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Why are "const Eigen::Matrix<>&" and "const Ref<Eigen::Matrix<> >" apparently incompatible?

Tags:

c++

eigen

eigen3

Here is my example code:

(Please note the section surrounded by #if ENABLE_MY_COMPILE_ERROR)

#include <Eigen/Core>
#include <iostream>

#define ENABLE_MY_COMPILE_ERROR 1

void f1(const Eigen::Ref<Eigen::MatrixXd> a, 
        const Eigen::Ref<Eigen::MatrixXd> b, 
        Eigen::Ref<Eigen::MatrixXd> c)
{
   c = a * b;
}

int main(int argc, const char *argv[])
{
   Eigen::Matrix3d M;
   Eigen::Vector3d x;
   Eigen::Vector3d y;

   M.setRandom();
   x.setRandom();

   std::cout<<"M = \n"<<M<<std::endl;
   std::cout<<"x = \n"<<x<<std::endl;
   std::cout<<"M * x = \n"<<M * x<<std::endl;

   {
      y.setZero();
      f1(M,x,y);

      std::cout<<"y = \n"<<y<<std::endl;
   }

   {
      Eigen::Matrix3d& MRef = M;

      y.setZero();
      f1(MRef,x,y);

      std::cout<<"y = \n"<<y<<std::endl;
   }

#if ENABLE_MY_COMPILE_ERROR
   {
      const Eigen::Matrix3d& MRef = M;

      y.setZero();
      f1(MRef,x,y);

      std::cout<<"y = \n"<<y<<std::endl;
   }
#endif
}

Here is the compile error I get when ENABLE_MY_COMPILE_ERROR != 0:

In file included from ../../../../external/src/eigen-current/Eigen/Core:334:0,
                 from Eigen_Ref.C:1:
../../../../external/src/eigen-current/Eigen/src/Core/PlainObjectBase.h: In constructor ‘Eigen::Ref<PlainObjectType, Options, StrideType>::Ref(const Eigen::DenseBase<OtherDerived>&, typename Eigen::internal::enable_if<(bool)((Eigen::internal::is_lvalue<Derived>::value && (bool)(typename Eigen::internal::traits<Eigen::Ref<_PlainObjectType, _Options, _StrideType> >::match<Derived>::MatchAtCompileTime))), Derived>::type*, int) [with Derived = Eigen::Matrix<double, 3, 3>; PlainObjectType = Eigen::Matrix<double, -1, -1>; int Options = 0; StrideType = Eigen::OuterStride<>; typename Eigen::internal::enable_if<(bool)((Eigen::internal::is_lvalue<Derived>::value && (bool)(typename Eigen::internal::traits<Eigen::Ref<_PlainObjectType, _Options, _StrideType> >::match<Derived>::MatchAtCompileTime))), Derived>::type = Eigen::Matrix<double, 3, 3>]’:
../../../../external/src/eigen-current/Eigen/src/Core/PlainObjectBase.h:726:12: error: ‘Eigen::PlainObjectBase<Eigen::Matrix<double, 3, 3> >::<anonymous enum> Eigen::PlainObjectBase<Eigen::Matrix<double, 3, 3> >::ThisConstantIsPrivateInPlainObjectBase’ is private
Eigen_Ref.C:47:18: error: within this context

Thus, apparently the following are incompatible:

  • const Eigen::Ref<Eigen::MatrixXd> a

  • const Eigen::Matrix3d& MRef = M

My questions are:

  • Is this by design, or is this a bug/shortcoming of Eigen::Ref<>?

  • If by design, then what is a good way to cleanly pass around Matrix objects while preserving const correctness?

For example, if if have a class with a member function:

const Matrix3d& SomeClass::getTheSuperDuperMatrix() const 
{
   return this->superDuperMat;
}

and I plan on using this function's return value several times in a block of code, I'd like to create a reference to it with a short name and I'd like that to be a constant reference.

const Matrix3d& M = someClassInstance.getTheSuperDuperMatrix();

But then I will get the above compile error if I pass M into function which accepts a const Ref<Matrix3d> argument, such as:

f1(M,x,y);

FYI, I am currently using the following version of Eigen:

Version: 3.2.91
Revision 5696:af94f93db432
like image 608
arr_sea Avatar asked Dec 11 '13 01:12

arr_sea


1 Answers

This is because const Eigen::Ref<Eigen::MatrixXd> is not a const reference. It must be declared like this:

Eigen::Ref<const Eigen::MatrixXd>
like image 175
ggael Avatar answered Sep 17 '22 21:09

ggael