negation operator overloading in a class with a 2d array

Question

I created a class called Matrix with a 2D array. I can run it with the constructor, copy constructor and destructor. When I introduce the unary negation operator, I get a run time error.

https://gist.github.com/anonymous/7823794

#ifndef MATRIX_H
#define MATRIX_H
class Matrix
{
public:
    Matrix(int rSize=3, int cSize=3);
    Matrix(const Matrix& m);
    ~Matrix();
    bool setValue(int rSize, int cSize, int value);
    bool getValue(int rVal, int cVal, int& value)const;
    const Matrix operator- ();
private:
    int rowSize;
    int columnSize;
    int** arr;
};
#endif



#include<iostream>
#include"Matrix.h"
using namespace std;
Matrix::Matrix(int rSize,int cSize)
{
    columnSize = cSize;
    rowSize = rSize;
    arr = new int* [rowSize];
    for(int i=0; i<rowSize; i++)
        arr[i] = new int[columnSize];
    for(int j=0; j<rowSize; j++)
    {
        for(int k=0; k<columnSize; k++)
            arr[j][k] = 0;
    }

}
Matrix::Matrix(const Matrix& m)
{
    columnSize = m.columnSize;
    rowSize = m.rowSize;
    arr = new int* [rowSize];
    for(int i=0; i<rowSize; i++)
    {
        arr[i] = new int [columnSize];
    }
    for(int i=0; i<rowSize; i++)
    {
        for(int j=0; j<columnSize; j++)
            arr[i][j] = m.arr[i][j];
    }
}
Matrix::~Matrix()
{
    for(int i = 0; i < rowSize; ++i)
        delete [] arr[i];
    delete [] arr;
}
bool Matrix::setValue(int rVal, int cVal, int value)
{
    if((rVal<0)||(cVal<0)||(rVal>rowSize-1)||(cVal>columnSize-1))
        return false;
    arr[rVal][cVal] = value;
    return true;
}
bool Matrix::getValue(int rVal, int cVal, int& value)const
{
    if((rVal<0)||(cVal<0)||(rVal>rowSize-1)||(cVal>columnSize-1))
        return false;
    value = arr[rVal][cVal];
    return true;
}
const Matrix Matrix:: operator- ()
{
    Matrix m(*this);
    for (int i = 0; i< rowSize; i++)
    {
        for(int j=0; j<columnSize; j++)
            m.arr[i][j] = (this->arr[i][j])* -1 ;
    }
    return m;
}





#include<iostream>
#include"Matrix.h"
using namespace std;
void main()
{
    Matrix m(4, 5);
    Matrix m2(m);
    m2.setValue(1,2,12);
    int x;
    m2.getValue(1,2,x);
    Matrix m3;
    m3 = -m2;
}

Edit: Following @unwind and @interjay advice, I implemented the assignment operator first and returned a value, not a reference, for the negation operator and it works. Thank you all for your help

You should paste the code here, so it's available if someone in the distant future sees your post. — kviiri
– kviiri, Commented Dec 6, 2013 at 13:30
@Abhineet Is there an easier way to indent than adding four spaces to each line manually? — FaizHasan123
– FaizHasan123, Commented Dec 6, 2013 at 13:51
@user3012044 Select the code and press the {} button in the toolbar or Ctrl+K. — interjay
– interjay, Commented Dec 6, 2013 at 13:53
The simple answer is to use std::vector rather than managing memory yourself. If you really want to juggle pointers for educational reasons, always remember the Rule of Three — Mike Seymour
– Mike Seymour, Commented Dec 6, 2013 at 14:04
Select the text you want to indent and press "CTRL+k". It will automatically indent the codes. — Abhineet
– Abhineet, Commented Dec 6, 2013 at 14:12

interjay · Accepted Answer · 2013-12-06 13:37:57Z

1

You don't have an assignment operator in your class, which will cause double deletion of the member pointer after an assignment (and you're assigning in the line m3 = -m2;).

You should obey the rule of three.

unwind is also correct that the negation operator should return by value: You should never return a reference to a local variable from a function as that leads to undefined behavior.

answered Dec 6, 2013 at 13:37

interjay

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2 Comments

FaizHasan123 Over a year ago

Should I also return the'+' and '-' operators by value or can I return those by reference?

interjay Over a year ago

@user3012044 By value. You can't return a local variable or a temporary by reference because it will stop existing when the function returns.

Community · Accepted Answer · 2017-05-23 12:13:06Z

1

Declaring the operator to return const Matrix& and then returning a local variable is wrong. That's a dangling reference, surely?

See for reference this answer where the operator returns an actual new value, not a reference. This makes a lot more sense to me.

edited May 23, 2017 at 12:13

CommunityBot

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answered Dec 6, 2013 at 13:36

unwind

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