🔗 C++ Interfaces (via Abstract Classes) – Design Contracts for OOP

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🧲 Introduction – Why Interfaces Matter in C++

In C++, interfaces are implemented using abstract classes that contain only pure virtual functions. While C++ doesn’t have a separate interface keyword like Java or C#, abstract classes serve the same purpose—providing a contract that derived classes must fulfill.

🎯 In this guide, you’ll learn:

• What an interface is in C++
• How to implement interfaces using abstract classes
• Benefits and real-world use cases
• Best practices for designing clean, modular APIs

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🔍 What Is a C++ Interface?

An interface in C++ is defined as an abstract class where:

• All functions are pure virtual (= 0)
• Typically no data members
• No constructor or implementation logic (optional, but best practice)

📌 Purpose: To enforce a structure/behavior that all derived classes must follow.

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💻 Code Examples – With Output

✅ Example 1: Basic Interface Implementation

#include <iostream>
using namespace std;

class IPrintable {
public:
    virtual void print() = 0;  // Pure virtual
    virtual ~IPrintable() {}   // Virtual destructor
};

class Document : public IPrintable {
public:
    void print() override {
        cout << "Printing Document..." << endl;
    }
};

int main() {
    IPrintable* obj = new Document();
    obj->print();
    delete obj;
    return 0;
}

🟢 Output:

Printing Document...

🔍 Explanation:

• IPrintable acts as an interface
• Document must override all methods in the interface
• A virtual destructor ensures safe polymorphic cleanup

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🧱 Interface Design Pattern

✅ Interface Skeleton:

class InterfaceName {
public:
    virtual void method1() = 0;
    virtual void method2() = 0;
    virtual ~InterfaceName() {}
};

🔒 No implementation or data members.
✅ Followed by multiple derived classes providing specific behavior.

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📘 Interfaces vs Abstract Classes

Feature	Interface (in C++)	Abstract Class
Purpose	Behavior contract only	Contract + partial implementation
Pure virtual methods	✅ All	✅ At least one
Data members allowed	❌ Avoided	✅ Allowed
Reuse implementation	❌ No	✅ Yes
Instantiable?	❌ No	❌ No

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💡 Best Practices & Tips

📘 Best Practice: Use I prefix (e.g., IShape, ISerializable) to clearly denote interfaces.

💡 Tip: Use virtual destructors in interfaces to ensure proper cleanup when deleting through a base pointer.

⚠️ Pitfall: Do not add data members or implementation logic to interfaces—this breaks interface separation.

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🛠️ Real-World Use Cases for Interfaces

📄 Document Processing: IPrintable, ISaveable, ISerializable
🎮 Game Engines: IEntity, IController, IInteractable
🔌 Plugins/Frameworks: IPlugin, IModule, IService
🌐 Network Stacks: ISocket, IProtocolHandler for dynamic swapping of implementations

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📌 Summary – Recap & Next Steps

🔍 Key Takeaways:

• Interfaces in C++ are built using abstract classes with pure virtual functions
• Provide a clean, modular way to define behavior contracts
• Enable powerful polymorphism and decoupled architecture

⚙️ Real-World Relevance:
Used in frameworks, APIs, engine architectures, and large-scale modular systems where components must follow strict behavioral contracts.

✅ Next Steps:

• Learn about Virtual Destructors and VTable Internals
• Explore Interface Inheritance and Multiple Interface Implementation

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❓FAQ – C++ Interfaces via Abstract Classes

❓Does C++ have a keyword for interface like Java?
❌ No. Interfaces are created using abstract classes with only pure virtual functions.

❓Can a class implement multiple interfaces in C++?
✅ Yes. C++ supports multiple inheritance from abstract classes (interfaces).

❓Can interfaces contain data members?
⚠️ Not recommended. Interfaces should only declare behavior, not state.

❓Why use virtual destructors in interfaces?
✅ To ensure that derived class destructors are called properly when deleting via interface pointers.

❓How is an interface different from a base class?
An interface has no implementation, while a base class can provide some logic.

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