BITAC Assistant Programmer Exam: 27.10.2023 (BUTEX)প্রশ্ন ও সমাধান

A virtual function is a member function declared in a base class with the virtual keyword that a derived class can override; when called through a base-class pointer or reference, C++ resolves which version to execute at runtime, based on the actual object type — this is runtime (dynamic) polymorphism.

#include <iostream>
using namespace std;

class Animal {
public:
    virtual void sound() {              // virtual function
        cout << "Animal makes a sound" << endl;
    }
};

class Dog : public Animal {
public:
    void sound() override {             // overriding
        cout << "Dog barks" << endl;
    }
};

int main() {
    Animal *a;
    Dog d;
    a = &d;
    a->sound();     // calls Dog::sound() at runtime, not Animal::sound()
    return 0;
}

Output:

Dog barks

Without virtual, a->sound() would call Animal::sound() based purely on the pointer's declared type (static binding) — virtual defers that decision to runtime using a vtable (virtual function table) that stores the correct overridden function's address for each class.

classDiagram
class Animal { +sound() virtual }
class Dog { +sound() override }
Animal <|-- Dog

Exam takeaway: virtual + a base-class pointer/reference + an override in the derived class = runtime polymorphism, implemented internally via a vtable/vpointer mechanism.