How does the friend mechanism of C++ functions break encapsulation?

In C, the friend mechanism breaks encapsulation and allows functions or classes to access private members of other classes. By using the friend keyword, you can declare a function as a friend of a class, allowing you to access its private data and methods. The friend mechanism is often used to resolve cyclic dependencies between classes, such as allowing constructors to become friends of each other and access each other's private member initialization objects. It should be noted that the friend mechanism will reduce encapsulation, so it should be used with caution and only the functions or classes that are really needed are designated as friends.

Friend mechanism of C function: a weapon to break encapsulation

Introduction

The friend mechanism is a mechanism in C that breaks encapsulation. It allows a function or class to access the private members of other classes or functions. This is useful in certain situations, such as when dealing with circular dependencies between classes or when you need to access specific members of other classes.

Usage

To declare a function as a friend of a class, you can use the friend keyword. For example:

class MyClass {
private:
    int private_member;
public:
    friend void myFriendFunction(MyClass& obj);
};

void myFriendFunction(MyClass& obj) {
    // 可以访问 MyClass 类的私有成员 private_member
    std::cout << obj.private_member << std::endl;
}

Practical case: circular dependency

Consider the following example, where two classes depend on each other:

class A {
private:
    std::vector<B*> b_objects;
};

class B {
private:
    A* a_object;
};

If circular dependencies are not allowed, Then the two classes cannot be initialized because the private members of the classes refer to each other. The friend mechanism can solve this problem by making the constructor of the class a friend of the other party, thereby allowing access to the other party's private members.

class A {
private:
    std::vector<B*> b_objects;
    friend class B; // 允许 B 类访问 A 的私有成员
};

class B {
private:
    A* a_object;
    friend class A; // 允许 A 类访问 B 的私有成员
};

Note:

• The friend mechanism is a mechanism that breaks encapsulation, so it should be used with caution.
• Friend functions can access all private members of the specified class, including data and methods.
• When defining friends, please ensure that only functions or classes that are truly needed are designated as friends.

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