Dilemma with "operator overloading"...

**Petike** · 08-24-2008

Hi all,
let's say we have a class Integer and we want to be able to add some two Integers (our objects) with operator "+" - so we overload the operator in our class:

Code:

class Integer
{
public:
    // Operator "+"
    const Integer
    operator+(const Integer& right) const
    {
        return Integer(i + right.i)
    }
private:
    int i;   // Some integer
}

But my question is:
Why the return value in operator+ function is const? Should I understand it, that the return value cannot be ever changed?
(Because in my opinion, from logic sight it doesn't get any meaning)

Thanks.

Petike

**matsp** · 08-24-2008

I agree that it doesn't make much sense to make it const Integer.

--
Mats

**Benzakhar** · 08-24-2008

I think "the return value cannot be ever changed" makes sense. Does "(a+b) = 7;" make sense to you? The "const" helps prevents people doing something like that by accident.

I think you can make a plus sign return something you can change, and it may make sense to lists for example:

newlist = (lista + listb).sort();

I'm not sure if the compiler will actually let you do that though. If the lists are big I wouldn't recommend doing it that way.

**CornedBee** · 08-24-2008

Should I understand it, that the return value cannot be ever changed?

While this is true, it only applies to the temporary being returned. You'll usually then assign the value to a non-const Integer.

By the way, the canonical way of overloading the binary operators is to do this:

Code:

class whatever
{
  whatever& operator +=(const whatever &rhs) {
    // Perform += here, i.e. add rhs's value to this one.
    return *this;
  }
};

(const?) whatever operator +(const whatever &lhs, const whatever &rhs)
{
  whatever tmp(lhs);
  tmp += rhs;
  return tmp;
}

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