在构建容器并希望为该类实现迭代器时，Friend可以派上用场。

在做TDD的时候，我经常使用c++中的'friend'关键字。

朋友能知道我的一切吗?

更新:我从Bjarne Stroustrup网站上找到了这个关于“朋友”关键字的有价值的答案。

“好友”是一种授予访问权限的显式机制，就像会员资格一样。

您使用私有/受保护/公共权限控制成员和函数的访问权限? 所以假设这3个层面中的每一个都很清楚，那么很明显我们遗漏了一些东西……

例如，将成员/函数声明为protected是非常通用的。你的意思是，这个功能对每个人来说都是遥不可及的(当然，继承的孩子除外)。但是异常呢?每个安全系统都让你有某种类型的“白名单”，对吗?

所以，“朋友”让你拥有坚如磐石的对象隔离的灵活性，但也允许为你认为合理的事情创造一个“漏洞”。

我想人们之所以说不需要它，是因为总有一种设计可以不需要它。我认为这类似于讨论全局变量:你永远不应该使用它们，总有办法不用它们……但在现实中，你会看到在某些情况下，这是(几乎)最优雅的方式……我认为朋友之间也是如此。

它实际上没有任何好处，除了让你在不使用设置函数的情况下访问成员变量

但这并不是正确的看法。 其理念是控制WHO可以访问什么，是否有设置功能与此关系不大。

In C++ "friend" keyword is useful in Operator overloading and Making Bridge. 1.) Friend keyword in operator overloading :Example for operator overloading is: Let say we have a class "Point" that has two float variable"x"(for x-coordinate) and "y"(for y-coordinate). Now we have to overload "<<"(extraction operator) such that if we call "cout << pointobj" then it will print x and y coordinate (where pointobj is an object of class Point). To do this we have two option: 1.Overload "operator <<()" function in "ostream" class. 2.Overload "operator<<()" function in "Point" class. Now First option is not good because if we need to overload again this operator for some different class then we have to again make change in "ostream" class. That's why second is best option. Now compiler can call "operator <<()" function:

   1.Using ostream object cout.As: cout.operator<<(Pointobj) (form ostream class).
   2.Call without an object.As: operator<<(cout, Pointobj) (from Point class).

Beacause we have implemented overloading in Point class. So to call this function without an object we have to add"friend" keyword because we can call a friend function without an object. Now function declaration will be As: "friend ostream &operator<<(ostream &cout, Point &pointobj);" 2.) Friend keyword in making bridge : Suppose we have to make a function in which we have to access private member of two or more classes ( generally termed as "bridge" ) . How to do this: To access private member of a class it should be member of that class. Now to access private member of other class every class should declare that function as a friend function. For example : Suppose there are two class A and B. A function "funcBridge()" want to access private member of both classes. Then both class should declare "funcBridge()" as: friend return_type funcBridge(A &a_obj, B & b_obj);I think this would help to understand friend keyword.

我发现了一个使用好友访问的方便地方:私有函数的Unittest。

c++的创建者说这并没有破坏任何封装原则，我将引用他的话:

“朋友”是否违反封装? 不。但事实并非如此。“好友”是一种授予访问权限的显式机制，就像会员资格一样。你不能(在符合标准的程序中)在不修改类源的情况下授予自己对类的访问权。

很明显…