Compilation error C2664 and C2440 when constructing a template function in C++

Question

I am using Spectra C++ library, which is built on top of the Eigen C++ library.

I will need to construct a correct SymGEigsSolver object based on the given the SelectionRule and GeigsMode parameters in a method. The definition of SymGEigsSolver is as follows:

template<typename Scalar, int SelectionRule, typename OpType, typename BOpType, int GEigsMode>
class Spectra::SymGEigsSolver< Scalar, SelectionRule, OpType, BOpType, GEigsMode >

However when I tried to compile the below function:

typedef SparseMatrix<double, Eigen::RowMajor> SpMat;
typedef Spectra::SparseSymMatProd<double, Eigen::Upper, Eigen::RowMajor> SpSymMatProd;
typedef Spectra::SparseCholesky<double, Eigen::Upper> SpCholesky;
typedef Spectra::SparseRegularInverse<double, Eigen::Upper, Eigen::RowMajor> SpRegularInverse;


template < typename Scalar,
    int SelectionRule,
    typename OpType,
    typename BOpType,
    int GEigsMode >
    SymGEigsSolver<Scalar, SelectionRule, OpType, BOpType, GEigsMode>& CreateSolverEngine
    (const SpMat& A, const SpMat&  B, int selectRule, int gMode,  int nMode, int ncv)
{
    SpSymMatProd op(A);

    if (gMode == Spectra::GEIGS_CHOLESKY)
    {
        SpCholesky  Bop(B);
        SymGEigsSolver<Scalar, SelectionRule, SpSymMatProd, SpCholesky, GEigsMode>
            ges1(op, Bop, nMode, ncv);
        return ges1;
    }
    if (gMode == Spectra::GEIGS_REGULAR_INVERSE)
    {
        SpRegularInverse Bop2(B);
        SymGEigsSolver<Scalar, SelectionRule, SpSymMatProd, SpRegularInverse, GEigsMode>
            ges(op, Bop2, nMode, ncv);
        return ges;
    }

    throw std::out_of_range("out of range for "+gMode);


}

I got compilation errors such as:

Error   C2664   'Spectra::SymGEigsSolver<Scalar,SelectionRule,OpType,BOpType,GEigsMode>::SymGEigsSolver(OpType *,BOpType *,int,int)'
: cannot convert parameter 1
from 'SpSymMatProd' to 'SpSymMatProd *'

And

C2440   'return' : cannot convert from
'Spectra::SymGEigsSolver<Scalar,SelectionRule,OpType,BOpType,GEigsMode>'
to
'Spectra::SymGEigsSolver<Scalar,SelectionRule,OpType,BOpType,GEigsMode>&'

And

C2664   Spectra::SymGEigsSolver<Scalar,SelectionRule,OpType,BOpType,GEigsMode>::SymGEigsSolver(OpType *,BOpType *,int,int)'
: cannot convert parameter 1 from 'SpSymMatProd' to 'SpSymMatProd *'

The first and third error messages are pretty readable. The second one isn't, but for that you might need to make a [mcve]. — nwp, Mar 13 '18 at 08:39
You are trying to return by reference a local variable, which will no longer exist after the function finishes. You are also passing as argument an object itself, where a pointer is expected as a function parameter. — Daniel Langr, Mar 13 '18 at 08:45
@DanielLangr, from what I can tell there is this copy elision that will allow me to return a local variable. Anyway I've figured out the solution — Graviton, Mar 13 '18 at 08:58
@Graviton Copy elision is involved only if you return _by value_, not _by reference_. — Daniel Langr, Mar 13 '18 at 09:14

score 0 · Accepted Answer · answered Mar 13 '18 at 08:58

I figure out the answer by myself-- I mistype & at the method name. This is the correct solution:

typedef SparseMatrix<double, Eigen::RowMajor> SpMat;
typedef Spectra::SparseSymMatProd<double, Eigen::Upper, Eigen::RowMajor> SpSymMatProd;
typedef Spectra::SparseCholesky<double, Eigen::Upper> SpCholesky;
typedef Spectra::SparseRegularInverse<double, Eigen::Upper, Eigen::RowMajor> SpRegularInverse;


template < typename Scalar,
    int SelectionRule,
    typename OpType,
    typename BOpType,
    int GEigsMode >
    SymGEigsSolver<Scalar, SelectionRule, OpType, BOpType, GEigsMode> CreateSolverEngine
    (const SpMat& A, const SpMat&  B, int selectRule, int gMode,  int nMode, int ncv)
{
    SpSymMatProd op(A);

    if (gMode == Spectra::GEIGS_CHOLESKY)
    {
        SpCholesky  Bop(B);
        SymGEigsSolver<Scalar, SelectionRule, SpSymMatProd, SpCholesky, GEigsMode>
            ges1(op, Bop, nMode, ncv);
        return ges1;
    }
    if (gMode == Spectra::GEIGS_REGULAR_INVERSE)
    {
        SpRegularInverse Bop2(B);
        SymGEigsSolver<Scalar, SelectionRule, SpSymMatProd, SpRegularInverse, GEigsMode>
            ges(op, Bop2, nMode, ncv);
        return ges;
    }

    throw std::out_of_range("out of range for "+gMode);


}

I don't have Spectra installed, but I find it surprising that this compiles. How does your change solve the C2664-issue? (Wouldn't that require writing `&op`, etc -- which would give you a pointer to a local variable, however ...) Also, the return types of both `if`-branches are different. Maybe Spectra does some implicit type conversion at all these cases. — chtz, Mar 13 '18 at 11:37

Compilation error C2664 and C2440 when constructing a template function in C++

1 Answers1