C++ Interfaces

Interfaces play an important role in designing clean and well-organized code structures in C++. In C++, an interface is a conceptual construct that specifies a set of methods that must be implemented by any class that claims to conform to the Interface. It acts as a blueprint that enforces consistent behavior among its implementers while shielding the details of the actual implementation. By doing so, C++ interfaces enhance code modularity, maintainability, and reusability, aligning perfectly with the principles of object-oriented programming.

Interface as a Concept

Although C++ lacks a defined interface keyword, the idea of an interface is accomplished through abstract classes and pure virtual functions. An abstract class has one or more pure virtual functions - functions specified in the base class but not implemented. These pure virtual functions serve as the methods of the Interface, setting up the requirements for any class that derives from the abstract base.

class Interface {
public:
virtual void method1() = 0; // Pure virtual function
virtual void method2() = 0; // Pure virtual function
// ... more pure virtual functions
};

Implementing the Interface

Classes that implement the interface must override all pure virtual functions declared in the abstract base. These classes provide the necessary implementations for the interface methods, adhering to the prescribed contract.

C++

class ConcreteClass: public Interface {
public:
void method1() override {
// Implementation for method1
}
void method2() override {
// Implementation for method2
}
};

Example 1:

#include <iostream>

// Define the Shape interface
class Shape {
public:
virtual double getArea() const = 0; // Pure virtual function for area
virtual double getPerimeter() const = 0; // Pure virtual function for perimeter
};

// Implement the Interface for a Circle
class Circle: public Shape {
private:
double radius;

public:
Circle(double r) : radius(r) {}

double getArea() const override {
return 3.14159265359 * radius * radius;
}

double getPerimeter() const override {
return 2 * 3.14159265359 * radius;
}
};

// Implement the Interface for a Rectangle
class Rectangle: public Shape {
private:
double length;
double width;

public:
Rectangle(double l, double w) : length(l), width(w) {}

double getArea() const override {
return length * width;
}

double getPerimeter() const override {
return 2 * (length + width);
}
};

int main() {
// Create instances of Circle and Rectangle
Circle circle(5.0);
Rectangle rectangle(4.0, 6.0);

// Calculate and display the area and perimeter of each shape
std::cout << "Circle Area: " << circle.getArea() << ", Perimeter: " << circle.getPerimeter() << std::endl;
std::cout << "Rectangle Area: " << rectangle.getArea() << ", Perimeter: " << rectangle.getPerimeter() << std::endl;

return 0;
}

Output:

Circle Area: 78.5398, Perimeter: 31.4159
Rectangle Area: 24, Perimeter: 20

Pure Virtual Function:

A pure virtual function is a function in C++ specified in a base class that does not have an implementation, signified by "= 0" in its declaration. It serves as a contract that derived classes must implement.

C++

"virtual": It indicates that this function is virtual, enabling polymorphism.
"void": Specifies the return type.
"myFunction": Name of the function.
"= 0": Marks it as pure virtual, meaning it has no implementation in the base class and must be overridden in derived classes.

C++ Abstract class

A class is abstracted in C++ by defining at least one of its functions as a>strong>pure virtual function. "= 0" is used in the declaration of a pure virtual function. Derived classes must provide its implementation.

Example 2:

Let us look at an abstract class in C++ that has one abstract function, draw(). Derived classes provide its implementation: Rectangle and Circle. Both classes have different implementations.

#include <iostream>
using namespace std;
class Shape
{
public:
virtual void draw()=0;
};
class Rectangle: Shape
{
public:
void draw()
{
cout < <"drawing rectangle..." < <endl;
}
};
class Circle: Shape
{
public:
void draw()
{
cout <<"drawing circle..." < <endl;
}
};
int main( ) {
Rectangle rec;
Circle cir;
rec.draw();
cir.draw();
return 0;
}

Output:

drawing rectangle...
drawing circle...

Benefits of C++ Interfaces:

There are several benefits of C++ interfaces. Some main benefits of the C++ interfaces are as follows:

Consistency and Contract: Interfaces ensure that classes adhering to the same interface exhibit consistent behavior. This contract-like structure aids in preventing deviations from the expected functionality, which is crucial for maintaining code reliability.
Modularity and Separation of Concerns: Interfaces encourage the separation of Interface from implementation. The users of a class only interact with the interface methods, leaving the implementation details hidden. This separation enhances code modularity, making it easier to manage complex systems.
Polymorphism and Flexibility: Interfaces facilitate polymorphism, allowing different classes to be treated uniformly through their shared interface methods. This feature enables developers to write generic code that can work with various implementations without knowing their specifics.
Extensibility and Reusability: C++ interfaces are important for designing extensible systems. When new requirements emerge, new classes adhering to existing interfaces can be added, seamlessly integrating into the system architecture without disturbing the existing codebase.
Testing and Mocking: Interfaces ease unit testing and mocking, as you can create mock implementations of interfaces to isolate individual components during testing. It helps in identifying issues early in the development process.
Code Documentation: Interfaces inherently document the expected behavior of classes. By inspecting the interface methods, developers gain insights into the functionality without delving into implementation intricacies.

Real-World Use Cases:

C++ interfaces find applications in various real-world scenarios. Consider a GUI framework where different types of buttons (simple, toggle, radio) share a common behavior (e.g., click event). By defining an interface for button behaviors, the framework ensures that all button types implement the necessary methods, maintaining consistency across the board.

Similarly, in a game engine, interfaces could define the behaviors of game entities (such as characters, enemies, and items). This abstraction allows developers to introduce new entities with unique behaviors by implementing the required interface methods.

Rules for Using Interfaces

Certainly, here are the essential rules for using interfaces in C++ briefly:

Use Abstract Classes: Define interfaces as abstract classes with pure virtual functions.
Prefix Interface Names: Consider prefixing interface names with 'I' or similar to indicate their role.
No Member Data: Interfaces should not contain member data (attributes).
Public Access: Declare interface methods as public for external usage.
Explicit virtual and override: Use virtual when overriding, and override to indicate explicit overriding.
Implement All Methods: Any class deriving from an interface must implement all interface methods.
Keep It Focused: Keep interface methods related to a specific theme or purpose.
Document Interfaces: Document the purpose and usage of interfaces.
Consider Segregation: Avoid forcing classes to implement unrelated methods (Interface Segregation Principle).
Stable Interfaces: Avoid making breaking changes to interfaces once in use.