While testing an object streaming class I've been working on, I came up with this
monster:

Class A { public: int i ; } ;
Class B : public A { public: int i ; } ;
Class C : public A, public B { public: int i ; } ;
Class D : public A, public B, public C { public: int i ; } ;

This compiles okay ( with a 'repeated base' warning and a couple of derogatory
comments from the compiler ) but when I try to access D::A::i in a method ( not
shown ) of Class D, I get an ambiguous base error. I am also having problems with
my compiler ( Borland C++Builder 3 ) not accepting the use of multiple scope
resolution operators such as B::A. It doesn't recognize 'A' as a 'namespace or
class'.

It seems to me that I should be able to access every 'i' in this hierarchy from 'D'
by using the appropriate scope resolution operators. However, I can't seem to do
this with C++Builder which is supposed to be ANSI/ISO compliant. Am I wrong in
thinking that I can access these base classes in this way? As a C++ devotee, I
refuse to believe that A's 'i' in the immediate base of D is lost forever due to
ambiguity. Also, I know I've used multiple scope resolution operators in the past (
with Borland C++5.02 ) to access base class members and this method is supported in
Bjarne Stroustrup's C++PL3 ( p. 394, sect 15.2.3 ). It doesn't seem to be working
with C++Builder though.

 Thanks,

  Dave

P.S.
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Dave J.G. wrote:

> While testing an object streaming class I've been working on, I came up with this
> monster:
>
> Class A { public: int i ; } ;
> Class B : public A { public: int i ; } ;
> Class C : public A, public B { public: int i ; } ;
> Class D : public A, public B, public C { public: int i ; } ;

You don't need to go that far to have problems. If you stop with C, you
already have an ambiguity.

> This compiles okay ( with a 'repeated base' warning and a couple of derogatory
> comments from the compiler ) but when I try to access D::A::i in a method ( not
> shown ) of Class D, I get an ambiguous base error. I am also having problems with
> my compiler ( Borland C++Builder 3 ) not accepting the use of multiple scope
> resolution operators such as B::A. It doesn't recognize 'A' as a 'namespace or
> class'.
>
> It seems to me that I should be able to access every 'i' in this hierarchy from 'D'
> by using the appropriate scope resolution operators.

No, you can't access some of them. That's because the base class
C::A is ambiguous. There is no way to refer to it that wouldn't
also refer to C::B::A.

--

Valentin Bonnard                mailto:bonnardv@pratique.fr
info about C++/a propos du C++: http://pages.pratique.fr/~bonnardv/
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Dave J.G. <see@signa.ture> wrote:
: While testing an object streaming class I've been working on, I came up with this
: monster:

: Class A { public: int i ; } ;
: Class B : public A { public: int i ; } ;
: Class C : public A, public B { public: int i ; } ;
: Class D : public A, public B, public C { public: int i ; } ;

: This compiles okay ( with a 'repeated base' warning and a couple of
derogatory : comments from the compiler ) but when I try to access
D::A::i in a method ( not : shown ) of Class D, I get an ambiguous
base error.

No wonder. Class has 8 members i, four of which which are
from class A:
A = (Ai)
B = ((Ai) Bi)
C = ((Ai) ((Ai) Bi) Ci)
D = ((Ai) ((Ai) Bi) ((Ai) ((Ai) Bi) Ci))
So the compiler cannot figure out which of the i-s from class A you want.
Try virtual base clases.

--
Jeroen Nijhof      J.H.B.Nijhof@aston.ac.uk
Accordion Links    http://www-th.phys.rug.nl/~nijhojhb/accordions.html
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Dave J G <see@signa.ture> writes:

> Class A { public: int i ; } ;
> Class B : public A { public: int i ; } ;
> Class C : public A, public B { public: int i ; } ;
> Class D : public A, public B, public C { public: int i ; } ;

> This compiles okay [...] but when I try to access D::A::i [...] I
> get an ambiguous base error.

The Nov'97 DWP states, in [class.mi]/3:

  [...] a class can be an indirect base class
  more than once and can be a direct and an indirect base class.   There
  are limited things that can be done with such a class.  The non-static
  data members and member functions of the direct base class  cannot  be
  referred  to  in  the scope of the derived class.

So, you can't disambiguate D::A, because this could be interpreted as
D::A, D::B::A, D::C::A, D::C::B::A.

--
Alexandre Oliva
mailto:oliva@dcc.unicamp.br mailto:aoliva@acm.org
http://www.dcc.unicamp.br/~oliva
Universidade Estadual de Campinas, SP, Brasil
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