Detailed explanation of C++ friend functions: What is the difference between friend functions and member functions?

Friend functions allow external functions to access private or protected members of a class by declaring them with the friend keyword in the class definition. Unlike member functions, friend functions are declared outside the class and can access private and protected members of the class, while member functions are declared inside the class and can access all members of the class. Friend functions are used as ordinary function calls, while member functions are called with class objects. Friend functions are used when external access to private or protected members is required, and member functions are used when member functions are used inside the class.

C Detailed explanation of friend functions: The difference between friend functions and member functions

Friend functions

A friend function is a special function that allows external functions to access private and protected members in a class. It is declared using the friend keyword in the class definition.

Syntax:

class MyClass {
  friend void myFriendFunction();
  ...
};

Member functions

Member functions are functions that belong to a class and can access its private and protected members.

Grammar:

class MyClass {
  void myMemberFunction();
  ...
};

The difference between friend function and member function

Features	Friend function	Member function
Accessibility	Can access private and protected members of the class	Can access all members of the class
Declaration location	Outside the class	Inside the class
Scope	Global	Internal class
Calling method	Call like a normal function	Call using class objects

Practical case

Consider a Student class that has a private marks Member:

class Student {
private:
  int marks;
  ...
};

We can use the friend function calculateAverage() to calculate the student's average grade, which has access to marks:

// 友元函数
friend double calculateAverage(Student& student);

// 计算学生的平均成绩
double calculateAverage(Student& student) {
  return student.marks / 3;
}

We can also use member functions to calculate the average grade, but it can only be used in the Student class:

// 类的成员函数
double getAverage() {
  return marks / 3;
}

Conclusion

Friend functions provide a mechanism that allows external functions to access private and protected members of a class without making these members visible to the outside world. Member functions have access to all members of the class, but can only be used within the class.

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