operator overloading about Abstract class (cpp)

Question

In student.h file

class Student {
 protected:
  string Name;
  int Stu_num;
 public:
  virtual void print() = 0;
  // constructor...
}

class Grad_Student: public Student {
 private:
  string Lab;
 public:
  void print();
 // constructor...
}

class Undergrad_Student: public Student {
 private:
  string Major;
 public:
  void print();
  // constructor...
}

in student.cpp file

bool operator == (const Student& x, const Student& y) {
 if (typeid(x) != typeid(y)) return false;
 else // dosomething..
}

I want to compare child of Student class; Grad and Undergrad. in main.cpp when i comapre two student class, it doesn't work..

Grad_Student grad1 = Grad_Student("Max", 11, "Hubo");
Student *std1 = &grad1;

Grad_Student grad2 = Grad_Student("Max", 11, "Hubo");
Student *std2 = &grad2;

cout << (std1 == std2) << endl; // it always prints 0.
cout << (*std1 == *std2) << endl; 
// I think this line should work, but makes error.
// error: invalid operands to binary expression ('Student' and 'Student')

Should i overload operator== in student class? give me a hint...

Answer 1

So, your overloaded equality operator does nothing... You need to compare something in the body and return a boolean result.

Also, your Undergrad_Student currently doesn't derive from Student.. Furthermore, since you're comparing 2 Student objects you can just make the equality operator overload a member function taking 1 argument.

Something like this:

class Student {
public:
    explicit Student( int id )
        : id_{ id }
    { }

    bool operator==( const Student& other ) const {
        return id_ == other.id_;
    }

    virtual void print() const = 0;
    virtual ~Student( ) = default;

protected:
    int id_;
}; 

class Grad: public Student {
public:
    explicit Grad( int id )
        : Student{ id }
    { }

    void print() const override {}
};

class Undergrad : public Student {
public:
    explicit Undergrad( int id )
        : Student{ id }
    { }

    void print() const override {}
};

int main( ) {
    Grad s1{ 11 };
    Undergrad s2{ 12 };

    // Prints false.
    std::cout << std::boolalpha << ( s2 == s1 ) << '\n';

    Grad s3{ 42 };
    Undergrad s4{ 42 };

    // Prints true.
    std::cout << std::boolalpha << ( s3 == s4 ) << '\n';
}