c语言中++b与b++

Friend Functions in C++ are a category of functions that can access private and protected members of a class while being a public function/outside the class.

C ++中的“朋友函数”是一类函数，可以作为类的公共功能/外部访问类的私有成员和受保护成员。

You may wonder; is this even possible? Well, modern C++ says yes!

您可能想知道； 这有可能吗？ 好吧，现代C ++可以！

Let us look at how we can use these functions in C++.

让我们看看如何在C ++中使用这些函数。

C ++中的朋友功能 (Friend Function in C++)

We call a function a friend function if the function definition is prefixed with the friend keyword.

如果函数定义以friend关键字为前缀，则将函数称为朋友函数 。

We cannot use the friend prefix outside a class, so it can only be used in the member function declaration.

我们不能在类外部使用friend前缀，因此只能在成员函数声明中使用。

For example, if we want to use the friend keyword in the declaration, we can use it like this:

例如，如果我们想在声明中使用friend关键字，可以这样使用：

#include <iostream>using namespace std;class MyClass {private:int a, b;public:MyClass(int a, int b) {// Constructorthis->a = a;this->b = b;}void set_a(int val);void set_b(int val);int get_a() {return a;}int get_b() {return b;}// Declare a friend function of this classfriend void my_fun(MyClass& my_obj, int val, char option);
};void my_fun(MyClass& my_obj, int val, char option) {// Friend function that sets the private variables// based on the optionif (option == 'a') {// Set amy_obj.a = val;}else {// Set bmy_obj.b = val;}
}int main() {MyClass my_obj(1, 2);cout << "my_obj.a = " << my_obj.get_a() << " and my_obj.b = " << my_obj.get_b() << endl;// Friend functions can be accessed from outside the class!my_fun(my_obj, 20, 'a');my_fun(my_obj, 40, 'b');cout << "my_obj.a = " << my_obj.get_a() << " and my_obj.b = " << my_obj.get_b() << endl;return 0;
}

Here, my_fun() is a friend function of MyClass, so it can access the private members too!

在这里， my_fun()是MyClass的朋友函数，因此它也可以访问私有成员！

Notice that we can access a friend function from outside the class as well.

注意，我们也可以从类外部访问friend函数 。

The above friend function sets the value of the private members a or b directly!

上面的friend函数直接设置私有成员a或b的值！

Output

输出量

my_obj.a = 1 and my_obj.b = 2
my_obj.a = 20 and my_obj.b = 40

A friend function can either be within the same class or even inside another class. But it must be a member function.

朋友函数可以在同一类中，甚至可以在另一类中。 但是它必须是成员函数。

Let’s look at these two cases one by one.

让我们一一看一下这两种情况。

C ++中同一类中的Friend Function (Friend Function within the same class in C++)

The above example showed us that we can use a friend function inside the same class.

上面的例子向我们展示了我们可以在同一个类中使用一个朋友函数。

class MyClass {private:int a, b;public:MyClass(int a, int b) {// Constructorthis->a = a;this->b = b;}void set_a(int val);void set_b(int val);int get_a() {return a;}int get_b() {return b;}// Declare a friend function of this class// within the same class itself!friend void my_fun(MyClass& my_obj, int val, char option);
};

We’ll move on to the next case; when a friend function is inside another class!

我们将继续进行下一个案例。 当一个朋友函数在另一个类里面时！

This is actually inside another topic, called Friend Classes, but they are directly related to each other.

这实际上在另一个主题（称为Friend Classes）中 ，但是它们彼此直接相关。

C ++中的朋友类–另一个类中的朋友函数 (Friend Classes in C++ – Friend Functions in another class)

Any class B which has friend functions of another class A is called a Friend Class of A.

它具有另一个类A的朋友功能的任何类B被称为A的朋友类。

Similar to friend functions, any member function inside the friend class B can access private and protected members of A.

类似于好友函数，好友类B中的任何成员函数都可以访问A的私有成员和受保护成员。

Let’s understand this more clearly using an example.

让我们通过一个例子更清楚地了解这一点。

I will use the previous example, but I will add another class Student, and make it as the friend class of MyClass.

