Question

Distinguish between virtual functions and pure virtual functions.

Short answer

Virtual functions have optional base class implementations; pure virtual functions have none, requiring derived classes to implement them.

Step-by-step solution

Define Virtual Functions

A virtual function in C++ is a member function in the base class that you expect to override in derived classes. When you declare a function as virtual, you tell the compiler to support late binding, allowing the function to be called according to the actual object (at run time) that's being referenced rather than the type of reference or pointer.

Understand Usage of Virtual Functions

Virtual functions enable polymorphism, a feature that allows one interface to be used for a general class of actions. In simple terms, polymorphism enables you to use virtual functions to call derived class methods through base class pointers or references.

Define Pure Virtual Functions

A pure virtual function is a virtual function with no implementation in the base class, and it's declared by assigning 0 in its declaration, e.g., `virtual void functionName() = 0;`. A class containing at least one pure virtual function becomes an abstract class that cannot be instantiated.

Evaluate Purpose of Pure Virtual Functions

Pure virtual functions are used when you want to ensure that derived classes implement specific functions. This requirement enforces a contract for derived classes to provide an implementation, effectively delegating the responsibility for function behavior by design.

Compare Virtual and Pure Virtual Functions

Both virtual and pure virtual functions serve to ensure correct function overriding in derived classes. The key difference is that virtual functions can have an implementation in the base class, while pure virtual functions do not and their presence forces derived classes to provide an implementation.

Key concepts

Polymorphism

Polymorphism is a powerful concept in programming, especially in object-oriented languages like C++. It provides a way to perform a single action in different ways. Polymorphism allows objects of different classes to be treated as objects of a common superclass. This is achieved through the use of virtual functions. A key feature of polymorphism is that it promotes flexibility and integration across different classes.

Polymorphism comes in two main types:

• Compile-time Polymorphism: Also known as static polymorphism, it happens at compile time and involves method overloading or operator overloading.
• Run-time Polymorphism: This is the more common form in C++ and involves using virtual functions and inheritance, allowing method overriding based on the object's actual derived type.

When a base class declares a function as virtual, it enables run-time polymorphism. Polymorphism ensures that the correct function is called for an object, eliminating the need for complex conditional statements. This is particularly useful when you have different implementations of a function and want to decide which implementation to invoke at runtime based on the actual object type being referred to by a base pointer or reference. This capability makes programs more flexible and easier to manage.

Pure Virtual Functions

Pure virtual functions in C++ are used to ensure that certain methods must be implemented in derived classes. A class with at least one pure virtual function becomes an abstract class.

Defining a Pure Virtual Function:
A pure virtual function is declared in a way that indicates it will not have implementation in its declaring class. This is done by adding `= 0` at the end of the function declaration, like this: ```cpp virtual void functionName() = 0; ```

• The `virtual` keyword indicates that the function supports late binding.
• The `= 0` syntax denotes that the function has no body or implementation at the base level.
• Derived classes are then obliged to provide an implementation for the pure virtual function.

Pure virtual functions are integral to creating abstract classes, which serve as blueprints for other classes. These functions allow you to define interfaces in your program, ensure that derived classes adhere to certain behavior or method signatures, and help implement design patterns like the Strategy or Template Method.

Abstract Class

An abstract class in C++ is a class that cannot be instantiated, which means you cannot create objects of an abstract class directly. Typically, an abstract class contains one or more pure virtual functions.

Characteristics of Abstract Classes:

• Abstract classes are designed to act as base classes. Their primary role is to provide a common interface for derived classes.
• A class becomes abstract by declaring at least one of its functions as pure virtual.
• Abstract classes can have regular member functions and member data along with pure virtual functions.
• While you cannot instantiate an abstract class directly, you can have pointers and references to it, which allows run-time polymorphism.

By forcing derived classes to implement certain functions, abstract classes help enforce consistency across different implementations of an interface. This provides a backbone structure that derived classes must follow, thereby ensuring that they fulfill specific contracts laid out by the abstract class. This kind of structure is pivotal in designing frameworks and systems that rely on a common set of operations or behaviors. As a result, abstract classes are an essential feature in achieving cleaner, more organized, and maintainable code.