Detailed explanation of C++ friend functions: How to use friend functions for cross-class access?

Friend functions in C allow non-member functions to access private and protected members of a class, which is very useful in implementing cross-class operations, ADT and unit testing: Declaring friend functions: Use the friend keyword to declare an out-of-class function. Use friend functions: Directly access a private member of a class as if it were a member function. Practical case: Use friend function to obtain private age information without exposing the age attribute.

Detailed explanation of C friend function: making cross-class access a reality

In C programming, friend function is a powerful mechanism that allows Functions outside a class access private and protected members of the class. This is very useful in some situations, such as:

• Implementing the operation of non-member functions on certain class attributes.
• Implement abstract data types (ADT) across classes.
• In the Unit testing framework, use friend functions to bypass access restrictions to test private methods.

Declaration of friend function

class MyClass {
public:
    // ...
private:
    // ...
    friend void print_my_class(const MyClass&); // 声明友元函数
};

The above code declares print_my_class as a friend function of MyClass. This means that the print_my_class function can access private and protected members of MyClass.

Use of friend functions

To use friend functions, you only need to directly access the private members of the class in the friend function:

void print_my_class(const MyClass& obj) {
    std::cout << "Private member: " << obj.private_member << std::endl;
    std::cout << "Protected member: " << obj.protected_member << std::endl;
}

Practical case

Suppose we have a Person class, representing a person, which has a private age member. We want to create a friend function get_age to get the age of the Person object:

class Person {
public:
    // ...
private:
    int age;
    friend int get_age(const Person&); // 声明友元函数
};

int get_age(const Person& person) {
    return person.age;
}

In the main function, we create a Person object and use the friend function get_age to access its private members:

int main() {
    Person person(25);
    std::cout << "Age: " << get_age(person) << std::endl;
    return 0;
}

Output:

Age: 25

The above is the detailed content of Detailed explanation of C++ friend functions: How to use friend functions for cross-class access?. For more information, please follow other related articles on the PHP Chinese website!