[bug] typedefed base class not visible [VC8 RTM.050727-4200]

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Fernando Cacciola - 03 Nov 2005 19:04 GMT

Hi people,

I'm testing

Microsoft Visual Studio 2005
Version 8.0.50727.42 (RTM.050727-4200)

with our code base.

I'm getting lots of errors on code that compiled fine in 7.1.
Most of it boils down to the following new compiler bug:

struct A
{
int foo() const { return 1; }
};

struct B : A
{
typedef A base ;
};

struct D : B
{
} ;

int main(int argc, char* argv[])
{
B b ;
D d ;
int r0 = b.base::foo();
int r1 = d.base::foo(); // error C2039: 'A' : is not a member of 'D'

return 0;
}

As you can see, vc8 can't see the type B::base, which is A, from D.

Worst.. I get the same error even if D is defined as:

struct D : B
{
typedef B::base base ;
} ;

which explicitely redeclares B::base within D

I can of course do something like

struct D : B
{
typedef B Base ;
} ;

and then change the client code from

d.base::foo()

to

d.Base::base::foo()

which compiles OK, but I'd really hate to need to refactor ALL client code..
any ideas?

TIA

Signature

Fernando Cacciola
SciSoft
http://fcacciola.50webs.com/

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Tamas Demjen - 03 Nov 2005 19:27 GMT

> I'm getting lots of errors on code that compiled fine in 7.1.
> Most of it boils down to the following new compiler bug:
[quoted text clipped - 8 lines]
> typedef A base ;
> };

I think the new compiler is right, 7.1 is wrong, and the problem is in
your code. You're doing private inheritance. By writing

struct B : A

you are essencially making foo() invisible. You should try

struct B : public A

Tom

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Fernando Cacciola - 03 Nov 2005 19:36 GMT

> I think the new compiler is right, 7.1 is wrong, and the problem is in
> your code. You're doing private inheritance. By writing
>
> struct B : A
>
> you are essencially making foo() invisible.

No. Is public inheritance because B is struct, not a class.

Test the code in

http://www.comeaucomputing.com/tryitout/

to see it's well-formed.

Comeau C++ is-and has always been-the best reference for testing std
conformance.

Signature

Fernando Cacciola
SciSoft
http://fcacciola.50webs.com/

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Tamas Demjen - 03 Nov 2005 23:36 GMT

> No. Is public inheritance because B is struct, not a class.

Sorry, you're right, I was way off. I should've tried it before posting.

I discovered a workaround, I don't know if it works for you though:

struct D : B
{
typedef B base; // <-- add this line
} ;

Tom

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Doug Harrison [MVP] - 03 Nov 2005 20:56 GMT

>Hi people,
>
[quoted text clipped - 33 lines]
>
>As you can see, vc8 can't see the type B::base, which is A, from D.

I think it's probably a bug, and you can report it here:

http://lab.msdn.microsoft.com/productfeedback/Default.aspx

Comeau does like it, a lot more than I'm liking trying to figure this out
from reading the standard, anyway. :)

>Worst.. I get the same error even if D is defined as:
>
[quoted text clipped - 22 lines]
>which compiles OK, but I'd really hate to need to refactor ALL client code..
>any ideas?

That's not the nicest syntax, so I'd have to wonder why you're doing things
this way. What happens if someone says d.foo() instead of d.base::foo()?
Would it be appropriate for B to bring A::foo into scope with a
using-declaration, so that if would overload with B::foo? If not, what
about introducing a forwarding function A_foo into B?

Signature

Doug Harrison
Visual C++ MVP

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Fernando Cacciola - 03 Nov 2005 21:36 GMT

> I think it's probably a bug, and you can report it here:
>
> http://lab.msdn.microsoft.com/productfeedback/Default.aspx

OK. I'll do it.

>> d.Base::base::foo()
>>
[quoted text clipped - 7 lines]
> B::foo? If not, what about introducing a forwarding function A_foo
> into B?

Well, what I posted was a simplified version of the actual code.
Anyway, this comes from something that once ocurred to me to be a good
idea.. I'm not sure anymore it really is.. still, I used it a lot those days
and it just linger there in this rather old code base.

I needed a way to offer a form of "late friendship", that is, a way to give
class A access to parts of class B "reserved for friends", but without
having B know about A (mainly becasue of the limitations of real friendship
between template classes)
A solution is to hide the reserved part of B, but only partially:

Consider the follow as part of some framework (or class library):

struct A
{
int foo() ;
}

class B : A
{
typedef A door ;

private:

int foo() ;
} ;

Framework users don't ever deal with A, only with B (that's the type exposed
by and documented in the framework), so b.foo() is "private" to them.
The framework itself, OTOH, gains access to foo() through the special
syntax: b.door::foo();

Of course, nothing prevents a framework user to "discover the door" and use
it, so the whole technique is probably just nonsense; but I have to live
with it now ;)

As you can see, the whole idea is to require the user code to explicitely
walk through the base class to get access to the "reserved" interface.

NOTE: FWIW, I discover this techique in the CGAL library www.cgal.org, is
not my invention.

Best

Signature

Fernando Cacciola
SciSoft
http://fcacciola.50webs.com/

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[bug] typedefed base class not visible [VC8 RTM.050727-4200]

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