How to load a ublas::compressed_matrix<ScalarType> ublas_matrix;

Hello,
I want to export and convert a matrix and load the matrix in an  ublas::compressed_matrix<ScalarType> ublas_matrix;
I have a function to convert the original matrix from LDU to CSR format but I have no idea about ho to load the ublas_matrix?! I am an absolute beginner.

Function to convert the original matrix:
void ldu2csr
( 
    const Foam::lduMatrix & matrix, 
    scalar* vals,
    uint* c_idx,
    uint* r_idx                                             
) 
{
    int n_rows = matrix.diag().size();

    //
    //    Calculate each row size. Sizes are shifted, because in the next part
    //    array r_idx is modified twice.
    //
    for (int i=0; i < matrix.upper().size(); i++) 
    {
        int ri1 = matrix.lduAddr().lowerAddr()[i]; 
        int ri2 = matrix.lduAddr().upperAddr()[i];
        r_idx[ri1+2] ++;
    r_idx[ri2+2] ++;
    } 

    //    
    //    Compute row offsets. Offsets are shifted by one position, 
    //  because they are used as START positions while filling values.
    //
    r_idx[0] = 0;
        r_idx[1] = 0;

    for (int i=1; i<n_rows; i++) 
    {
        r_idx[i+1] += r_idx[i];
    } 

    //
    //    Fill in CSR matrix.
    //    Order below is important to keep column indices sorted.
    //

    //  lower triangle
    for (int i=0; i < matrix.lower().size() ; i++) 
    {
        int row    = matrix.lduAddr().upperAddr()[i] +1;
        int column = matrix.lduAddr().lowerAddr()[i];

        int idx = r_idx[row];
        vals[idx] = matrix.lower()[i];
        c_idx[idx] = column;
        r_idx[row]++;
    } 
        
    // diagonal
    for (int i=0; i<matrix.diag().size(); i++) 
    {
        int idx = r_idx[i+1];
        vals[idx] = matrix.diag()[i];
        c_idx[idx] = i; // i is row and column index
        r_idx[i+1]++;
        } 

    // upper triangle
    for (int i=0; i < matrix.upper().size() ; i++) 
    {
    int row    = matrix.lduAddr().lowerAddr()[i] +1;
    int column = matrix.lduAddr().upperAddr()[i];

    int idx = r_idx[row];
    vals[idx] = matrix.upper()[i];
    c_idx[idx] = column;
    r_idx[row]++;
    } 
}
My idea of a function to load the ublas_matrix:
void load_umatrix(    const Foam::lduMatrix & matrix, boost::numeric::ublas::compressed_matrix<scalar>& ublas_matrix){    uint N_DIAG = matrix.diag().size();
    uint lduMatrixSIZE = matrix.lower().size() + matrix.upper().size() + matrix.diag().size(); // nnz
        
    // allocat mem for CSR sparse matrix 
    scalar * v = (scalar *)calloc(lduMatrixSIZE, sizeof(scalar));
    uint * c = (uint *)calloc(lduMatrixSIZE, sizeof(uint));
    uint * r = (uint *)calloc(N_DIAG + 2, sizeof(uint));

    ldu2csr(matrix,v,c,r);    ublas_matrix.set(r,c,v,N_DIAG,N_DIAG,lduMatrixSIZE); //???   
    // free and release the matrix mem
    free(v); free(r); free(c);  //colloc()    

}
I keep getting error messages related to "incomplete types" and that the matrix cannot be defined.
Any hint to fix the problem is highly appreciated.
Klaus