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

From:

"Alf P. Steinbach" <alfps@start.no>

Newsgroups:

comp.lang.c++

Date:

Thu, 29 Oct 2009 16:43:28 +0100

Message-ID:

<hccd72$mcp$1@news.eternal-september.org>

* 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 functionality.

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

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;
        }

Not related to the compilation error, but a logical error: this begin() function
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 calls 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 constructor that
takes a B object. A B object cannot be implicitly converted down to the more
specialized C object (the inheritance goes the wrong way).

return 0;
}

Cheers & hth.,

- Alf