Debugging Grimoire

C++ Inheritance and Derived Classes

The Inheritance Mechanism

• Allows deriving new classes from existing base classes.
• Reuses existing code, avoiding tedious and error-prone duplication.
• Derived classes extend or alter base class functionality.
• Creates a hierarchy of related types sharing code and interface.

Base Class: student

#include <cstring>
#include <iostream>

class student {
public:
  enum year { fresh, soph, junior, senior, grad };
  student(char *nm, int id, double g, year x);
  void print() const;

protected:
  int student_id;
  double gpa;
  year y;
  char name[30];
};

Derived Class: grad_student

class grad_student : public student {
public:
  enum support { ta, ra, fellowship, other };
  grad_student(char *nm, int id, double g, year x, support t, char *d, char *th);
  void print() const;

protected:
  support s;
  char dept[10];
  char thesis[80];
};

Constructors

student::student(char *nm, int id, double g, year x)
    : student_id(id), gpa(g), y(x) {
  strcpy(name, nm);
}

grad_student::grad_student(char *nm, int id, double g, year x, support t,
                           char *d, char *th)
    : student(nm, id, g, x), s(t) {
  strcpy(dept, d);
  strcpy(thesis, th);
}

• grad_student constructor invokes the base student constructor.
• Base class constructed first.
• student_id and gpa are protected, so accessible to derived class.

Print Functions

void student::print() const {
  std::cout << name << " , " << student_id << " , " << y << " , " << gpa << std::endl;
}

void grad_student::print() const {
  student::print();
  std::cout << dept << " , " << s << std::endl << thesis << std::endl;
}

• grad_student::print reuses student::print and adds extra info.

main() Function

int main() {
  student s("Mae Pohl", 100, 3.425, student::fresh), *ps = &s;
  grad_student gs("Morris Pohl", 200, 3.2564, student::grad, grad_student::ta,
                  "Pharmacy", "Retail Pharmacies"), *pgs;
  
  ps->print();  // student::print
  ps = pgs = &gs;
  ps->print();  // still student::print due to pointer type
  pgs->print(); // grad_student::print
}

• Demonstrates polymorphism via pointer to base class.
• ps points to both student and grad_student objects.
• Without virtual, the base class's print() is called.
• pgs calls the derived version because it's typed as grad_student*.

Benefits Recap

• Reuse of tested code.
• Reflects domain relationships.
• Allows treating derived types as base types.
• Simpler, more maintainable code.

Reminder: Mark that print() function as virtual if you want actual polymorphism, otherwise it’s just pretending like your last situationship.