Virtual functions in C++ - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a virtual function in C++?

A virtual function is a member function in a base class that you expect to be overridden in derived classes. It allows C++ to support runtime polymorphism by deciding which function to call at runtime based on the object's actual type.

beginner

Why do we use virtual functions?

We use virtual functions to achieve dynamic dispatch, meaning the program decides at runtime which function to call depending on the actual object type, not the pointer or reference type. This helps in writing flexible and extendable code.

intermediate

What happens if a base class function is not declared virtual but overridden in a derived class?

If the base class function is not virtual, then calling the function through a base class pointer or reference will always call the base class version, even if the object is of a derived class. This is called static binding.

beginner

How do you declare a virtual function in C++?

You declare a virtual function by adding the keyword virtual before the function's return type in the base class. For example:

virtual void speak();

intermediate

What is a pure virtual function and how is it declared?

A pure virtual function is a virtual function with no implementation in the base class, making the class abstract. It is declared by assigning 0 to the function in the base class, like this: <pre>virtual void draw() = 0;</pre>

What keyword is used to declare a virtual function in C++?

A abstract

B override

C dynamic

D virtual

The keyword virtual is used to declare virtual functions in C++.

If a base class function is virtual, which function is called when using a base class pointer to a derived object?

A Base class function

B Derived class function

C Compiler error

D Depends on pointer type

With virtual functions, the derived class function is called at runtime, enabling polymorphism.

What happens if you call a non-virtual function through a base class pointer to a derived object?

A Base class function is called

B Derived class function is called

C Runtime error

D Function is ignored

Non-virtual functions use static binding, so the base class function is called.

How do you declare a pure virtual function?

A virtual void func() = 0;

B void func() = 0;

C virtual void func();

D void func() override;

A pure virtual function is declared by assigning 0 after the declaration.

What kind of class contains at least one pure virtual function?

A Static class

B Concrete class

C Abstract class

D Final class

A class with at least one pure virtual function is called an abstract class.

Explain how virtual functions enable polymorphism in C++.

Think about how the program chooses which function to run when using a base class pointer.

Describe the difference between a virtual function and a pure virtual function.

Consider what happens if you want to force derived classes to implement a function.