How do I expose a static_cast?

[news.ir.com.au]

Hi,

In the following code I get the compiler error:

error C2243: 'static_cast' : conversion from 'class B *' to 'class A *'
exists, but is inaccessible

I understand why I get this error and can usually get around the situation
by inserting a "using A::..." statement inside class B, however, due to this
being a static cast, what is the syntax?

--------

class A
{
};

class B : private A
{
};

int main(int argc, char* argv[])
{
B* b;
A* a;

a = static_cast<A*> (b);

return 0;
}

Thanks,
David

[Rolf Magnus]

Re: How do I expose a static_cast?

news.ir.com.au wrote:
[color=blue]
> Hi,
>
> In the following code I get the compiler error:
>
> error C2243: 'static_cast' : conversion from 'class B *' to 'class A
> *' exists, but is inaccessible
>
> I understand why I get this error and can usually get around the
> situation by inserting a "using A::..." statement inside class B,
> however, due to this being a static cast, what is the syntax?[/color]

The syntax for what?
The A part of B objects is private, i.e. inaccessible to the outside
world. Therefore, you cannot convert a B pointer into an A pointer. I'm
not sure if you meant that by "I understand why I get this error".
Anyway, I don't understand what your question now is.
[color=blue]
> class A
> {
> };
>
> class B : private A
> {
> };
>
> int main(int argc, char* argv[])
> {
> B* b;
> A* a;
>
> a = static_cast<A*> (b);
>
> return 0;
> }
>
> Thanks,
> David[/color]

[news.ir.com.au]

Re: How do I expose a static_cast?

Rolf,

Thanks for your reply.

What I meant is, I can understand that I get this error due to class A being
unaccessible to class B. However it is possible to explictely allow members
the be accessed with the "using" keyword.

Eg. If I was to add the method A::Test(), it is now possible to access
A::Test() inside class B by adding the following statement to class B:

using A::Test();

My question is, since the above can be done, is it possible to do the same
for static_cast?

I've tried the obvious of:

using A::static_cast;

and other variations such as:

using A::operator static_cast;
using A::operator static_cast<>;
..
..
..

Please let me know if you still do not understand.

Thanks,

David


"Rolf Magnus" <ramagnus@t-online.de> wrote in message
news:c28fn8$2b4 $02$1@news.t-online.com...[color=blue]
> news.ir.com.au wrote:
>[color=green]
> > Hi,
> >
> > In the following code I get the compiler error:
> >
> > error C2243: 'static_cast' : conversion from 'class B *' to 'class A
> > *' exists, but is inaccessible
> >
> > I understand why I get this error and can usually get around the
> > situation by inserting a "using A::..." statement inside class B,
> > however, due to this being a static cast, what is the syntax?[/color]
>
> The syntax for what?
> The A part of B objects is private, i.e. inaccessible to the outside
> world. Therefore, you cannot convert a B pointer into an A pointer. I'm
> not sure if you meant that by "I understand why I get this error".
> Anyway, I don't understand what your question now is.
>[color=green]
> > class A
> > {
> > };
> >
> > class B : private A
> > {
> > };
> >
> > int main(int argc, char* argv[])
> > {
> > B* b;
> > A* a;
> >
> > a = static_cast<A*> (b);
> >
> > return 0;
> > }
> >
> > Thanks,
> > David[/color]
>[/color]

[Andrey Tarasevich]

Re: How do I expose a static_cast?

news.ir.com.au wrote:[color=blue]
> ...
> Eg. If I was to add the method A::Test(), it is now possible to access
> A::Test() inside class B by adding the following statement to class B:
>
> using A::Test();
>
> My question is, since the above can be done, is it possible to do the same
> for static_cast?[/color]

Formal answer - no.

But if you really want 'B*' to be convertible to 'A*' maybe you should
just make 'A' _public_ base class of 'B'. Although, come to think about
it, public inheritance usually implies a lot more than a mere pointer
convertibility. ..

You can also use "brute force" to break through private inheritance by
using C-style cast

B* b;
A* a;
...
a = (A*) b;

This will work. But this is as ugly as it ever gets.

Maybe more elegant solution would be to introduce a member function into
class 'B', which will return a pointer to its 'A' base

class B : private A {
...
public:
A* get_A() { return this; }
};

Anyway, it would be useful if you could explain in more detail why
exactly you need this type of functionality.

(And would you please stop top-posting?)

--
Best regards,
Andrey Tarasevich

[Clark Cox]

Re: How do I expose a static_cast?

In article <nLP1c.225$IH5. 11956@news.optu s.net.au>,
"news.ir.com.au " <davidp_au@yaho o.com> wrote:
[color=blue]
> Rolf,
>
> Thanks for your reply.
>
> What I meant is, I can understand that I get this error due to class A being
> unaccessible to class B. However it is possible to explictely allow members
> the be accessed with the "using" keyword.
>
> Eg. If I was to add the method A::Test(), it is now possible to access
> A::Test() inside class B by adding the following statement to class B:
>
> using A::Test();
>
> My question is, since the above can be done, is it possible to do the same
> for static_cast?[/color]

No. The closest you could do would be to add some function to B, like
the following:


A *B::GetAPointer ()
{
return this;
}

[Tom Plunket]

Re: How do I expose a static_cast?

> What I meant is, I can understand that I get this error due to[color=blue]
> class A being unaccessible to class B. However it is possible to
> explictely allow members the be accessed with the "using"
> keyword.[/color]

static_cast is not a member of A or B though. There's nothing to
use.

Might this work?

class B : private A
{
public:
operator A&() { return *this; }
};


?

I'm not really sure why you'd want to do this though.
Alternatively you could make A publically inherited (since you
seem to want it to be anyway), or you could make A a sub-object
of B (that is, B has-a A) and then offer a GetA() method... This
seems like an odd request without knowing more context.

