reusing the returned object from operator function

Question

Consider the following operator overloading scenario

class integer
{
   public:
   int a;
   integer()
   {
        a = 0;
   }
  friend integer operator+(integer &a ,int b);
};
integer operator+(integer &a,int b)
{
    a.a = a.a + b;
    return a;
}
int main()
{
   integer obj;
   integer obj2;
   obj+2; //Line 1.  this works.

The operator+ function returns an object of integer type.So why can't i execute a function call like

(obj+2)+3; //no match for 'operator+' (operand types are 'integer' and 'int')

While this works

obj2 = obj+2;
obj2+3;

(obj+2) returns a temporary. Temporaries can't bind to a non-const reference. — Igor Tandetnik
– Igor Tandetnik, Commented Apr 14, 2016 at 5:08
Taking a step back, you've named your operator +, but it behaves like +=. This will end in tears. — Igor Tandetnik
– Igor Tandetnik, Commented Apr 14, 2016 at 5:09
Simply making operator+ take a const integer & a and making it not modify the reference, but modify a new integer that you return will make this work as you want, and as it should with regards to what @IgorTandetnik said. — xaxxon
– xaxxon, Commented Apr 14, 2016 at 6:09

M.M · Accepted Answer · 2016-04-14 06:31:42Z

3

integer &a means that a can only refer to an lvalue. But obj+2 is not an lvalue.

To fix this, you should change your parameter to integer a, which:

can bind to rvalues
makes your function actually perform operator+'s function.

If your first parameter is integer& it would suggest you are trying to write operator+=. For that function, the solution would be to also return integer&, so that obj+=2 is an lvalue.

answered Apr 14, 2016 at 6:31

M.M

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Richard Hodges · Accepted Answer · 2016-04-14 06:48:54Z

2

As a corollary to M.M's answer (which you should accept), here is the fixed code:

class integer
{
   public:
   int a;
   integer()
   {
        a = 0;
   }
  friend integer operator+(integer a ,int b);
};

// fix: arg 1 passed by value
integer operator+(integer a,int b)
{
    a.a = a.a + b;
    return a;
}

int main()
{
   integer obj;
   integer obj2;
   obj+2;
  (obj+2)+3;
}

answered Apr 14, 2016 at 6:48

Richard Hodges

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