Is this key-oriented access-protection pattern a known idiom?

Question

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-control than making Foo a friend of Bar and avoids more complicated proxying patterns.

Does anyone know whether this approach already has a name, i.e., is a known pattern?

Answer 1

Thanks to your other question it looks like this pattern is now known as the "passkey" pattern.

In C++11, it gets even cleaner, because instead of calling

b.protectedMethod(SomeKey()); 

you can just call:

b.protectedMethod({});