Get into the habit of writing together! This is the 7th day of my participation in the "Nuggets Daily New Plan · April Update Challenge", click to view the details of the event .

1️⃣Foreword

Knowledge points in this chapter:

What is Friendship?

What are the characteristics of Friends?

Why the concept of friend Yuan?

What are the categories of friends?

How to create and use friends?

2️⃣ Answer

✨What is Friends?

A friend is an ordinary function or class defined outside the class

It needs to be declared inside the class body

In order to distinguish it from the member functions of this class, we will prefix the description with a keywordfriend

Simply put, a friend is a relationship

A class-function or class-class relationship

✨What are the characteristics of Friendship?

Friends can access private members of the class. (Similar: Friends are trustworthy and can disclose some of their privacy to friends.)

Friendships cannot be inherited (similar: your dad's friends are not necessarily your friends)

Friendships are unidirectional (similar: class A is a friend of class B, but B is not necessarily a friend of A)

Friendship relationships cannot be passed (similarly: that is, B is a friend of A, C is a friend of B, but C is not necessarily a friend of A.)

✨Why the concept of Friendship?

We know that classes have encapsulation, that is, data hiding.

Only the member functions of the class can access the private members of the class.

Other functions in the source file cannot access private members.

Then generally we will set up some public interfaces to open to the outside world.

That is, non-member functions can access public members of the class.

But sometimes, class access can be a bit rigid in some applications.

If the data members are defined as public, which destroys the hidden feature.

So there is the concept of friends .

Friends are granted access to private parts of the class from the outside (i.e. "backdoors" some functions outside the class)

This also increases the flexibility of the class's public interface.

But it should be noted that friends destroy the overall operability of the class, and also destroy the encapsulation of the class.

So we have to choose to use friends

✨What are the categories of Friends?

Classification:

friend class

friend function

友元关系可以是一个函数，该函数被称为友元函数。

友元也可以是一个类，该类被称为友元类。

✨如何创建和使用友元？

关键字：frend

我们需要在类体内进行说明，为了与该类的成员函数加以区别，在说明时前面加以关键字friend

下面着重看看友元类以及友元函数！

3️⃣友元类

首先我们定义一个类：

这个myBuilding类就相当于我的家，现在定义另一个类，是我的朋友类

并且我的朋友想访问我家的客厅和卧室

很显然，不可访问！

这是因为，在myBuilding这个类中，我的卧室是一个私有的，外部无法访问。

但是如果我们在myBuilding类中声明myFriend这个类是友元关系的，即friend class myFriend;

那么此时myFriend这个类就是一个友元类，它可以访问类似私有成员。

所以完整代码如下：

运行结果为：

朋友在访问：我的客厅
朋友在访问：我的卧室
复制代码

4️⃣友元函数

一旦我们将一个类声明为另一个类的友元，就意味着这个类的所有成员函数都是另一个类的友元函数，即都可以访问另一个类中私有成员或保护成员。

但有时候我们并不想让类的所有成员都能访问另一个类的私有成员，那么此时我们可以将类的部分成员函数声明为友元函数，这样就只有友元函数可以访问另一个类的私有成员， 而不是整个类都能访问另一个类的私有成员。

因此总结的说，友元函数就是能够访问另一个类的私有成员的一个函数。友元函数从语法上看，它与普通函数一样，即在定义上和调用上与普通函数一样。

For example, in myBuildingthis class, we declare that it visit1is a friend function, so we visit1can access myBuildingthe private membersm_BedRoom