Inheritance Forms, Virtual Base Classes & The Diamond Problem

Explanation

Inheritance establishes 'is-a' relationships. In multipath multiple inheritance, duplicated ancestor copies create memory bloat and compiler ambiguity. C++ resolves this via Virtual Base Classes, where the compiler synthesizes an internal virtual base pointer table (vbtable), binding derived instances to a single shared ancestor subobject in memory.

Key points

• Inheritance types: Single, Multilevel, Multiple, Hierarchical, Hybrid.
• The Diamond Problem: Arises in multipath multiple inheritance where Class D inherits from Class B and C, which both inherit from Class A. D receives two conflicting copies of A's members.
• Virtual Base Classes: Class B : virtual public A and Class C : virtual public A ensures that Class D inherits only ONE shared instance of Class A.
• Constructors in Inheritance: Base constructors execute first, followed by derived constructors. In virtual inheritance, the most derived class directly calls the virtual base constructor.

Example

Code examples

// Resolving The Diamond Problem Using Virtual Base Classes in C++
#include <iostream>
using namespace std;

class Student {
protected:
    int rollNumber;
public:
    void setRoll(int r) { rollNumber = r; }
};

class Test : virtual public Student {
protected:
    float marks1, marks2;
public:
    void setMarks(float m1, float m2) { marks1 = m1; marks2 = m2; }
};

class Sports : virtual public Student {
protected:
    float score;
public:
    void setScore(float s) { score = s; }
};

class Result : public Test, public Sports {
public:
    void display() {
        // No ambiguity! Exactly one copy of rollNumber exists
        float total = marks1 + marks2 + score;
        cout << "Roll: " << rollNumber << ", Total: " << total << endl;
    }
};

Exam questions

• [8 Marks] What is the diamond problem in C++? Explain with an inheritance diagram how virtual base classes resolve member duplication.
• [7 Marks] Explain the visibility of base class members under public, protected, and private derivation modes.
• [7 Marks] Write a program demonstrating constructor call sequences in multiple and multilevel inheritance.