Re: conversion to non-scalar type error in inheritance hierarchy

From:
kmw <wuerzner@gmail.com>
Newsgroups:
comp.lang.c++
Date:
Thu, 29 Oct 2009 08:42:49 -0700 (PDT)
Message-ID:
<8ca19f66-d358-486e-8b49-a28666837266@s31g2000yqs.googlegroups.com>
Well, yes. Your code gives me the same error. I see the conversion
problem now. Thanks for clarifying. Of course I don't want explicit
conversion from the base class into the inherited classes. But how can
I achieve such kind of "parallel" inheritance? That is, 'C' is
accepted as 'B' in 'A1'. I know I could do it with reference returns
but that's not in accordance to the stl-iterator approach.

Cheers,
Kay

On Oct 29, 4:29 pm, Victor Bazarov <v.Abaza...@comAcast.net> wrote:

kmw wrote:

Hello,

I am working on container class with different implementations and
want to add stl-style iterators. Now, I am confronted with a
"conversion to non-scalar type" error which I do not understand. Any
comments are appreciated. Many thanks in advance!

The interface for the different implementations is defined in the base
class A. A1 is one implementation. B is the iterator base class and C
its implementation in A1. I really do not see a problem since C
inherits from B but the compiler says:

test.cpp: In function =91int main()':
test.cpp:34: error: conversion from =91A<int>::B' to non-scalar type
=91A1<int>::C' requested


You do not see the problem? How about here:

    class B {};
    class C : public B {};

    B foo();

    int main()
    {
       C c = foo();
    }

Do you get the same error?

Best regards,
Kay

Code:
template <class X>
class A
{
   public:
           class B
           {
                   public:
                           virtual ~B () {}
                           virtual void met=

hod () {}

           };
           virtual ~A () {}
           virtual B begin ()=0;
};
template <class X>
class A1 : public A<X>
{
   public:
           class C : public A<X>::B
           {
                   public:
                           ~C () {}
                           void method ( ) =

{}

           };
           ~A1 () {}
           typename A<X>::B begin ( )
           {
                   C test;
                   return test;
           }
};
int main ( )
{
   A1<int> test;
   A1<int>::C test_it = test.begin ();

   return 0;
}


There is no conversion from the base class to the derived class. You
have to define this conversion if you want it to exist.

V
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