CODEWITHSUNDEEP
c++classoperator-keyword
ITEM-3
ITEM-3

Reputation: 3

C++ classes: create copy of "this" class within member function

I'm trying to write a function for the += operator of a C++ class which utilizes the already written + operator function. So far, I have been unsuccessful in associating the this pointer with the + operator. These are some of the attempts I've made that compiled in g++ but did not produce the desired result. Twice I attempted simply to make a copy of the this class, but that did not appear to work.

intstr& intstr::operator+=(const intstr& i)
{
  intstr *a;
  a = new intstr;
  *a = *this + i;
  return *a;
}

intstr& intstr::operator+=(const intstr& i)
{
  intstr *a, b(*this);
  a = new intstr;
  *a = b + i;
  return *a;
}

intstr& intstr::operator+=(const intstr& i)
{
  intstr *a, *b;
  a = new intstr;
  b = this;
  *a = *b + i;
  return *a;
}

intstr& intstr::operator+=(const intstr& i)
{
  intstr *a;
  a = new intstr;
  *a = this->operator+(i);
  return *a;
}

In the test code, all I've done is replace the working line of code a = a + i with a += i, so I doubt the problem lies there, but it is possible. Is the only way to do this to copy the code from the + operator into the += function?

Upvotes: 0

Views: 737

Answers (3)

Loki Astari
Loki Astari

Reputation: 264551

Usually you do it the other way around:

intstr& intstr::operator+=(intstr const& rhs)
{
     // Do this stuff to add rhs into this object.
     return *this;
}

// Then + is implemented in terms of +=
intstr intstr::operator+(intstr const& rhs)
{
    instr  result(*this);   // Make a copy as the result
    result += rhs;
    return result;
}

// Then your free functions as needed.

Upvotes: 0

Vlad from Moscow
Vlad from Moscow

Reputation: 311048

The operator can look like

intstr& intstr::operator+=( const intstr& i )
{
   *this = *this + i;

   return *this;
}

If the operator + is declared as a class member function then you can also write

intstr& intstr::operator+=( const intstr& i )
{
   *this = operator +( i ); // or *this = this->operator +( i );  

   return *this;
}

It would be a mistake to allocate dynamically an object of type intstr within the operator. At least there is no such a need to do this.

Upvotes: 1

David Rodríguez - dribeas
David Rodríguez - dribeas

Reputation: 208396

Usually the approach is the opposite: you implement operator+= and then you implement operator+ using that implementation (create a copy of the first argument, then use += to increment by the second argument and return that).

Other than that, why are you calling new in all versions? For operator+= you don't need to create any new object at all. The operator should modify the value of the left-hand-side operand by incrementing it with the value of the right-hand-side. No new objects need to be created anywhere (and less so dynamically allocated with new!)

Upvotes: 2

Related Questions