Virtual function implementation C++ not working - c++

Question

I'm new to C++, and I'm trying to write a simple code to compare two objects of subclasses of a parent class called Comparable. I want each subclass to have its own implementation of a method to compare objects based on the data they hold, so I used the virtual keyword:
class Comparable {
public:
virtual int compare(Comparable *other);
};
For example, my subclass HighScoreElement would have its own implementation of compare that would compare the score of the object to the score of another HighScoreElement.
Here is my subclass HighScoreElement:
class HighScoreElement: public Comparable {
public:
virtual int compare(Comparable *other);
HighScoreElement(string user_name, int user_score); // A constructor
private:
int score;
string name;
};
But in my compare implementation in HighScoreElement, I first try to check if the current object's data is the same as other's data. But since the pointer to other is of class Comparable and not HighScoreElement, I can't reference other->score at all in my code, even though HighScoreElement is a subclass of Comparable.
Here is the full code so far:
#include <iostream>
using namespace std;
class Comparable {
public:
virtual int compare(Comparable *other);
};
class HighScoreElement: public Comparable {
public:
virtual int compare(Comparable *other);
HighScoreElement(int user_score, string user_name);
private:
string name;
int score;
};
HighScoreElement::HighScoreElement(int user_score, string user_name) {
name = user_name;
score = user_score;
}
int HighScoreElement::compare(Comparable *other) {
if (this->score == other->score) { // Compiler error right here, other->score is invalid.
// Code to do the comparing if two scores are equal...
}
}
I get a compiler error immediately when I write this code:
if (this->score == other->score)
because other doesn't have data called score, but its subclass, HighScoreElement, does. How can I fix my function implementation so that I can reference the data of "other?" I know my question may sound vague, but any help would be appreciated!

Answer 1

You could implement a virtual function GetScore(), possibly pure virtual in the base class, and use that instead of accessing the field score in your compare function. Make it a const method. On the other hand, Compare could be a method implemented in the base class, that uses this->GetScore() and other->GetScore()
Code stub:
class A {
virtual int getScore() const = 0;
inline bool compare(const A* in) {return (in && this->getScore() == in->getScore());}
//return false also if "in" is set to NULL
}
class B : public A {
int score;
inline int getScore() const {return score;}
}

Answer 2

You can cast the pointer passed to HighScoreElement::compare using "dynamic_cast" (it throws a bad_cast exception on failure).
int HighScoreElement::compare(Comparable *other) {
HighScoreElement *h = NULL;
try
{
ptr = dynamic_cast<HighScoreElement *>(other);
}
catch(std::bad_cast const &)
{
// Handle the bad cast...
}
if (this->score == ptr->score) {
// Code to do the comparing if two scores are equal...
}
}

Answer 3

If you are prepared to accept null pointers, you can use dynamic casts. You can have an overload for the case when you are comparing a HighScoreElement pointer to avoid an unnecessary cast.
#include <iostream>
using namespace std;
class Comparable {
public:
virtual int compare(Comparable *other) = 0; // made pure virtual to compile without definition
};
class HighScoreElement: public Comparable {
public:
virtual int compare(Comparable *other);
int compare(HighScoreElement *other); // comparing to a HighScoreElement ptr, no need to dynamic cast
HighScoreElement(int user_score, string user_name);
private:
string name;
int score;
};
HighScoreElement::HighScoreElement(int user_score, string user_name) {
name = user_name;
score = user_score;
}
int HighScoreElement::compare(Comparable *other) {
HighScoreElement * pHSE = dynamic_cast<HighScoreElement*>(other);
if (pHSE) {
return compare(pHSE);
} else {
return -1; // or however you want to handle compare to non HighScoreElement
}
}
int HighScoreElement::compare(HighScoreElement *other) {
if (this->score == other->score) {
;
}
}

Answer 4

Are you sure it's not
compare( Comparable other )
If (this->score == other.score)