Passing a custom Matrix class to a Lapack subroutine

Question

I would like the lapack package for some very useful functions which I would not like/being able to implement myself that well. The problem is that I cannot pass my custom Matrix class to the lapack function, I don't get the wanted behaviour. Here is my Matrix class

#ifndef MATRIX_T_H
#define MATRIX_T_H

#include <iostream>
#include <vector>
#include <functional>
#include <fstream>
#include "Complex.hpp"
#include "abs.hpp"

#define This (*this)
#define cout std::cout
#define endl std::endl
#define pa(a) std::pair<a>
#define O(a) std::optional<a>


template <class T>
class Column;

template <class T>
class Row;

template<class T>
struct pivot {
    uint index;
    T val;
};

template<class T>
class Matrix
{
public:
//**CONSTRUCTORS AND DESTRUCTORS*********************************************

    Matrix() : m_rows(0), m_cols(0) {}
    explicit Matrix(int nrows, int ncols=0);
    explicit Matrix (const T& val, uint nrows, uint ncols=0);
    explicit Matrix(T (&fun) (int,int), uint nrows, uint ncols=0);
    explicit Matrix(std::function<T(int,int)>& fun, uint nrows, uint ncols=0)
        : m_rows(nrows), m_cols(ncols==0 ? nrows : ncols)
    {
        createMatrix();
        fill(fun);
    }
    Matrix(const std::vector<std::vector<T>>& matrix);
    Matrix(const Matrix<T>& other); //Copy constructor
    Matrix(Matrix<T>&& other);      //Move constructor
    ~Matrix();
//****************************************************************************
//*****************  methods *************************************************
    void createMatrix(uint size) {
        m_rows = size;
        m_cols = size;
        createMatrix();
    }
    void createMatrix(uint n_rows, uint n_cols) {
        m_rows = n_rows;
        m_cols = n_cols;
        createMatrix();
    }
    T **getMatrix() {return m_matrix;}
    T *getLinearMatrix() {return m_line_matrix;}


//*****************  operators *******************************************
    operator T** () {return m_matrix;}
    operator T* () {return m_line_matrix;}

// MORE CODE

//********************************************************************


protected:
    T **m_matrix = nullptr;
    T *m_line_matrix = nullptr;
    bool m_created_matrix = false;
    uint m_rows;
    uint m_cols;

protected:
    void createMatrix();
    void deleteMatrix();
};


template<class T>
Matrix<T>::Matrix(int nrows, int ncols)
    : m_rows(nrows), m_cols(ncols==0 ? nrows : ncols) {
    createMatrix();
}

template<class T>
Matrix<T>::Matrix(const T &val, uint nrows, uint ncols)
    : m_rows(nrows), m_cols(ncols==0 ? nrows : ncols) {
    createMatrix();
    for (uint i=0; i<m_rows; i++)
        m_matrix[i][i] = val;
}

template<class T>
Matrix<T>::Matrix(T (&fun)(int, int), uint nrows, uint ncols)
    : m_rows(nrows), m_cols(ncols==0 ? nrows : ncols) {
    createMatrix();
    fill(fun);
}

template<class T>
Matrix<T>::Matrix(const std::vector<std::vector<T> > &matrix) : m_rows(matrix.size()), m_cols(matrix[0].size()) {
    createMatrix();
    for (int i=0; i<m_rows; i++)
        for (int j=0; j<m_cols; j++)
            m_matrix[i][j] = matrix[i][j];
}

template<class T>
Matrix<T>::Matrix(const Matrix<T> &other) : m_rows(other.rows()), m_cols(other.cols()) {
//    cout << "Matrix::copy_constructor" << endl;
    createMatrix();
    copyMatrix(other.m_matrix);
}

