Question

I have this method (Matrix::WriteToArray(double &CopyOfArray)) that I want to write a copy of an array in the Matrix object to an array of doubles (i.e. CopyOfArray).
I am having trouble compiling though.

Any help is appreciated.
Thanks

Error:

$ make
g++ -g -Wall -c main.cpp
main.cpp: In function ‘int mrstart(double, double*, Matrix&, Matrix&)’:
main.cpp:459:13: error: ‘cff’ declared as reference but not initialized
main.cpp:465:45: error: invalid type argument of unary ‘*’
main.cpp:467:73: error: invalid type argument of unary ‘*’
main.cpp:470:77: error: invalid type argument of unary ‘*’
Makefile:20: recipe for target `main.o' failed
make: *** [main.o] Error 1

Here are the supporting files:
Main.cpp

int mrstart(double hcen, double mr[],  Matrix &a,  Matrix &HT)
{
    double *cff;
    a.WriteToArray(&cff);
    /*...*/
}

Matrix.cc

int Matrix::WriteToArray(double &CopyOfArray){
    int i;
    for(i=0;i<n_rows;i++){
        CopyOfArray[i]=array[i*n_cols];
        i++;
    }
    return *CopyOfArray;
}

Matrix.hh

#ifndef MATRIX_H
#define MATRIX_H
// Matrix class of variable size
class Matrix {

private:
    int n_rows;
    int n_cols;
    double *array;

public:
    // Constructors
    Matrix(); // default constructor
    Matrix(int rows, int cols); // two-argument constructor
//  Matrix(const Matrix &arr); // copy constructor


    // Destructor
    ~Matrix();

    // Mutators
//  void add(Matrix m2);
//  void subtract(Matrix m2);
    void setelem(int r, int c, double val);

    // Accessors
//  void add(Matrix m2);
//  void subtract(Matrix m2);
    int getrows();
    int getcols();
    double getelem(int r, int c);
    bool equals(Matrix m2);
    char display();
    int WriteToArray(double &CopyOfArray);

};
#endif

Answer 1

You want

int Matrix::WriteToArray(double CopyOfArray[], const int size){
    //make sure size >= n_rows then copy
}

and call it like so

double cff[MAX_SIZE] = {};
a.WriteToArray(cff);

You should really use std::vector and not worry about dynamic allocation.

EDIT: ok you can do manual allocation if you really want to but be careful to release it:

double* cff = 0;
a.WriteToArray(cff);
//do stuff with cff
delete [] cff;

And inside you write function

int Matrix::WriteToArray(double *dest){
 dest = new double[n_rows];
 //copy data into dest
}

The main thing is to make sure you delete dest when you are done using it in main so there is no memory leakage.