C++ Tutorial Index

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C++ Control Statements

C++ If C++ Nested if C++ If-else C++ If-else-if C++ Switch C++ Break C++ Continue C++ Goto C++ For loop C++ While loop C++ Do while loop

C++ Functions

C++ Call by Value C++ Call by Reference C++ Recursion Function C++ Inline function C++ Friend function

C++ Arrays

Single dimension array Two dimension array

C++ Strings

C++ Strings

C++ Inheritance

C++ Inheritance Single level Inheritance Multilevel Inheritance Multiple Inheritance Hierarchical Inheritance Hybrid Inheritance

C++ Polymorphism

C++ Polymorphism C++ Overloading C++ Overriding C++ Virtual Function

C++ Pointers

C++ Pointers C++ this pointer

C++ Exception Handling

C++ Exception Handling

C++ Constructors

C++ Constructors Default Constructor Parameterize Constructor Copy constructor Constructor Overloading Destructor

C++ File Handling

C++ File Handling C++ Writing to file C++ Reading file C++ Close file


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C++ Interfaces

The C++ programming language provides programmers with a variety of capabilities and functions. It also enables object-oriented programming, which is essential while working on a project. It will be simple for you to learn C++ programming if you have already learnt C programming. Because some ideologies in C++ are very much same as those in C programming. Now, from here we'll go over a C++ concept known as Interface.

Interfaces are present in various languages but here we will only be focussed on the interfaces in C++ language, what they are, how they are used and how they are helpful for us.

In C++, an interface is just a means to define the behaviour of a class without committing to any specific implementation. It is a feature that C++ classes and objects provide as a complement. We'll show you how to take the next step in exploring the depths of object-oriented programming using classes and objects. It's worth noting that in C++, there's no distinction between interfaces and abstract classes.

So moving ahead, let's start C++ Interfaces.

What are interfaces in the C++ language :

A class having a pure virtual function is referred to as an abstract class in C++ programming.

Interfaces are very much related to the classes and the objects. As a result, it's reasonable to assume that the terms "Interfaces" and "Abstract Classes" are interchangeable. The difference between data abstraction and abstract classes is that the goal of data abstraction is to keep important details separate from other data. Interfaces and abstract classes are similar in such way that they express the same message.

It's crucial to understand the difference between abstract classes and data abstraction in C++, as data abstraction merely removes the relevant data from the implementation approaches.

C++ Pure Virtual Functions :

An abstract class in C++ is just a class having a pure virtual function, as previously stated. First we should know about virtual and pure virtual functions to grasp more knowledge.

Virtual function-

A virtual function is a member function declared in a base class that a derived class epitomises (bypasses). When you refer to a derived class object that used a pointer or a reference to the base class, you may call a virtual function for that object and have it to execute the derived class's version of the function. It is feasible to create a virtual function-based base class. Abstract classes are not at all those that have virtual functions. There will be no influence on the compilation if the derived class does not redefine the virtual function of the base class. Also

  • • Virtual functions guarantee that the correct function is called for an object, irrespective of the type of reference (or pointers) used to call it.
  • They're mostly utilised to achieve polymorphism at runtime.
  • The virtual keyword is used to declare functions in base classes.
  • The call to a function is resolved at runtime.

Drawbacks of Virtual Functions :

  1. The virtual mechanism causes the function call to take somewhat longer, making it more difficult for the compiler to optimise because it does not know which function will be called at build time and is thus slower.
  2. Virtual functions can make it a little more difficult to find out where a function is being called from in a complicated system.

Example –

#include <iostream>
using namespace std;
class Base {
    virtual void prints() {
        cout << "Base Func." << endl;
class Derived : public Bases {
    void prints() {
        cout << "Derived Func." << endl;
int main() {
    Derived derived01;

    // pointer of the Base type, points to the derived01
    Base* base01 = &derived01;

    // calls the member function of the earlier Derived class

    return 0;


Derived Func.

In this example, the prints() method of Base has been designated virtual in this case. So, even if we use a reference of Base type that links to the Derived object derived01, this function is overwritten. Thus in the main, the pointer of the Base type points towards the Derived object derived01. Then further calling the member function prints() of this Derived class we printed the required output.

