Overloaded ( ) operator

Question

The spec is to overload the parenthesis () operator to access and assign elements of a matrix, however, I am struggling to figure out what my overload function needs to return.

The Brief Main:

//The .h files and everything has been included, my other functions work just not this one
cout << "Test (): " << endl;
m0(0,0) = 17.25;
m0(1,5) = -2.33;
m0(5,5) = 233;   // There is class exception handling I just haven't included it

The Function Implementation:

Matrix* Matrix::operator () (const unsigned int m, const unsigned int n) //We were told they HAD to be unsigned int
{
    Matrix *ptr = &matrix[m][n];
     return ptr;
}

The Error:

main.cpp: In function ‘int main()’:
main.cpp:59:15: error: lvalue required as left operand of assignment
     m0(0,0) = 17.25;
               ^~~~~
main.cpp:60:16: error: lvalue required as left operand of assignment
     m0(1,5) = -2.33;
                ^~~~
main.cpp:61:15: error: lvalue required as left operand of assignment
     m0(5,5) = 233; // should throw an exception

I understand what the error is saying, so please don't just tell me to read it again and fix it. I'm not actually sure how to fix what my function is returning and make it so the double values in the main can actually be assigned to something.

Thanks in advance

Answer 1

If you check this line in your program:

m0(0,0) = 17.25;

You are trying to assign a double value to the value returned by the () operator.

That means that you should return something that allows a double value assigned to it - and the easiest way to do it, is to return a double.

Also, because you want to assign something to it, you should return a reference - otherwise, you would just return a simple temporary value, and assignment isn't allowed for that. Even if the compiler would allow it, you wouldn't see the changes in your matrix, as you would change a temporary value.

I also see that in your current implementation, you are returning a pointer. Pointers are similar to references, but if you think about it, you can't directly assign a value to the memory pointed by a pointer - you have to dereference it first:

double* d = ...; // points to some memory
*d = 5;

And that's why a reference is better in this case.