TITLE: controlling access without outright friendship

(Newsgroups: comp.std.c++, 21 Mar 99)

[ This is a related technique to that described in the
  tip priv_interface. -adc ]


BONNARD: Valentin Bonnard <Bonnard.V@wanadoo.fr>

[snip]

Sometimes I want to allow a class (and only this class) to call 
some functions (for example to give it access to a registery).

One way to do that is to only give this class ctor a pointer 
or reference to the registery, but this isn't allways possible 
(when the register and remove functions are part of another 
class). In this case I use a private key:

class Granting
{
public:
    class Key
    {
        friend class Priviledge;
        Key () {}
        Key (const Key&) {}
    };

    void sensitive_function (Key);
};

class Priviledge
{
public:
    void do_stuff ()
    { g.sensitive_function (Granting::Key ()); }
private:
    Granting& g;
};

Priviledge isn't a friend of Granting, not even close; but 
it can do more than another class or free function.