Data Abstraction in C++: Concepts, Examples, and Implementation

To maximize the benefits of data abstraction in C++, it’s important to follow well-established design strategies and principles. Two of the most impactful are the DRY (Don’t Repeat Yourself) principle and the use of higher-order functions.

The DRY Principle

The DRY (Don't Repeat Yourself) principle encourages the developer to avoid re-writing code. This can be done by encapsulating repeatable functionality as objects that can be reused i.e. methods, functions classes etc. The use of DRY will probably result in a smaller app that can be maintained, reduce the chance of inconsistency in the code, and make extending the finalized code 'easier'. For these reasons alone, DRY is preferred to code duplication.

Example:
Rather than define its copies of duplicated code blocks in each of the parts of a corresponding app, the duplicated/shared code can be wrapped within a single function or class method, so any needed change or bugfix needs only to be modified in one place.

Higher-Order Functions

Higher-order functions are functions that take functions as arguments followed by returning functions as output. While higher-order functions are no less useful in C++ than in some other languages, C++ does provide features such as function pointers, functors and lambda expressions to provide a way to abstract behaviour and control-flow. If you're leveraging higher-order functions then you'll be writing code that is significantly more flexible and reusable.

Example:
You might use the std::sort algorithm with a custom comparator function, abstracting away the sorting logic and allowing different sorting behaviors to be injected as needed.

Separating Interface from Implementation

The default design principle of having data abstracted is to make the interface - what behaviours/methods is exposed - separate from the implementation - how those interfaces/behaviours are implemented.

Separating the implementation from the interface enables a programmer to subsequently alter a consumers specific implementation details of any functionality without breaking the consumer, thereby increasing the robustness and adaptability of the software you develop.

Abstraction is achieved in Java using interfaces and abstract classes and allows you to develop code that is both flexible and maintainable. Though both provide a template for other classes, both serve different purposes. It is important you understand the differences and when to use abstract classes and interfaces in order to facilitate good object-oriented programming in Java.

What is Abstract Class?

An abstract class in C++, is a class that is intended to be used as a base class for other classes and it cannot be directly instantiated. It will have at least one pure virtual function - a function that is declared with = 0 in its declaration. Because of this, the function is declared as abstract and an implementation has to be provided by that derived class. Abstract classes allow for the implementation of certain methods, but can also allow the development of a generic interface for a set of derived classes. Abstract classes are valuable when multiple derived classes have identifiable implementations for specific operations but share common functionality.

Syntax of Abstract Classes

To create an abstract class in C++ requires the definition of at least one pure virtual function. A pure virtual function defined by the syntax virtual returnType functionName() = 0; makes the class abstract; therefore, it cannot be instantiated directly. Here is the basic syntax:

class AbstractClassName {
public:
    virtual void functionName() = 0; // Pure virtual function
    void normalFunction() {
    	// Regular function implementation
    }
};

In this example, functionName() is defined as a pure virtual function, and AbstractClassName is an abstract class. Derived classes must override functionName() in order for an object of the derived class to be created.

Code Example of Abstract Class

Let's begin with a simple example to better understand how abstract classes work in C++. In this instance, we create an abstract class Shape which has a pure virtual function area(). Then we create two derived classes Rectangle and Circle which implement their own area() functions.

#include <iostream>
using namespace std;
 
// Abstract class
class Shape {
public:
    // Pure virtual function
    virtual double area() = 0;
};
 
// Derived class: Rectangle
class Rectangle : public Shape {
private:
    double length;
    double width;
public:
    Rectangle(double l, double w) : length(l), width(w) {}
    double area() override {
    	return length * width;
    }
};
 
// Derived class: Circle
class Circle : public Shape {
private:
    double radius;
public:
    Circle(double r) : radius(r) {}
    double area() override {
    	return 3.14 * radius * radius;
    }
};
 
int main() {
    Shape* shape1;
    Shape* shape2;
    Rectangle rect(5, 3);
    Circle circ(4);
 
    shape1 = &rect;
    shape2 = &circ;
 
    cout << "Area of Rectangle: " << shape1->area() << endl;
    cout << "Area of Circle: " << shape2->area() << endl;
 
    return 0;
}

How The Code Works

• Shape is an abstract class because it has a pure virtual function area().
• Rectangle and Circle derive from Shape and implement their own version of area().
• In main, we create instances of Rectangle and Circle but use Shape type pointers to access those instances. We are demonstrating polymorphism.
• This design allows us to calculate the area for different shapes using a common interface. This design is an example of using abstraction.

