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

From:

Victor Bazarov <v.Abazarov@comAcast.net>

Newsgroups:

comp.lang.c++

Date:

Thu, 29 Oct 2009 11:29:30 -0400

Message-ID:

<hcccca$j59$1@news.datemas.de>

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 ?int main()?:
test.cpp:34: error: conversion from ?A<int>::B? to non-scalar type
?A1<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 method () {}
        };
        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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