如何正确访问好友功能？(How to access friend functions correctly?)

 您对friend功能的理解是错误的 。 如果将函数friend声明为另一个类，则表示friend函数可以访问该类的私有成员，这并不意味着friend函数成为类成员 。 
 
 这里doStaff()是B friend ，而不是B的成员。 但是你在B的对象上使用它，因此编译器说它不是该类的成员。 使用friend给doStaff权限访问B的private成员。 
 
 您所关注的教程清楚地说明了这一点。 duplicate()方法是Rectangle的朋友，因此它可以访问private成员的width和height ，但duplicate()不用作Rectangle的类方法。 
 
 总之，你的问题是错误的。 您可以像任何免费 （非类方法）功能一样访问friend功能。 问题是friend函数如何访问它所属的类的成员，并且如上所述，它可以访问该类中的所有内容，这是拥有朋友函数的动机，即允许访问类的私有成员一个局外人。 
 
 类似地， friend类可以访问它所熟悉的类的所有内容。 
 
 因此，对于您的问题，要么从doStaff移除friend以使其成为B的成员。 但我想你的意图是使用friend方法，在这种情况下你可以做到以下几点： 
 
void doStuff(B b, int i)
{
  b.m_stuff = i; // it is friend function of B, so can access m_staff
}
 
 或者你可以成为A doStaff()成员然后你可以写 
 
class A{
  //other members
  void doStaff(B& b, int i){ b.m_staff=i;}

void A::testStuff() {

  B b {B()};
  doStuff(b,1); //although u can just write "b.m_staff = 1;" right here
}

Your understanding regarding friend function is wrong. If you declare a function friend to another class, it means the friend function can access private members of that class, it does not mean the friend function becomes a class member.
 
Here doStaff() is friend to B, not a member of B. But you are using it on an object of B, hence compiler saying that it is not member of that class. using friend gives doStaff permission to access B's private members in it.
 
The tutorial you are following says it clearly. duplicate() method is a friend to Rectangle, hence it can access the private members width and height, but duplicate() is not used as a class method of Rectangle.
 
In summary, your question is wrong. You can access a friend function just like any free (non-class method) function. The question is how friend functions accesses the member of a class to which it is friend, and as said, it can access everything in that class, which is the motivation of having friend functions, namely to give access of private members of a class to a outsider.
 
Similarly a friend class can access everything of the class to which it is friend. 
 
So for your problem, either you remove friend from doStaff to make it a member of B. But I guess your intention was to use friend method, in that case you can do the following:
 
void doStuff(B b, int i)
{
  b.m_stuff = i; // it is friend function of B, so can access m_staff
}
 
or you can make doStaff() member of A and then you can write
 
class A{
  //other members
  void doStaff(B& b, int i){ b.m_staff=i;}

void A::testStuff() {

  B b {B()};
  doStuff(b,1); //although u can just write "b.m_staff = 1;" right here
}