error: passing ‘const MyDiagonalTen’ as ‘this’ argument discards qualifiers

[alexpghayes]

I'm having trouble getting the following example to work (although the first two examples in the README work fine for me). Very new to C++ so could be a bug or I could be missing something obvious.

#include <Eigen/Core>
#include <Spectra/SymEigsSolver.h>
#include <iostream>

using namespace Spectra;

// M = diag(1, 2, ..., 10)
class MyDiagonalTen
{
public:
    using Scalar = double;  // A typedef named "Scalar" is required
    int rows() { return 10; }
    int cols() { return 10; }
    // y_out = M * x_in
    void perform_op(const double *x_in, double *y_out) const
    {
        for (int i = 0; i < rows(); i++)
        {
            y_out[i] = x_in[i] * (i + 1);
        }
    }
};

int main()
{
    MyDiagonalTen op;
    SymEigsSolver<MyDiagonalTen> eigs(op, 3, 6);
    eigs.init();
    eigs.compute(SortRule::LargestAlge);
    if (eigs.info() == CompInfo::Successful)
    {
        Eigen::VectorXd evalues = eigs.eigenvalues();
        std::cout << "Eigenvalues found:\n"
                  << evalues << std::endl;
    }

    return 0;
}

I see:

g++ tmp.cpp -o tmp && "/dabox/hayes/spectra/examples/"tmp
tmp.cpp: In member function ‘void MyDiagonalTen::perform_op(const double*, double*) const’:
tmp.cpp:17:34: error: passing ‘const MyDiagonalTen’ as ‘this’ argument discards qualifiers [-fpermissive]
   17 |         for (int i = 0; i < rows(); i++)
      |                                  ^
tmp.cpp:12:9: note:   in call to ‘int MyDiagonalTen::rows()’
   12 |     int rows() { return 10; }
      |         ^~~~
In file included from /usr/local/include/Spectra/SymEigsBase.h:22,
                 from /usr/local/include/Spectra/SymEigsSolver.h:12,
                 from tmp.cpp:2:
/usr/local/include/Spectra/MatOp/internal/ArnoldiOp.h: In instantiation of ‘Spectra::ArnoldiOp<Scalar, OpType, Spectra::IdentityBOp>::Index Spectra::ArnoldiOp<Scalar, OpType, Spectra::IdentityBOp>::rows() const [with Scalar = double; OpType = MyDiagonalTen; Spectra::ArnoldiOp<Scalar, OpType, Spectra::IdentityBOp>::Index = long int]’:
/usr/local/include/Spectra/LinAlg/Arnoldi.h:119:59:   required from ‘Spectra::Arnoldi<Scalar, ArnoldiOpType>::Arnoldi(ArnoldiOpType&&, Spectra::Arnoldi<Scalar, ArnoldiOpType>::Index) [with Scalar = double; ArnoldiOpType = Spectra::ArnoldiOp<double, MyDiagonalTen, Spectra::IdentityBOp>; Spectra::Arnoldi<Scalar, ArnoldiOpType>::Index = long int]’
/usr/local/include/Spectra/LinAlg/Lanczos.h:52:62:   required from ‘Spectra::Lanczos<Scalar, ArnoldiOpType>::Lanczos(T&&, Spectra::Lanczos<Scalar, ArnoldiOpType>::Index) [with T = Spectra::ArnoldiOp<double, MyDiagonalTen, Spectra::IdentityBOp>; Scalar = double; ArnoldiOpType = Spectra::ArnoldiOp<double, MyDiagonalTen, Spectra::IdentityBOp>; Spectra::Lanczos<Scalar, ArnoldiOpType>::Index = long int]’
/usr/local/include/Spectra/SymEigsBase.h:237:37:   required from ‘Spectra::SymEigsBase<OpType, BOpType>::SymEigsBase(OpType&, const BOpType&, Spectra::SymEigsBase<OpType, BOpType>::Index, Spectra::SymEigsBase<OpType, BOpType>::Index) [with OpType = MyDiagonalTen; BOpType = Spectra::IdentityBOp; Spectra::SymEigsBase<OpType, BOpType>::Index = long int]’
/usr/local/include/Spectra/SymEigsSolver.h:158:69:   required from ‘Spectra::SymEigsSolver<OpType>::SymEigsSolver(OpType&, Spectra::SymEigsSolver<OpType>::Index, Spectra::SymEigsSolver<OpType>::Index) [with OpType = MyDiagonalTen; Spectra::SymEigsSolver<OpType>::Index = long int]’
tmp.cpp:27:47:   required from here
/usr/local/include/Spectra/MatOp/internal/ArnoldiOp.h:120:50: error: passing ‘const MyDiagonalTen’ as ‘this’ argument discards qualifiers [-fpermissive]
  120 |     inline Index rows() const { return m_op.rows(); }
      |                                                  ^
tmp.cpp:12:9: note:   in call to ‘int MyDiagonalTen::rows()’
   12 |     int rows() { return 10; }
      |         ^~~~

[Done] exited with code=1 in 2.437 seconds

[yixuan]

Hi Alex, can you try to simply add a const keyword to the rows() and cols() member functions? I will fix the example later.

[alexpghayes]

Thanks, fixed it for me!