我将使用前面的示例，但是我将添加另一个类Student ，并将其作为MyClass的朋友类。

#include <iostream>
#include <string>using namespace std;class Student;class MyClass {private:int a, b;public:MyClass(int a, int b) {// Constructorthis->a = a;this->b = b;}void set_a(int val);void set_b(int val);int get_a() {return a;}int get_b() {return b;}// Declare a friend function of this classfriend void my_fun(MyClass& my_obj, int val, char option);// Make Student a friend Class of MyClassfriend Student;
};

Now, let’s write the Student class:

现在，让我们编写Student类：

class Student {private:string name;   int marks;public:Student(string s_name, int s_marks) {// Constructorname = s_name;marks = s_marks;}int get_marks() {return marks;}string get_name() {return name;}void set_marks(Student& stud, int s_marks) {stud.marks = s_marks;}void set_name(Student& stud, string s_name) {stud.name = s_name;}void change_a(MyClass& my_obj, int val) {// You need to pass the object by reference.// Otherwise, it will not reflect the changes on// the original objectmy_obj.a = val;}void change_b(MyClass& my_obj, int val) {// You need to pass the object by reference.// Otherwise, it will not reflect the changes on// the original objectmy_obj.b = val;}
};

As you can see, I am using change_a() and change_b() to change the MyClass object attributes directly!

正如你所看到的，我使用change_a()和change_b()来改变MyClass对象属性直接！

Let’s now run the complete snippet below.

现在，让我们运行下面的完整代码段。

#include <iostream>
#include <string>using namespace std;class Student;class MyClass {private:int a, b;public:MyClass(int a, int b) {// Constructorthis->a = a;this->b = b;}void set_a(int val);void set_b(int val);int get_a() {return a;}int get_b() {return b;}// Declare a friend function of this classfriend void my_fun(MyClass& my_obj, int val, char option);// Make Student a friend Class of MyClassfriend Student;
};class Student {private:string name;   int marks;public:Student(string s_name, int s_marks) {// Constructorname = s_name;marks = s_marks;}int get_marks() {return marks;}string get_name() {return name;}void set_marks(Student& stud, int s_marks) {stud.marks = s_marks;}void set_name(Student& stud, string s_name) {stud.name = s_name;}void change_a(MyClass& my_obj, int val) {// You need to pass the object by reference.// Otherwise, it will not reflect the changes on// the original objectmy_obj.a = val;}void change_b(MyClass& my_obj, int val) {// You need to pass the object by reference.// Otherwise, it will not reflect the changes on// the original objectmy_obj.b = val;}
};void my_fun(MyClass& my_obj, int val, char option) {// Friend function that sets the private variables// based on the optionif (option == 'a') {// Set amy_obj.a = val;}else {// Set bmy_obj.b = val;}
}int main() {MyClass my_obj(1, 2);cout << "my_obj.a = " << my_obj.get_a() << " and my_obj.b = " << my_obj.get_b() << endl;// Friend functions can be accessed from outside the class!my_fun(my_obj, 20, 'a');my_fun(my_obj, 40, 'b');cout << "my_obj.a = " << my_obj.get_a() << " and my_obj.b = " << my_obj.get_b() << endl;// Class Student objectsStudent stud("Amit", 34);cout << "stud.name = " << stud.get_name() << " and stud.marks = " << stud.get_marks() << endl;// Change my_obj.a and my_obj.b using the friend classstud.change_a(my_obj, 100);stud.change_b(my_obj, 200);cout << "After using the Friend Class methods,\n";cout << "my_obj.a = " << my_obj.get_a() << " and my_obj.b = " << my_obj.get_b() << endl;return 0;
}

Output

输出量

my_obj.a = 1 and my_obj.b = 2
my_obj.a = 20 and my_obj.b = 40
stud.name = Amit and stud.marks = 34
After using the Friend Class methods,
my_obj.a = 100 and my_obj.b = 200

Indeed, the changes made by the friend class methods are indeed reflected in my_obj.

实际上， friend类方法所做的更改确实反映在my_obj 。

结论 (Conclusion)

Hopefully, that gives you a good idea of what friend functions in C++ are.

希望您能对C ++中的朋友功能有一个很好的了解。

C++ has given us this flexibility to even modify private and protected variables using friend methods. Make sure you use them wisely!

C ++给了我们这种灵活性，甚至可以使用friend方法来修改私有和受保护的变量。 确保您明智地使用它们！

参考资料 (References)

• cplusplus.com article on Friend Functions关于朋友功能的cplusplus.com文章

翻译自: https://www.journaldev.com/36329/friend-function-in-c-plus-plus

c语言中++b与b++