friend

Matthieu M. brought up a pattern for access-protection in this answer that i'd seen before, but never conciously considered a pattern: class SomeKey { friend class Foo; SomeKey() {} // possibly make it non-copyable too }; class Bar { public: void protectedMethod(SomeKey); }; Here only a friend of the key class has access to protectedMethod() : class Foo { void do_stuff(Bar& b) { b.protectedMethod(SomeKey()); // fine, Foo is friend of SomeKey } }; class Baz { void do_stuff(Bar& b) { b.protectedMethod(SomeKey()); // error, SomeKey::SomeKey() is private } }; It allows more fine-granular access

The background of this question is based on a practical sample where I wanted to remove a «friend» dependency from a pair of classes that are used to manage read/write locked access to a shared resource. Here's an abstraction of the original structural design for that scenario: Marked in red, there's this ugly «friend» dependency I want to remove from the design. In short, why do I have this thing there: ClassAProvider shares a reference to a ClassA over a number of concurrently accessing Client instances Client instances should access ClassA solely through the ClassAAccessor helper class that

After searching aroung SO, one question taught me that the lexical scope of an inline friend function is the class it's defined in, meaning it can access e.g. the typedef s in the class without qualifying them. But then I wondered what is the actual scope of such a function? GCC at least rejects all my attempts to call it. Can a function such as in the example ever be called through means other than ADL, which is not possible here thanks to no arguments? Standard quotations are appreciated, as I currently can't access my copy of it. The following code namespace foo{ struct bar{ friend void baz

类的友元函数是定义在类外部，但有权访问类的所有私有成员和保护成员。尽管友元函数的原型有在类的定义中出现过，但友元函数并不是成员函数。 友元可以是一个函数，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。在这种情况下，整个类及其所有成员都是友元。如果要声明函数为一个类的友元，需要在类定义中该函数原型前使用关键字friend，如下所示： class Box { double width; public: double length; friend void printWidth(Box box); void setWidth(double wid); }; 声明类ClassTwo的所有成员函数作为类的ClassOne的友元，需要在类ClassOne的定义中放置如下声明： friend class ClassTwo; /*** friend.cpp ***/ #include<iostream> using namespace std; class Box { double width; public: friend void printWidth(Box box); void setWidth(double wid); }; void Box::setWidth(double wid) { width = wid; } void printWidth(Box box) {