Detailed explanation of C++ friend functions: What problems are friend functions used to solve?

Friend functions are special functions in C that can access private members of other classes. They solve the access restrictions caused by class encapsulation and are used to solve problems such as data operations between classes, global function access to private members, and code sharing across classes or compilation units. Usage: Use the friend keyword to declare a friend function to access private members. Note: Use friend functions with caution to avoid errors caused by bypassing the encapsulation mechanism. Use only when necessary, limit access, and use modifier functions sparingly.

C Detailed explanation of friend function: a powerful tool for lifting access restrictions

Introduction

Friend function is a special type of function in C that can access private members of another class. It allows data and methods that are originally inaccessible to the outside world to be accessed by the outside world, thus solving the access restriction problems caused by some class encapsulation.

Purpose

Friend functions are usually used to solve the following problems:

• When two or more classes need to operate each other's private data time.
• When you need to provide a global function to access private members of a class to perform specific operations.
• When code needs to be shared between different classes or different compilation units.

Syntax

The syntax for declaring a friend function is as follows:

class ClassName {
    // ...成员声明
    friend FunctionName;
};

where FunctionName is the friend function The name.

Practical case

Suppose we have two classes Student and Teacher, they need to access each other's private data . We can use friend functions to achieve this:

class Student {
private:
    int marks;
};

class Teacher {
private:
    int salary;
public:
    friend void calculateBonus(const Student& student, const Teacher& teacher);
};

void calculateBonus(const Student& student, const Teacher& teacher) {
    std::cout << "Student's marks: " << student.marks << std::endl;
    std::cout << "Teacher's salary: " << teacher.salary << std::endl;
}

int main() {
    Student student;
    student.marks = 90;
    Teacher teacher;
    teacher.salary = 50000;
    calculateBonus(student, teacher);
    return 0;
}

In this example, the calculateBonus function is declared as a friend of the Student and Teacher classes metafunction, so it can access the private members marks and salary of these two classes.

Usage Precautions

You need to be careful when using friend functions, because they bypass the encapsulation mechanism of the class and may cause unexpected errors. Therefore, the following points should be considered when declaring friend functions:

• Use friend functions only when absolutely necessary.
• Restrict the access rights of the friend function to only allow it to access necessary private data.
• Use modifier functions carefully in friend functions to avoid accidentally modifying private data.

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