func( M.row(2) ) << endl;
^
1 error generated.

Answer 1

If you read the part highlighted by clang
EIGEN_STATIC_ASSERT((EIGEN_IMPLIES(MaxRowsAtCompileTime==1 && MaxColsAtCompileTime!=1, (Options&RowMajor)==RowMajor)
you see that, if a matrix has one row at compile time and more than one column, it must be RowMajor. That means, if you set Order = RowMajor or just leave out the , Order in
using vect_t = Matrix<double, 1, 3, Order>;
your code should compile fine (it looks like everything else are just follow-up errors).

Answer 2

Not exactly sure what is causing your compiler error directly but from what you have shown in your source code as well as your compiler errors. This is the part of the compiler error message that I'm basing this off of.
/home/dan/Downloads/eigen_3.3.3/Eigen/src/Core/PlainObjectBase.h:899:7: error: static_assert failed
"INVALID_MATRIX_TEMPLATE_PARAMETERS"
So when looking at the source code you are trying to use the #define func3
that you have declared as such:
template<class D> vect_t func(const MatrixBase<D>& f) {
return f;
}
with the using directives that you have declared as:
constexpr Eigen::StorageOptions Order = ColMajor;
using vect_t = Matrix<double, 1, 3, Order>;
using matr_t = Matrix<double, Dynamic, 3, Order>;
So let's expand this to the full form to see what the compiler is trying to parse or deduce as your parameters and return type.
template<class D>
Matrix<double, 1, 3, constexpr Eigen::StorageOptions Order = ColMajor>
func( const MatrixBase<D>& f ) {
return f;
}
Then in your main you use this with:
int main() {
// matr_t M = matr_t::Random(5,3);
Matrix<double, Dynamic, 3, constexpr Eigen::StorageOptions Order = ColMajor> M
= Matrix<double, Dynamic, 3, constexpr Eigen::StorageOptions Order = ColMajor>::Random(5, 3);
// Then you call your function passing it this:
std::cout << func( M.row(2) ) << std::endl;
return 0;
}
func() is returning Matrix<double, 1, 3, Order>;
and it takes const MatrixBase<D>&
however it appears you are passing it: M.row(2)
that is constructed from: Matrix<double, Dynamic, 3, constexpr Eigen::StorageOptions Order = ColMajor>::Random(5, 3)
In the function itself you are returning f which happens to be
the function's const ref of MatrixBase<D> parameter yet the declaration is expecting to return a Matrix<double, 1, 3, Order>
So I think the compiler is having a hard time trying to convert a
Matrix<double, Dynamic, 3, Order> to a Matrix<double, 1, 3, Order>
Maybe this will help you to understand the compiler error a little better; as you can see when you look at the rest of the compilation error message you can tell that this is evident when it is trying to do the specializations of this template.