What is an Abstract Class in C++?

Expected Interview Answer

Step-by-Step Explanation

1. Step 1

   Declare a pure virtual function

   Inside the class, write 'virtual returnType name() = 0;' to make the function pure virtual.

2. Step 2

   Class becomes abstract

   Any class with at least one pure virtual function is abstract and cannot be instantiated directly.

3. Step 3

   Add shared members if useful

   Include data members, constructors, and concrete methods to provide common state and default behaviour.

4. Step 4

   Derive concrete classes

   Create derived classes that override every pure virtual function so they become instantiable.

5. Step 5

   Use through base pointers

   Store derived objects behind an abstract base pointer or reference to call the overridden methods polymorphically.

Code Example

#include <iostream>
using namespace std;

class Shape {
public:
    virtual double area() const = 0;  // pure virtual -> abstract
    virtual ~Shape() = default;
};

class Circle : public Shape {
    double r;
public:
    Circle(double radius) : r(radius) {}
    double area() const override { return 3.14159 * r * r; }
};

int main() {
    // Shape s;             // ERROR: cannot instantiate abstract class
    Shape* s = new Circle(2.0);
    cout << "Area: " << s->area() << "\n";
    delete s;
}

Follow-up Questions

• What is a pure virtual function and how is it declared?
• Can an abstract class have a constructor?
• What is the difference between an abstract class and an interface in C++?
• What happens if a derived class does not override all pure virtual functions?
• Can a pure virtual function have a definition in C++?