Re: Friends and Visual C++ Express 2005

"t" <tmt513@Yahoo.com> wrote in message
news:1190451610.080366.244830@r29g2000hsg.googlegroups.com...
: On Sep 22, 1:30 am, Barry <dhb2...@gmail.com> wrote:
:
: > Well, use the former example rather than this one
: > void g(A a, A2 a2)
: > {
: > a2.x; // compiles, but shouldn't ?
: >
: > }
: >
: > here a2.x is can be interpreted this way:
: >
: > int A::*pm = &A::x; // (1)
: > a2.*x; // (2)
: >
: > then the access control actually takes place on (1),
: > since g is a friend of A, then taking a private pointer to member of
A
: > is legal.
: >
: > So, to conclude, when we do inheritance, the members of the base
class
: > are not members of the derived class. The derived class only
*inherits*
: > them.
: >
: > --
: > Thanks
: > Barry
:
: Is there a good reason for friendship to be like this? That friends
: of A have access to the A parts of subclasses of A. This doesn't seem
: "natural" or "right" to me w/ the way my mental model of C++ has been
: building up. Maybe it's because I don't understand the underlying
: mechanics of C++. It seems like there are some linguistic rules in
: play that don't fit well w/ my mental model (I tend to think
: geometrically rather than algebraically).
:
: Or is this one of those rules that have no good basis but I simply
: have to remember?

Well, if a class D publicly derives from A, instances of D *want* to
behave as if they were of type A. Otherwise, the subclass can
use private inheritance, or containement. If an instance of D
*is* an instance of A, friends as well may treat it as an A.
This seems fully logical and intuitive to me.

However, we do not want friendship to be "viral", which is what
the restrictions are about: a subclass shall not benefit from
or suffer from the consequences of the friedships of its parents.

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