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news.ir.com.au
Guest
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 Posted: Thu Mar 04, 2004 11:52 pm Post subject: How do I expose a static_cast? |
 |
|
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
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Rolf Magnus
Guest
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| Posted: Thu Mar 04, 2004 11:56 pm Post subject: Re: How do I expose a static_cast? |
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news.ir.com.au wrote:
| Quote: | 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?
|
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.
| Quote: | 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
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news.ir.com.au
Guest
|
| Posted: Fri Mar 05, 2004 12:22 am Post subject: Re: How do I expose a static_cast? |
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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 (AT) t-online (DOT) de> wrote
| Quote: | news.ir.com.au wrote:
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?
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.
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
|
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Andrey Tarasevich
Guest
|
| Posted: Fri Mar 05, 2004 12:43 am Post subject: Re: How do I expose a static_cast? |
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news.ir.com.au wrote:
| Quote: | ...
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.
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
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Clark Cox
Guest
|
| Posted: Fri Mar 05, 2004 1:07 am Post subject: Re: How do I expose a static_cast? |
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In article <nLP1c.225$IH5.11956 (AT) news (DOT) optus.net.au>,
"news.ir.com.au" <davidp_au (AT) yahoo (DOT) com> wrote:
| Quote: | 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?
|
No. The closest you could do would be to add some function to B, like
the following:
A *B::GetAPointer()
{
return this;
}
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Tom Plunket
Guest
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| Posted: Fri Mar 05, 2004 1:21 am Post subject: Re: How do I expose a static_cast? |
|
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| Quote: | 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.
|
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!
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Chris ( Val )
Guest
|
| Posted: Fri Mar 05, 2004 11:55 am Post subject: Re: How do I expose a static_cast? |
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"Andrey Tarasevich" <andreytarasevich (AT) hotmail (DOT) com> wrote
| Quote: | 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_cast<A*>( b );
| Quote: | 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
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Rob Williscroft
Guest
|
| Posted: Fri Mar 05, 2004 12:19 pm Post subject: Re: How do I expose a static_cast? |
|
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Chris ( Val ) wrote in
news:c29ps9$1re7h0$1 (AT) ID-110726 (DOT) news.uni-berlin.de:
| Quote: |
"Andrey Tarasevich" <andreytarasevich (AT) hotmail (DOT) com> wrote in message
news:104fj94jio1esd2 (AT) news (DOT) supernews.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_cast<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_cast<> 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/
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Chris ( Val )
Guest
|
| Posted: Fri Mar 05, 2004 12:29 pm Post subject: Re: How do I expose a static_cast? |
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|
"Rob Williscroft" <rtw (AT) freenet (DOT) REMOVE.co.uk> wrote
| Quote: | Chris ( Val ) wrote in
news:c29ps9$1re7h0$1 (AT) ID-110726 (DOT) news.uni-berlin.de:
"Andrey Tarasevich" <andreytarasevich (AT) hotmail (DOT) com> wrote in message
news:104fj94jio1esd2 (AT) news (DOT) supernews.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_cast<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_cast<> 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
|
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Andrey Tarasevich
Guest
|
| Posted: Fri Mar 05, 2004 9:43 pm Post subject: Re: How do I expose a static_cast? |
|
|
Chris ( Val ) wrote:
| Quote: | |
| 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_cast<A*>( b );
...
|
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_cast' is implementation-defined.
--
Best regards,
Andrey Tarasevich
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