How do I properly implement a display function of a class? - c++

Question

I have a class called magicSquare with a constructor and a display function called display. The constructor creates the magic square, and the display function displays the results. In my main function, I created an instance of magicSquare called ms and gave it a value 7. To display it, shouldn't it work if I just did ms.display()?
class magicSquare
{
private:
int size, square;
vector<vector <int> > finalvec;
public:
magicSquare(int a):finalvec(a, std::vector<int>(a))
{
int i = 0;
int j = a/2;
size = a;
square = a * a;
vector<int>vec(a);
vector<vector<int> > finalvec(a,vec);
for (int i = 0; i < size; i++)
{
for (int j = 0; j< size; j++)
cout << finalvec[i][j];
cout << endl;
}
for (int k=0; k < square; ++k)
{
finalvec[i][j] = k;
i--;
j++;
if (k%a == 0)
{
i = i+ 2;
--j;
}
else
{
if (j==a)
j = j- a;
else if (i<0)
i = i+ a;
}
}
}
void display()
{
for (int i = 0; i < size; i++)
{
for (int j = 0; j< size; j++)
cout << finalvec[i][j];
cout << endl;
}
}
};
int main()
{
magicSquare ms(3);
ms.display();
return 0;
}

Answer 1

Your Error
As pointed by #Retired Ninja, the vector > finalvec(a,vec); hide your member variable finalvec, as you redefine it as a new vector...
What can correct it
You could construct your vector inside a Member initializer list like this
magicSquare(int a) : finalvec(a, std::vector<int>(a, 0)) {
/* your constructor */
}
And delete the two line:
vector<int>vec(a);
vector<vector<int> > finalvec(a,vec);
In your code
How to not make this error
Seeing which value are a class member, method parameter or even context variable can be sometime difficult:
What i can recommend you is to do the following:
class member -> m_NAME_OF_YOUR_CLASS_MEMBER
method parameter -> t_NAME_OF_YOUR_METHODE_PARAMETER
context variable -> c_NAME_OF_YOUR_CONTEXT_VARIABLE
by doing this error like you've done is a bit harder to do!
EDIT: After testing your code
I see there's error in it, effectively, the first time you go in that line:
finalvec[i][j] = k;
i > size, so you access further that your vector allow it, which result in a segfault! please repair your code!
hope that can help