Problem with Template Function and overloaded equality operator

This is the error:

"main.cpp": main.cpp passing `const HugeInt' as `this' argument of `bool HugeInt::operator==(const HugeInt&)' discards qualifiers at line 29

This is where the error occurs.

Code:

---


template <class T>
bool isEqualTo( const T &left, const T &right )
{
  return ( left == right );
}

---

Code for overloaded function

Code:

---


bool HugeInt::operator==( const HugeInt &op2 )
{
    for( int i = 29; i >= 0; i-- ){
      if ( integer[i] != op2.integer[i])
        return false;
    }
    return true;
}

---

Anyone have a clue what's going on here. I don't really understand the error message so that's not helping me.

Make your operator == function const:

bool HugeInt::operator==( const HugeInt &op2 ) const

You're calling an overloaded operator for a const object with a non-const object. You can do one of two things to fix the problem. First, you can declare the overloaded operator const:

Code:

---

bool HugeInt::operator==( const HugeInt &op2 ) const
{
    for( int i = 29; i >= 0; i-- ){
      if ( integer[i] != op2.integer[i])
        return false;
    }
    return true;
}

---

Second, and this is the better method because comparison with operator == should be commutative, declare operator== as a friend function that takes two arguments and then define is as such:

Code:

---

bool operator==( const HugeInt& op1, const HugeInt &op2 )
{
  for( int i = 29; i >= 0; i-- ){
    if ( op1.integer[i] != op2.integer[i])
      return false;
  }
  return true;
}

---

I'm not sure I understand. Which object is constant and which is not?

>Which object is constant and which is not?
The object that you're using is constant, but the object that you say you're using is not. You can't call a non-const member function with a const object, so operator== needs to be declared const. Or you could make it a friend function that takes two arguments like everyone else. ;)

Ok I think I understand now. What was giving me problems was that i passed constant references to the the IsEqualTo function. But that doesn't make the function constant I guess. Is it possible to have a constant reference to a non constant object or are references always constant since they can't be reassigned?

The IsEqualTo function is a global function (it appears) and so it isn't const and shouldn't be const. A function being const only makes sense if it is a member function. That is why your operator== needed to be const. It was implemented as a member function. When you call a member function using a const reference or object, that function must be declared as const (this makes sense, you have a const reference so you promise not to change it, and you can only call const functions that promise not to change it).

A reference can be const or non-const, and you can initialize a const reference with a non-const object.

Code:

---

class Test {
public:
    void ModifyMe() { /* Do something to modify me */ }
    void DontModifyMe() const { /* Read-only operations */ }
};

int main()
{
    Test tInst;
    const Test constInst;

    Test& refToInst = tInst;  // Fine, non-const reference to non-const object.
    const Test& constRefToInst = tInst; // Fine, const reference to non-const object.

    //Test& refToConstInst = constInst;  // ERROR!! Non-const reference to const object.
    const Test& constRefToConstInst = constInst; // Fine, const reference to const object.

    tInst.ModifyMe(); // Fine, call non-const function from non-const object.
    //constInst.ModifyMe(); // ERROR!! Call non-const function from const object.
    refToInst.ModifyMe(); // Fine, call non-const function from non-const reference.
    //constRefToInst.ModifyMe(); // ERROR!! Call non-const function from const reference.
    //constRefToConstInst.ModifyMe(); // ERROR!! Call non-const function from const reference.

    tInst.DontModifyMe(); // Fine, call const function from non-const object.
    constInst.DontModifyMe(); // Fine, call const function from const object.
    refToInst.DontModifyMe(); // Fine, call const function from non-const reference.
    constRefToInst.DontModifyMe(); // Fine, call const function from const reference.
    constRefToConstInst.DontModifyMe(); // Fine, call const function from const reference.
}

---

So in your code:

Code:

---

template <class T>
bool isEqualTo( const T &left, const T &right )
{
  return ( left == right );
}

---

Note that the above is equivalent to:

Code:

---

template <class T>
bool isEqualTo( const T &left, const T &right )
{
  return ( left.operator==(right) );
}

---

You have a const reference, left, calling a member function, bool operator==( const HugeInt &op2 ), that wasn't const. That is like the line above:
//constRefToInst.ModifyMe(); // ERROR!! Call non-const function from const reference.

When you add the const to the operator==, you see that it works like this line:
constRefToInst.DontModifyMe(); // Fine, call const function from const reference.

Of course, then once you understood why it wasn't working before, you changed operator== to not be a member function like Prelude suggested, and you stopped having to worry about member function constness for this problem. :)

Thanks for the help. I understand the problem with my code but references still torment me sometimes :D