The Friend Keyword

The friend keyword stands as a unique construct, offering a departure from the standard encapsulation principles of object-oriented programming. It allows a function or another class to access private and protected members of a class.

main


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#include <iostream>

class Example {
private:
    const static int static_private_member = 0;
    friend void func(); // grants access to the private members to func()
};

void func()
{ 
    std::cout << Example::static_private_member; 
}

int main() 
{
    func();
}

Using this approach breaks encapsulation because it lets outside entities access the class members. However, there might be situations where it's necessary to do so. For instance:

KeyManager


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class KeyManager {
public:
    KeyManager(const std::string& key) : encryptionKey(key) {}
private:
    std::string encryptionKey;
};

The encryptionKey is kept private, and there is no accessor method provided because we want to prevent external access to it from outside the class. But what if there is a necessity to use an external algorithm for encrypting and decrypting, this is where the friend keyword comes into play.

KeyManager

CryptographicAlgorithm


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#include "CryptographicAlgorithm.h"

class KeyManager {
public:
    KeyManager(const std::string& key) : encryptionKey(key) {}
private:
    std::string encryptionKey;
    // Allow CryptographicAlgorithm access to private members
    friend class CryptographicAlgorithm; 
};

The most common use-case for friend keyword arises when quick fixes are required, and you intend to refactor it later. It's preferable to design your class relationships without relying on it, although specific scenarios may still occur.

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Sección 3. Capítulo 6

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Contenido del Curso

C++ OOP

1. Fundamentals of OOP in C++

2. Constructors and Destructors

3. Encapsulation Overview

Introduction to Encapsulation Access Modifiers Keywords Access Modifiers Practice Accessor and Mutator Methods Segmentation and Aggregation The Friend Keyword

4. Inheritance Overview

Introduction to Inheritance Access Modifiers in Inheritance Mulptiple Inheritance Constructor and Destructor in Inheritance Diamond Inheritance The Final Keyword

5. Polymorphism Overview

Introduction to Polymorphism Virtual and Override Keywords Pure Virtual Method Operator Overloading Summary and Overview

The Friend Keyword

main


              1234567891011121314151617
            
#include <iostream>

class Example {
private:
    const static int static_private_member = 0;
    friend void func(); // grants access to the private members to func()
};

void func()
{ 
    std::cout << Example::static_private_member; 
}

int main() 
{
    func();
}

Using this approach breaks encapsulation because it lets outside entities access the class members. However, there might be situations where it's necessary to do so. For instance:

KeyManager


              123456
            
class KeyManager {
public:
    KeyManager(const std::string& key) : encryptionKey(key) {}
private:
    std::string encryptionKey;
};

KeyManager

CryptographicAlgorithm


              12345678910
            
#include "CryptographicAlgorithm.h"

class KeyManager {
public:
    KeyManager(const std::string& key) : encryptionKey(key) {}
private:
    std::string encryptionKey;
    // Allow CryptographicAlgorithm access to private members
    friend class CryptographicAlgorithm; 
};

¿Todo estuvo claro?

¡Gracias por tus comentarios!

Sección 3. Capítulo 6