Pure virtual function-

Pure virtual function is a virtual function that is known to be existing but we can't implement that. It's just a mere declaration and is not a plan.

Virtual functions are utilised exclusively

  • if a method has no purpose in the base class
  • but it’s required to be implemented by all of its derived classes

It's worth remembering that a pure virtual function may only be declared, not defined. The definition of a function is not specified in the base class in the case of a pure virtual function. It will not produce an error if the derived class does not define the pure virtual function; rather, the derived class will become an abstract class.


Virtual float func() = 0;

Example –

In this example, we have assumed that we've descended from the Shapes class, the various shapes of figures and we want to determine the area of each of these forms.

In this situation, we may use the Shape to build a pure virtual function called area(). All derived classes of those shapes must implement the area() function since it is a pure virtual function.

The function body is not present in a pure virtual function, and it must conclude with = 0. As an example,

class Shapes {

      // we are creating a pure virtual function in c++
      virtual void area() = 0;

Note that the = 0 syntax does not imply that the function is set to zero but it is the predefined manner in which the pure virtual functions are defined in the C++ language.

The Significance of Interface in C++ :

Let us recognise the significance of pure virtual functions once we have developed a comprehension of them by addressing a small situational example. Assume you've made an Apples class containing data members iPhoneX, iPad Pro, and iPod, as well as member methods rate(), ring() and alarms().

Assume that the ratee of each product that is being sold is fixed and can’t be changed. We know that Apple goods come in a variety of pricing, such as the iPhone X, which costs $60,500, and the iPad Pro, which costs $45,000.

The only acceptable approach to implement the function rate() in the class Apples is to make it pure abstract, so that we don't have to implement it in the class Apples, but all the classes that inherit the Apples class must. We can assure that all Apples items have a set and unique pricing in this way.

In C++, there are two methods for implementing the friend function.

Here is a C++ application that will assist you in solving the problem indicated above:


using namespace std;

class Apples
// Pure Virtual Funct 
virtual void rate() = 0;

// Member funct
void ring() 
cout<<"Ring-tone is: trumpets"<<endl;
class iPhoneX: public Apples
void rate() 
cout<<"The rate is: 60,500"<<endl;

int main()

cout<<"Significance of interfaces in C++"<<endl<<endl;

iPhoneX i;
return 0;


Significance of interfaces in C++!
The rate is: 60,500
Ring-tone is: trumpets

Another example  –

Assume you've constructed an OS class having data members Windows, Linux, and Mac, as well as member methods like sizes(), types() and features().

Assume that each operating system's size is fixed and cannot be changed. Varied operating systems, on the other hand, have different sizes. You may implement the class OpS for the method sizes() in one approach by making it abstract. We can ensure that the size of all operating systems is set this way.

The code to address the problem is provided below.

#include <iostream>
using namespace std;
class OpS
virtual void sizes() = 0;
void types()
cout<<"It’s windows OS!"<<endl;
class WindowsOS: public OpS
void size()
cout<<"The size is 4.89GB!"<<endl;

int main()
cout<<"Interfaces in C++ !"<<endl<<endl;
WindowsOS data;
return 0;


Interfaces in C++!
The size is 4.890GB!
It’s windows OS!

In C++, several rules applied to interfaces are :

When dealing with interfaces/abstract classes in C++, there are a few principles to bear in mind.

  • Since it is not feasible to define a pure virtual function thus it can only be declared and not defined.
  • You can't provide the pure virtual function any other value than 0.
  • It is not feasible to establish a class instance.

Example- You can't build an Apples type object, like, using the software shown above. There would be a compilation fault as a result.

  • The derived class becomes an abstract class if it is unable to implement the base class's pure virtual function.
  • Using a reference to the base abstract class, it is possible to build a pointer that refers to the instance of the derived class.


Using the preceding programme as an example, you may do this task by writing the following statements:

Apples *object = new iPhoneX();
object -> rate();


OpS *obj = new WindowsOS();

In both these examples, we used a reference object which refers to the instance of the derived classes here.

Summarizing the content :

Hope the distinction between interfaces and abstract classes in C++ is clearer now. In addition, we now know about how to use classes to build pure virtual functions in a real-world application. Finally, we went through some of the principles that apply to abstract classes, leaving no space for confusion.