Output

Area of Rectangle: 15
Area of Circle: 50.24

Rules of Abstract Class

• When a derived class does not override all pure virtual functions, it will also become an abstract class.
• You may have regular member functions and regular member variables in an abstract class, which derived classes may inherit and utilize.
• When objects of a derived class are produced or destroyed, the constructors and destructors of the abstract class will be called. You can have both constructors and destructors in an abstract class.
• You can use pointers - and references - to an abstract class to allow polymorphism. The derived classes will work for dynamic binding of overridden methods.

What is an Interface? 

An interface is somewhat like a contract. It indicates what a class must do, but not how to do it. An interface indicates the methods that must be implemented, but does not give the implementation; an interface also allows passably multiple classes to implement it. This adds to polymorphism and multiple inheritance, plus making your code lighter and more modular.

Features of Interfaces

• No Implementation: Interfaces only declare method signatures and not their implementations. The implementing class must define the method bodies.
• Multiple Inheritance: a class can implement any number of interfaces, and it allows Java to effectively simulate multiple inheritance, which is not something that is allowed with classes.
• Default Methods: Java 8 introduced default methods in interfaces. They are methods with a body (and therefore, default behavior) which classes can use or override.
• Constant Variables: All variables in an interface are implicitly public, static, and final. This means they are constants and must be initialised when declared.
• Polymorphism: Interfaces allow objects of different classes to be treated as objects of the same interface type, promoting flexible and reusable code.
• Extends Another Interface: Additionally, an interface does not have to offer implementations in order to inherit and extend another interface's abstract methods.

Syntax of Interface

An interface in Java is declared with the interface keyword. The methods in the interface are declared with no body. This is the syntax in its most basic form.

interface InterfaceName {
	// Abstract method (no body)
	returnType methodName();
	
	// You can also have default methods with a body (Java 8 and later)
	default returnType defaultMethod() {
    	// method body
	}
	
	// Constants (public, static, final by default)
	int CONSTANT_VARIABLE = 100;
}

• Interface Declaration: You use the interface keyword to declare an interface.
• Method Declaration: Methods in an interface are abstract by default, so they don’t have a body and must be implemented by the class that implements the interface.‍
• Default Methods: These methods have a body and can provide default behaviour, allowing classes to use them directly or override them.

Code Example of an Interface

Let’s take a look at a simple example where we define an interface called Animal with an abstract method sound(). Two classes, Dog and Cat, implement the Animal interface, each providing its own version of the sound() method.

#include <iostream>
using namespace std;
 
// Abstract class acting as an interface
class Animal {
public:
    virtual void sound() = 0;  // Pure virtual function
};
 
class Dog : public Animal {
public:
    void sound() override {
    	cout << "Bark" << endl;
    }
};
 
class Cat : public Animal {
public:
    void sound() override {
    	cout << "Meow" << endl;
    }
};
 
int main() {
    Animal* myDog = new Dog();
    Animal* myCat = new Cat();
 
    myDog->sound();  // Outputs: Bark
    myCat->sound();  // Outputs: Meow
 
    delete myDog;
    delete myCat;
    return 0;
}

Explanation of the code

• The Animal class is declared as an abstract class with a pure virtual function sound(), which makes it an interface-like structure in C++.
• The sound() function is implemented by the Dog and Cat classes since they are extensions of Animal.
• You can see from the main() function that we create pointers to an Animal object that reference the Dog and Cat objects to show polymorphism.
• Call the sound() function for each object and print the sound produced, Bark for Dog and Meow for Cat.

Output

Bark
Meow

There are a number of ways to construct data abstraction in C++ that hide implementation specifics while revealing only the most important features. Among the well-known methods are:

Using Abstract Classes

Another best way to accomplish data abstraction is an abstract class. An abstract class allows developers to abstract the implementation and provide common interfaces. An abstract class in C++ will have one or more pure virtual functions (functions declared with = 0 ) , which make it abstract, and to not allow instantiation.

