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

From:
kmw <wuerzner@gmail.com>
Newsgroups:
comp.lang.c++
Date:
Thu, 29 Oct 2009 08:47:58 -0700 (PDT)
Message-ID:
<fd82ba29-2211-4069-925e-c333553d2983@v30g2000yqm.googlegroups.com>

It may be easiest to write a templated wrapper that uses some underlying
conventional named methods interface to provide STL iterator functionalit=

y.

Thanks for the hint. Could you give a small code snippet to ease my
understanding? That would be great!

I see the problem with the inheritance 'in the wrong way' now. Also,
thanks for this.

Cheers,
Kay

On Oct 29, 4:43 pm, "Alf P. Steinbach" <al...@start.no> wrote:

* kmw:

Hello,

I am working on container class with different implementations and
want to add stl-style iterators.


It may be easiest to write a templated wrapper that uses some underlying
conventional named methods interface to provide STL iterator functionalit=

y.

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

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;
           }


Not related to the compilation error, but a logical error: this begin() f=

unction

always produces an A<X>::B object. It doesn't matter that that object is
initialized from a C object. When it's returned it's a B object, and call=

s to

method will invoke B::method.

};
int main ( )
{
   A1<int> test;
   A1<int>::C test_it = test.begin ();


test.begin() produces a B object. Class C does not provide any constructo=

r that

takes a B object. A B object cannot be implicitly converted down to the m=

ore

specialized C object (the inheritance goes the wrong way).

   return 0;
}


Cheers & hth.,

- Alf

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