Virtual Functions

Virtual Functions and Polymorphism

Polymorphism means "many forms" — the same call can behave differently depending on the actual type of the object. Virtual functions make this possible.

The Problem Without virtual

class Shape {
public:
    string type() { return "shape"; }
};

class Circle : public Shape {
public:
    string type() { return "circle"; }
};

void describe(Shape& s) {
    cout << s.type() << endl;
}

Circle c;
describe(c);  // prints "shape" — wrong!

Without virtual, the compiler picks the method at compile time based on the declared type (Shape&), not the actual type (Circle).

The Fix: virtual

Mark the base class method as virtual:

class Shape {
public:
    virtual string type() { return "shape"; }
};

class Circle : public Shape {
public:
    string type() override { return "circle"; }
};

Circle c;
describe(c);  // prints "circle" — correct!

Now the method is selected at runtime based on the actual object type. This is runtime polymorphism.

override Keyword

The override keyword is placed after the parameter list in a derived class method. It tells the compiler you intend to override a virtual function from the base class. While not strictly required, it is strongly recommended because the compiler will produce an error if the method does not actually match a virtual function in the base — catching typos and signature mismatches at compile time:

class Circle : public Shape {
    string type() override { return "circle"; }  // OK: matches virtual in Shape
    string tipe() override { ... }  // error: no virtual tipe() in Shape
};

Pure Virtual Functions and Abstract Classes

A pure virtual function has no implementation in the base class — derived classes must provide one:

class Shape {
public:
    virtual int area() = 0;  // pure virtual
};

class Square : public Shape {
public:
    int side;
    Square(int s) : side(s) {}
    int area() override { return side * side; }
};

// Shape s;  // error: cannot instantiate abstract class

Your Task

Create a Shape base class with virtual string type() and virtual int area(). Create Rectangle (width, height) and Square (side) that override both methods. Write a printInfo(Shape& s) function and call it with each shape.