A high-level interface that can be shared by several derived classes can be defined using abstract classes. The derived classes are in charge of actually implementing the functions, and they are free to modify the behaviour to suit their requirements. By doing this, the abstract class's users are guaranteed to only engage with its interface and not the finer points of its implementation.

Example 

#include <iostream>
using namespace std;

// Abstract class with pure virtual function
class Shape {
public:
    // Pure virtual function for area calculation (abstract)
    virtual double calculateArea() = 0;

    // Virtual destructor
    virtual ~Shape() {}
};

// Derived class for a Circle
class Circle : public Shape {
private:
    double radius;

public:
    // Constructor to initialize radius
    Circle(double r) : radius(r) {}

    // Implementation of the pure virtual function
    double calculateArea() override {
        return 3.14159 * radius * radius;
    }
};

// Derived class for a Rectangle
class Rectangle : public Shape {
private:
    double width, height;

public:
    // Constructor to initialize width and height
    Rectangle(double w, double h) : width(w), height(h) {}

    // Implementation of the pure virtual function
    double calculateArea() override {
        return width * height;
    }
};

int main() {
    // Creating objects of derived classes
    Shape* shape1 = new Circle(5.0);       // Circle with radius 5
    Shape* shape2 = new Rectangle(4.0, 6.0); // Rectangle with width 4 and height 6

    // Displaying area using polymorphism
    cout << "Area of Circle: " << shape1->calculateArea() << endl;
    cout << "Area of Rectangle: " << shape2->calculateArea() << endl;

    // Clean up
    delete shape1;
    delete shape2;

    return 0;
}

How this Code Works

• Abstract Class: The Shape class defines a pure virtual function calculateArea(), which makes it an abstract class. Because Shape cannot be instantiated, it is an incomplete class.
• Pure Virtual Function: The calculateArea() function is pure virtual (= 0), forcing derived classes to implement it, and providing specific behaviour for different shapes.
• Derived Classes: Circle and Rectangle are derived from Shape and provide their own implementations of the calculateArea() function, hiding the details of how the area is calculated.
• Data Abstraction: The user interacts with the abstract class Shape using the calculateArea() function without needing to understand the specific implementation details of how the area is calculated for each shape.

Output

Area of Circle: 78.5397
Area of Rectangle: 24

Using Classes and Objects

Data abstraction is usually implemented in C++ using classes, which act as object creation blueprints. A class conceals the internal workings of the user while encapsulating data and operations that manipulate that data. Through public methods (getters, setters, etc.), the user can interact with class objects without having to understand how the data is internally processed or stored.

Encapsulation ensures that internal data members are concealed (typically designated as private) and that only the public interface that the class provides may access them. This preserves the object's integrity and guards against unwanted changes. 

Example

#include <iostream>
using namespace std;

// Class definition with data abstraction
class BankAccount {
private:
    double balance;  // Private data member

public:
    // Constructor to initialize balance
    BankAccount(double initial_balance) {
        balance = initial_balance;
    }

    // Public method to deposit money
    void deposit(double amount) {
        if (amount > 0) {
            balance += amount;
            cout << "Deposited: $" << amount << endl;
        } else {
            cout << "Invalid amount!" << endl;
        }
    }

    // Public method to withdraw money
    void withdraw(double amount) {
        if (amount > 0 && amount <= balance) {
            balance -= amount;
            cout << "Withdrew: $" << amount << endl;
        } else {
            cout << "Insufficient balance or invalid amount!" << endl;
        }
    }

    // Public method to get the balance (read-only access)
    double getBalance() {
        return balance;
    }
};

int main() {
    // Create a BankAccount object
    BankAccount account(1000.0);  // Initial balance of $1000

    // Accessing public methods to deposit and withdraw money
    account.deposit(500.0);      // Deposit $500
    account.withdraw(200.0);     // Withdraw $200

    // Displaying the final balance
    cout << "Current balance: $" << account.getBalance() << endl;

    return 0;
}

How this Code Works

• Data Abstraction is achieved by hiding the balance data member inside the private section of the class.
• Only specific methods (deposit, withdraw, and getBalance) provide controlled access to the balance, ensuring that the data cannot be directly modified from outside the class.
• The details of how the balance is changed or previously acquired are hidden from the user, so the interface is clean, and the data is still accurate.  

