Question

I have a virtual class Element that forces its derived classes to have a == operator:

So, I have derived classes (Integer, Word) that implement that operator.

I have a class Group that basically is a list of elements.

In a function, I want to compare if an element from a group its equal to an element of a different group, so I’m using:

if(actual == lookingfor)

where both actual and lookingfor are pointers to Element…but the comparison is being made at the level of pointers, so both pointers are always different.

How can I force that the operator == from the derived class of element be used?

EDIT:

class Element 
{ 
  virtual int operator==(Elemento *)=0; 
}

class Word : public Element { ... } 

int Word::operator==(Element * element) 
{ 
  Element * ptr = element; 
  Word * wordPtr = dynamic_cast< Word * >(ptr); 
  int equal = 0; 
  if(wordPtr)
  { 
    equal = strncmp(this->ptr,wordPtr->ptr,49)==0; 
  } 
  return igual; 
}

Answer 1

^{please not that the below implementation uses references instead of pointers as arguments to operator== defined as pure-abstract in struct Element. This is how it’s usually done and I’d recommend you to do the same in your code.}

#include <iostream>

struct Element {
  virtual ~Element () {}
  virtual bool operator== (Element&) const =0; 
};

struct Integer : Element {
  bool operator== (Element&) const {
    return false; /* dummy impl */
  }
};

struct Word : Element {
  bool operator== (Element&) const {
    return false; /* dummy impl */
  }
};

---

It’s not possible to overload the comparision of pointers in C++, you will have to dereference your pointers so that you’ll get valid objects which you can compare in a normal manner, and your own operator== will be called.

int
main (int argc, char *argv[])
{
  Element * e1 = new Integer;
  Element * e2 = new Word;

  if (*e1 == *e2)
    ;   

  delete e2; 
  delete e1; 
}

---

Another method is to call your operator== directly through your pointers, such as in the below example.

if (e1->operator== (*e2))
  ;

---