template<class T>
Matrix<T>::Matrix(Matrix<T> &&other) : m_created_matrix(true), m_rows(other.m_rows), m_cols(other.m_cols){
    delete[] m_matrix;
    delete[] m_line_matrix;
    m_matrix = other.m_matrix;
    m_line_matrix = other.m_line_matrix;
    other.m_line_matrix = nullptr;
    other.m_matrix = nullptr;
    other.m_created_matrix = false;
}

template<class T>
Matrix<T>::~Matrix() {
    deleteMatrix();
}


template<class T>
T *&Matrix<T>::operator [](int index) const {
    return m_matrix[index];
}


template<class T>
void Matrix<T>::createMatrix() {
    if (m_line_matrix!=nullptr || m_matrix != nullptr)
        deleteMatrix();

    m_line_matrix = new T[m_rows*m_cols]();
    if (m_line_matrix==0) {
        cout << "Could not allocate new memory. Needed " << m_rows*m_cols*sizeof(T) << " bytes" << endl;
        abort();
    }
    m_matrix = new T*[m_rows]();

    for (uint i=0; i<m_rows; i++)
        m_matrix[i] = m_line_matrix + m_cols*i;

    m_created_matrix = true;

}

template<class T>
void Matrix<T>::deleteMatrix() {
    delete [] m_line_matrix;
    delete[] m_matrix;
    m_line_matrix = nullptr;
    m_matrix = nullptr;
    m_created_matrix = false;
}

// MORE CODE
}

#undef This
#undef cout
#undef endl
#undef pa
#undef O

#endif // MATRIX_T_H

As you can see from createMatrix() I create an array dynamically and mimic the matrix structure by saving the addresses in a pointer to a pointer. So technically using the double operator [][] gives me the wanted element of the matrix.

The problem is when I try to pass it to the lapack function I am interested, to solve eigenvalue problems, which has signature

void LAPACK_dsygv(
    lapack_int const* itype, char const* jobz, char const* uplo,
    lapack_int const* n,
    double* A, lapack_int const* lda,
    double* B, lapack_int const* ldb,
    double* W,
    double* work, lapack_int const* lwork,
    lapack_int* info );

where lapack_int is simply an int value. I tried to pass my matrix in so many different ways, i.e.

• dsygv_(&itype, &jobz , &uplo , &ndim, amm.getLinearMatrix(), &nnn , axx.getLinearMatrix(), &nnn, &wr[0], &aux[0], &lwork , &info);, where amm and axx are instances of my Matrix class and aux and wr are instances of std::vector;
• dsygv_(&itype, &jobz , &uplo , &ndim, &(amm[0][0]), &nnn , &(axx[0][0]), &nnn, &wr[0], &aux[0], &lwork , &info);, same variables as before
• dsygv_(&itype, &jobz , &uplo , &ndim, *amm, &nnn , *axx, &nnn, wr, aux, &lwork , &info);, where now I have double amm[NMAX][NMAX] and the same for axx, while I have int wr[NMAX], aux[4*NMAX]

In all cases the values of the matrices and arrays are the same, but only in the last case it works. I guess it has to do with the fact that the function does not recognize a matrix[][] as a double**.

How can I solve this?

As I said I tried changing the way I pass the arguments, but not having access to the lapack library code directly (and I don't want to recompile everything by hand) I cannot just use gdb or other tools to check what has been passed to the function.

Answer 1

• To fix the 1st: dsygv_(&itype, &jobz , &uplo , &ndim, amm.getLinearMatrix(), &nnn , axx.getLinearMatrix(), &nnn, &wr[0], &aux[0], &lwork , &info), because *amm.getLinearMatrix(), *axx.getLinearMatrix() have type double, not double *. Here should be compile error, shouldn't it?
• To fix the 2nd dsygv_(&itype, &jobz , &uplo , &ndim, amm[0], &nnn , axx[0], &nnn, &wr[0], &aux[0], &lwork , &info); because &(amm[0][0]) is address of temporary double object amm[0][0]on the stack, so its value doesn't equal amm.getLinearMatrix()