Output

Deposited: $500
Withdrew: $200
Current balance: $1300

Access Specifiers (Private, Protected, and Public)

In C++, access specifiers allow you to control access to the members of a class (variables, methods) such as private, protected, and public. C++ does data hiding by making data members private so they cannot be accessed directly from outside the class. 

Public Specifier

With the public access specifier, class members are available everywhere in your code. A public member can be modified or references directly outside of your class. This is helpful when defining the interface or methods that the user or the user application will operate on. The downside of having too many public members means that you have lost some control over your object's internal state; use them sparingly.

Private Specifier

The private access specifier allows class members in the class itself to be only be accessible inside of the class. These members will never be accessed directly outside of the class or from the derived class. Private members help provide some level of data encapsulation and security to ensure that the internals of an object remain completely hidden and could only be affected through the public methods defined, like getters/setters . This is important to maintain the control mechanisms you develop around your objects state and behaviour.

Protected Specifier

The protected access specifier is a middle ground between public and private. Members declared as protected are accessible within the class and by derived classes but not from outside the class hierarchy. This allows derived classes to inherit and use certain members while keeping them hidden from external code. It's useful when you want to provide some level of access to derived classes but still maintain encapsulation from other parts of the program.

• Only the class's methods have the ability to access and change private members.
• The interface through which users interact with the object is provided by public members, who can be accessed from anywhere in the code.
• Derived classes can access protected members, but not from outside the class hierarchy.
• With these specifiers, you may manage the extent to which the object's internal information and behaviour are made public, ensuring that only the required operations are available and that the underlying implementation is concealed. 

Code Example

#include <iostream>
using namespace std;

class Car {
private:
    // Private data: cannot be accessed directly from outside the class
    string model;

public:
    // Public function to set the model of the car
    void setModel(string m) {
        model = m;
    }

    // Public function to get the model of the car
    string getModel() {
        return model;
    }

protected:
    // Protected data: accessible by derived classes
    int speed;

public:
    // Constructor to initialize the speed
    Car(int s) : speed(s) {}

    // Public function to display car speed
    void showSpeed() {
        cout << "Car speed: " << speed << " km/h" << endl;
    }
};

class SportsCar : public Car {
public:
    // Constructor to initialize model and speed
    SportsCar(string m, int s) : Car(s) {
        setModel(m);
    }

    // Public function to display model and speed
    void display() {
        cout << "Sports Car Model: " << getModel() << endl;
        showSpeed();
    }
};

int main() {
    // Create an object of the derived class
    SportsCar myCar("Ferrari", 250);

    // Access public functions to set and display car details
    myCar.display();

    // Cannot directly access private data (model) outside the class
    // cout << myCar.model;  // This would cause an error

    return 0;
}

How this Code Works

Here is a simple step-by-step explanation of how the code demonstrates data abstraction using access specifiers:

• The Car class has three different types of members. They are private, protected and public. 
• The model variable is private so it cannot be accessed directly from outside of the class.
• The only way to access or modify the model is through the public methods setModel() and getModel(), which provide controlled access.
• The setModel() method is public and allows setting the value of the private model variable.
• The getModel() method is also public and provides access to the model value from outside the class.
• The speed variable is protected, meaning it can only be accessed within the Car class and any class derived from the Car (e.g., SportsCar).
• This prevents direct access to speed from outside while still allowing derived classes to use it.
• The showSpeed() method is public and displays the speed of the car. Even though speed is protected, it is accessible through this public method.
• The SportsCar class is derived from Car, inheriting its members and methods.
• It has its own constructor to set the car's model and speed using setModel() and the base class constructor.
• The display() method in SportsCar accesses the getModel() method and showSpeed() method to display both the model and speed.
• The private model and protected speed variables hide the internal details of the car's attributes.
• The user can interact with the car's data only through the public interface (setModel(), getModel(), showSpeed(), display()).
• This approach abstracts the internal workings of the Car class and allows the user to interact with the car’s features without needing to know how the data is stored or manipulated inside the class.

