Question

When I try to use this line in main.cpp:

m3.array = m1.array+m2.array;

where m3, m1, and m2 are all objects of a class type Matrix, with an int[3][3] array –
I keep getting an error that deals with an incompatible assignment of type int[3][3] and int[3][3] to opperand ‘+’. I don’t know the exact error, because I’m not at a computer to compile the program.

Here’s the matrix.cpp I have:

#include <iostream>
#include <string>
#include "matrix.h"

using namespace std;

Matrix::Matrix()
{
    m1.array = 0;
}

istream& opeator >>(istream& inp, Matrix& m1)
{
         int i, j;

         for (i = 0; i < 3;i++)
         {
             for (j=0; j < 3;j++)
             {
                 inp >> m1.array[i][j];
             }
         }
         return inp;
}
ostream& operator <<(istream& outp, Matrix& m1)
{
         int i, j;
         for (i = 0;i<3;i++)
         {
             for (j = 0;j<3;j++)
             {
                 out<<m1.array[i][j]<<" "<<endl;
             }
         }
         return outp;
}

Matrix operator + (const Matrix& m1, const Matrix& m2)
{
        Matrix answer;
        int i,j;
        for (i = 0;i<3;i++)
        {
            for (j = 0;j<3;j++)
            {
                answer.array[i][j] = m1.array[i][j] + m2.array[i][j];
            }
        }
        return answer;
}

Matrix operator - (const Matrix& m1, const Matrix& m2)
{
        Matrix answer;
        int i, j;
        for (i = 0;i<3;i++)
        {
            for (j = 0;j<3;j++)
            {
                answer.array[i][j] = m1.array[i][j] - m2.array[i][j];
            }
        }
        return answer;
}

Matrix operator * (const Matrix& m1, const matrix& m2)
{
       Matrix answer;
       int i, j, k;
       for (i = 0;i<3;i++)
       {
           for (j = 0; j<3;j++)
           {
               for (k = 0; k<3;k++)
               {
                   answer.array[i][j] = m1.array[i][k] + m2.array[k][j];
               }
           }
       }
       return answer;
}

and the matrix.h:

#ifndef MATRIX_H
#define MATRIX_H

using namespace std;

class Matrix
{
      public:
             Matrix();
             friend istream& operator >>(istream&, Matrix&);
             friend ostream& operator <<(ostream&, const Matrix&);
             friend Matrix& operator +(const Matrix&, const Matrix&);
             friend Matrix& operator -(const Matrix&, const Matrix&);
             friend Matrix& operator *(const Matrix&, const Matrix&);
             int array[3][3];

};




#endif

Answer 1

Matrix operator + (const Matrix& m1, const Matrix& m2)

This tells the computer how to add two Matrix objects, good job.

m3.array = m1.array+m2.array;

m1 and m2 are Matrix objects, but m1.array is not. That is a int[3][3] object. Luckily, the fix is very very simple:

m3 = m1 + m2;