Re: problem: private inheritance. How to explain it in terms of C++ rules?

From:
"Alf P. Steinbach" <alfps@start.no>
Newsgroups:
comp.lang.c++
Date:
Tue, 17 Feb 2009 09:55:20 +0100
Message-ID:
<gndu1r$m38$1@news.motzarella.org>
* Marcel M?ller:

Michal wrote:

hallo group members, I wonder if the example below is consistent with a
C++ rules:


It is.

class A {
};

class B: private A {
};

int main(int argc, char *argv[])
{
  B *b = new B;
  A *a = (A*)b;
  A *c = b; /* produces follwowing error:
               p5.cpp: In function "int main(int, char**)":
               p5.cpp:15: error: "A" is an inaccessible base of "B"

               What is the reason for it???


Neither the implicit nor the explicit conversion to a private base class
is allowed outside the class. Otherwise, the base would no longer be
private.
Your cast to (A*) is a reinterpret cast


No, it's a very special case, ?5.4/7, that a C style cast can convert to a
pointer or reference to an inaccessible but unambigious base class as if
static_cast was used for an accessible base (it can also do more such stuff).

So since the cast here would have been a static_cast if the base was accessible
(because static_cast comes before reinterpret_cast in the list of possibilities,
it goes const_cast, static_cast, static_cast+const_cast, reinterpret_cast,
reinterpret_cast+const_cast), it is therefore a static_cast.

And technically utterly safe, although -- as illustrated here -- not very
safe against being misunderstood or misidentified, even by the original author.

and will badly fail in
conjunction with multiple inheritance, at least.


Depends.

Cheers & hth.,

- Alf

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