Output

Sports Car Model: Ferrari
Car speed: 250 km/h

Abstraction in Header files

In C++, header files declare classes, functions, variables, etc., without revealing the internal workings. This is a type of abstraction, where the header file is just the function prototypes, class declarations and constants, while the implementation is hidden away in amounting .cpp file. This separates the interface from the implementation in something called a module. The ability to separate the implementation from the interface increases the maintainability and modularity of code. Abstraction shields implementations from the users of the header because the users have no knowledge of how the code actually works. This makes it easier to support the code, and helps to promote encapsulation.

In this simple example, we demonstrate the concept of data abstraction by using a basic Hello World program. Even though the implementation is straightforward, the complexity of underlying system functions remains hidden from the user.

Code Example  

#include <iostream>
using namespace std;

int main()
{
    cout << "Hello World" << endl;
}

How the Code Works

• The program includes the necessary library for input and output (#include <iostream>).
• It uses using namespace std; to avoid needing to type std:: before standard functions like cout and endl.
• The program starts execution with the int main() function.
• It uses cout to output "Hello World" onto the screen.
• The program finishes and exits after printing, as it reaches the closing curly bracket }.

Output

Hello World

Data abstraction is a core concept of object-oriented programming (OOP). It is designed to hide an object's implementation from the user while only showing the user what is essential. By providing clear interfaces for interacting with objects without requiring knowledge related to the details of the object implementation, data abstraction (in C++) can manage complex systems. Data abstraction is therefore essential to creating software that is modular, scalable, and maintainable. The fundamental ideas of data abstraction in C++ are as follows:

1. Encapsulation

It is the act of combining the data and the functionalities that operate on it into one unit which typically takes the form of a class.  Encapsulation will ensure that within an object, the public methods afford controlled access to the data while keeping the particulars of the object hidden from outside access.  Encapsulation reduces the possibility of unwanted access from the outside and protects the data integrity.  Encapsulation and data hiding posses access restrictions to the data members, by making the data members private and allowing access through specific interfaces.

2. Abstraction

Abstraction aims to simplify complicated systems by revealing only the essential components and concealing the extraneous details. Abstract classes, which specify a common interface that other classes can implement, are frequently used in C++ to accomplish abstraction. An abstract class usually has one or more pure virtual function, and cannot be instantiated on its own. The concept allows for more flexible and reusable code, which lets programmers work with an object through its interface while providing the freedom to change its implementation, if needed. 

3. Classes and Objects

Classes act as objects with aspects of both data and operations that manipulate the data. Without having to comprehend an object's intrinsic structure, users can interact with it through well-defined interfaces thanks to the abstraction that classes provide. In order to reduce complexity, users are protected from the low-level specifics of how the data is handled or processed while interacting with objects of a class that are instantiated with specified data. 

4. Data Hiding

One way to restrict access to a class's private data is to use data hiding. In C++, access specifiers private, protected and public control access and modification to data members. The primary purpose of data hiding is to prevent sensitive information from being accessed or modified unintentionally or even illegally while also preserving the integrity and consistency of objects.  This process would prevent outside interference with the implementation and would also help facilitate the use of the objects in a proper manner. 

5. Interface and Implementation Separation

Data abstraction encourages the separation of an object's implementation and interface. The implementation specifies how these methods are carried out, whereas the interface specifies the methods that can be used to communicate with an object. More flexibility is made possible by this separation as, as long as the interface stays the same, changes to the implementation won't impact the class's users. This division also makes code maintenance and expansion easier because new features can be introduced, or current ones can be changed with little effect on other software components. 

Data abstraction that C++ provides helps to manage the various levels of complexity within the system, which allows programmers to define more efficient and maintainable software. Abstraction decreases code complexity, better code reuse, and enhances focus on the key aspects of a system by hiding unnecessary details. In order to accomplish this, the key concepts of encapsulation, abstraction, data hiding, and separation of interface and implementation are important. These concepts allow the programmer to work at higher levels of abstraction while maintaining flexibility and robustness in the code.

