CODEWITHSUNDEEP
c++c++11matrixcodeblocksdynamic-arrays
Nilesh Kumar
Nilesh Kumar

Reputation: 373

Object returned by an overloaded operator is losing data in c++

I've made a class that stores a pointer to a dynamic 2D array, I've overloaded the '+' operator so that it returns the addition of two matrices:

class matrix
{
    int * mat, row, col;
public:
    matrix(int r, int c)
    {
        row = r;
        col = c;
        mat = new int[row * col];
    }

    matrix operator + (matrix m)
    {
        matrix result(row, col);
        for (int i = 0; i < row; i++)
        {
            for (int j = 0; j < col; j++)
            {
                result.mat[i * result.col + j] = mat[i * col + j] + m.mat[i * m.col + j]
            }
        }
        //result.display(); //result contains expected values i.e. added values
        return result;
    }

    matrix(matrix &m)
    {
      this->mat = m.mat;
      this->row = row;
      this->col = col;
    }


  ~matrix()
   {
     delete[] mat;
   }

  void operator = (matrix m)
{
    if (row != m.row || col != m.col)
    {
        cerr << "Incompatible Matrix Assignment";
        return;
    }

    else
    {
        for(int i = 0; i < row; i++)
        {
            for(int j = 0; j <col; j++)
            {
                mat[i * col + j] = m.mat[i * m.col + j];
            }
        }
    }

    return;
} 

};

int main()
{
    matrix m1(2, 2); //m1.input(); //input has been taken
    matrix m2(2, 2); //m2.input(); //input has been taken
    matrix m3(2, 2);
    m3 = m1 + m2;
    //m3.output;
}

'+' function in class returns a local matrix variable 'result' it contains expected values but when result is returned by the function only first two values of mat(data member of matrix class) contains garbage i.e. m[0][0] and m[0][1], rest of the array variables have expected values. Make array size 3X3 or 4X4 it doesn't create a difference only first two variables contain garbage.

Upvotes: 0

Views: 166

Answers (1)

Yakk - Adam Nevraumont
Yakk - Adam Nevraumont

Reputation: 275730

Follow the rule of 0/3/5. In this case the rule of 0 is best.

Of these 5 special member functions:

  • dtor

  • copy assign/ctor

  • move assign/ctor

implement manually 0, 3 or 5 of them (either dtor and a assign/ctor pair, all 5, or none). Sometimes =delete can replace manually implementing them. If you implement a dtor and fail to do anything with the move/copy ctors, your code will probably be broken.

By far the easiest is to follow the rule of zero.

std::vector<int> mat;

then instead of mat = new int[row * col];:

mat.resize(row * col);

finally don't write a dtor (destructor).

By having a resource management class (vector) manage your resources, you can make your business logic class (matrix) not be full of error prone resource management code.

Upvotes: 1

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