-tom!

[Chris \( Val \)]

Re: How do I expose a static_cast?


"Andrey Tarasevich" <andreytarasevi ch@hotmail.com> wrote in message
news:104fj94jio 1esd2@news.supe rnews.com...
| news.ir.com.au wrote:
| > ...
| > Eg. If I was to add the method A::Test(), it is now possible to access
| > A::Test() inside class B by adding the following statement to class B:
| >
| > using A::Test();
| >
| > My question is, since the above can be done, is it possible to do the same
| > for static_cast?
|
| Formal answer - no.
|
| But if you really want 'B*' to be convertible to 'A*' maybe you should
| just make 'A' _public_ base class of 'B'. Although, come to think about
| it, public inheritance usually implies a lot more than a mere pointer
| convertibility. ..
|
| You can also use "brute force" to break through private inheritance by
| using C-style cast
|
| B* b;
| A* a;
| ...
| a = (A*) b;
|
| This will work. But this is as ugly as it ever gets.

This might be even more ugly, but at least we can spot it :-):
a = reinterpret_cas t<A*>( b );

| Maybe more elegant solution would be to introduce a member function into
| class 'B', which will return a pointer to its 'A' base
|
| class B : private A {
| ...
| public:
| A* get_A() { return this; }
| };
|
| Anyway, it would be useful if you could explain in more detail why
| exactly you need this type of functionality.

I prefer this, given the two options.

Cheers.
Chris Val

[Rob Williscroft]

Re: How do I expose a static_cast?

Chris ( Val ) wrote in
news:c29ps9$1re 7h0$1@ID-110726.news.uni-berlin.de:
[color=blue]
>
> "Andrey Tarasevich" <andreytarasevi ch@hotmail.com> wrote in message
> news:104fj94jio 1esd2@news.supe rnews.com...
>| news.ir.com.au wrote:
>| > ...
>| > Eg. If I was to add the method A::Test(), it is now possible to
>| > access A::Test() inside class B by adding the following statement
>| > to class B:
>| >
>| > using A::Test();
>| >
>| > My question is, since the above can be done, is it possible to do
>| > the same for static_cast?
>|
>| Formal answer - no.
>|
>| But if you really want 'B*' to be convertible to 'A*' maybe you
>| should just make 'A' _public_ base class of 'B'. Although, come to
>| think about it, public inheritance usually implies a lot more than a
>| mere pointer convertibility. ..
>|
>| You can also use "brute force" to break through private inheritance
>| by using C-style cast
>|
>| B* b;
>| A* a;
>| ...
>| a = (A*) b;
>|
>| This will work. But this is as ugly as it ever gets.
>
> This might be even more ugly, but at least we can spot it :-):
> a = reinterpret_cas t<A*>( b );
>[/color]

This is one of the things that C-style casts do that can't be
done by the other cast's. You're reinterpret_cas t<> will only
work if the A subobject in B is at offset 0. The C-style cast
will work regardless.

Rob.
--

http://www.victim-prime.dsl.pipex.com/

[Chris \( Val \)]

Re: How do I expose a static_cast?


"Rob Williscroft" <rtw@freenet.RE MOVE.co.uk> wrote in message
news:Xns94A37D5 3157B3ukcoREMOV Efreenetrtw@195 .129.110.204...
| Chris ( Val ) wrote in
| news:c29ps9$1re 7h0$1@ID-110726.news.uni-berlin.de:
|
| >
| > "Andrey Tarasevich" <andreytarasevi ch@hotmail.com> wrote in message
| > news:104fj94jio 1esd2@news.supe rnews.com...
| >| news.ir.com.au wrote:
| >| > ...
| >| > Eg. If I was to add the method A::Test(), it is now possible to
| >| > access A::Test() inside class B by adding the following statement
| >| > to class B:
| >| >
| >| > using A::Test();
| >| >
| >| > My question is, since the above can be done, is it possible to do
| >| > the same for static_cast?
| >|
| >| Formal answer - no.
| >|
| >| But if you really want 'B*' to be convertible to 'A*' maybe you
| >| should just make 'A' _public_ base class of 'B'. Although, come to
| >| think about it, public inheritance usually implies a lot more than a
| >| mere pointer convertibility. ..
| >|
| >| You can also use "brute force" to break through private inheritance
| >| by using C-style cast
| >|
| >| B* b;
| >| A* a;
| >| ...
| >| a = (A*) b;
| >|
| >| This will work. But this is as ugly as it ever gets.
| >
| > This might be even more ugly, but at least we can spot it :-):
| > a = reinterpret_cas t<A*>( b );
| >
|
| This is one of the things that C-style casts do that can't be
| done by the other cast's. You're reinterpret_cas t<> will only
| work if the A subobject in B is at offset 0. The C-style cast
| will work regardless.

Yes, you're right, in that the c-style cast is much more
powerful in this regard.

Thanks.
Chris Val

[Andrey Tarasevich]

Re: How do I expose a static_cast?

Chris ( Val ) wrote:[color=blue]
> |
> | You can also use "brute force" to break through private inheritance by
> | using C-style cast
> |
> | B* b;
> | A* a;
> | ...
> | a = (A*) b;
> |
> | This will work. But this is as ugly as it ever gets.
>
> This might be even more ugly, but at least we can spot it :-):
> a = reinterpret_cas t<A*>( b );
> ...[/color]

Yes, but this is not the same. The behavior of C-style cast in this case
is unambiguously defined by the language specification. And it performs
a correct derived-to-base conversion (ignoring any limitations imposed
by private inheritance).

On the contrary, the result of 'reinterpret_ca st' is implementation-defined.

--
Best regards,
Andrey Tarasevich