Now, let’s look at some of the differences between encapsulation and abstraction:

• Class internals are shielded against unintentional user-level mistakes.
• The low-level code does not have to be written by the programmer.
• Because code duplication is prevented, the programmer does not have to do fairly typical procedures to accomplish a comparable activity repeatedly.
• Code reuse and appropriate class division are essential to abstraction.
• Although this might not appear helpful for small projects, it offers structure and conformance for larger ones by providing documentation through the abstract class contract.
• It permits modifications to internal implementation details without impacting the abstraction's users.

To summarize, data abstraction in C++ is a key notion in object-oriented programming that can lower system complexity by presenting only the relevant data to the user while concealing much of the implementation. It makes the code modular and maintainable. Data abstraction also encourages the use of classes and abstract classes and access specifics such as public, private and protected. Abstract classes can help create common interfaces in a program, and derived classes help to specify and specialize abstract classes; encapsulating data allows us to protect our data using an interface that is well-defined, and to control access and modification to that data (the getter and setters promote restricted access to our information). This distinction between implementation and interface allows developers to concentrate on the high-level design and think about a system with a greater level of abstraction. This leads to code that is often more adaptable, usable, safe because it is encapsulated behind the interface, easier to maintain, and easier to scale.

1. How does encapsulation contribute to data abstraction in C++?

Encapsulation, by combining data with methods that manipulate that data, encapsulation helps C++ achieve data abstraction while concealing implementation detail. Encapsulation limits access to the class’s data members by constraining direct access (rather than being directly available to the user), normally by specifying access with the access specifiers: private, protected, and public. The purpose of data abstraction is to ensure that the internal state of an object is protected from misuse through limited access by the user via public methods and, therefore, interactions with the object are easier. 

2. What is the comparison between an abstract class and an interface in C++?

While an interface (usually implemented as an abstract class with just pure virtual functions) specifies a contract without implementation, an abstract class in C++ can have both conventional methods with implementations and pure virtual functions. In contrast to an interface, which usually does not have member variables, an abstract class may. The main distinction is that while an interface only outlines necessary behaviours that derived classes must implement, an abstract class might offer default behaviour. 

3. How does data abstraction support code maintainability and reusability?

Data abstraction in C++ improves code maintainability and reusability by displaying only the most significant elements and hiding the trivial details of implementations. This allows programmers to change or update a class's underlying processes without impacting the overall system's interface or other parts of the entire system. With high level functionality the code can be easier to read, debug and allow for additional functionality. It also allows for the same functionality to be used in many contexts or projects by abstracting common behaviours into reusable components or interfaces, which makes it more reusable and reduces code duplication.

4. In what scenarios might you use data abstraction in C++ for software design?

C++ data abstraction can be applied to a number of program design scenarios, such as:

• Developing reusable libraries: Data abstraction enables users to engage with libraries or frameworks through clearly defined interfaces without disclosing implementation specifics.
• Creating complicated systems: Abstraction hides superfluous complexities and streamlines interactions in large, multi-component systems, making them easier to maintain and operate.
• Putting polymorphism into practice: Data abstraction using abstract classes or interfaces permits polymorphic behaviour in situations when various object types exhibit similar behaviours, improving extensibility and flexibility.
• Encapsulating sensitive data: Data abstraction guarantees that an object's internal state is safeguarded and that access is only permitted via safe, regulated means in applications that are sensitive to security. 
• Supporting future changes: Abstraction makes it simpler to alter internal implementations of systems that might need to change in the future without interfering with the exterior interface.

5. What would happen if data abstraction were not used in a C++ program?

Without the abstraction represented by C++, the code in a C++ program could become more complicated and possibly more difficult to maintain, and probably easier to make mistakes. Without abstraction, the user would encounter the structure of the internal workings of the data structures and functions immediately, which can complicate implementing updates or modifications to the implementation because changes made to one component of the system may impact other components. A program without abstraction could result in tightly coupled components, with some code duplication, and the same effort could ultimately create problems when it comes to debugging and extending the program. The program could become less flexible, less modular, and less maintainable over time.