virtual
- C++ course
- Intermediate
- Lesson 14/116
- 5 min
Learn virtual methods and polymorphism with virtual.
virtual
By marking a base class member function virtual, you let a derived class run its own version. This is called polymorphism. When called through a pointer or reference, the object's real type is used.
Example: class Animal { public: virtual void speak() { std::cout << "Sound"; } };. Dog redefines it: void speak() { std::cout << "Woof"; }. The call Animal* a = &dog; a->speak(); prints Woof, because with virtual the real type of dog is chosen. Without virtual, it would print Sound.
Calling through a pointer
Polymorphism happens at runtime; it requires a base class pointer or reference. A direct call through an object does not work, because the compiler uses the static type. The -> operator accesses a member through a pointer.
Example: Dog dog; Animal* a = &dog; a->speak();. Here a is an Animal pointer but points to the dog object. With virtual, Dog::speak is called. A reference works too: Animal& r = dog; r.speak();. Both give the same result.
Practice Polymorphism with virtual speak
Define an Animal class: under public: virtual void speak() { std::cout << "Sound"; }. Derive Dog from Animal and redefine void speak() { std::cout << "Woof"; }. In main create Dog dog;, take Animal* a = &dog; and call a->speak().
Starter code
#include <iostream>
// Define Animal and Dog classes here
int main() {
// Create Dog, call speak via Animal*
return 0;
}Sample solution
#include <iostream>
class Animal {
public:
virtual void speak() {
std::cout << "Sound";
}
};
class Dog : public Animal {
public:
void speak() {
std::cout << "Woof";
}
};
int main() {
Dog dog;
Animal* a = &dog;
a->speak();
return 0;
}Expected output
Woof
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Write the virtual keyword
class Animal { public: ____ void speak() { } };class Animal { public: virtual void speak() { } };Explanation: virtual lets speak be redefined in a derived class.
2 What does Animal* a = &dog; a->speak(); print? (speak is virtual, Dog::speak prints "Woof")
Animal* a = &dog;
a->speak();- Sound
- Compile error
- Nothing
- Woof Correct answer
Explanation: With virtual, the real type of dog is chosen and Woof is printed.
3 Match each piece to its role
| Term | Meaning |
|---|---|
virtual |
Enables polymorphism |
-> |
Member access via pointer |
Animal* |
Base class pointer |
&dog |
Object the pointer points to |