Re: conversion to non-scalar type error in inheritance hierarchy
On Oct 29, 3:20 pm, kmw <wuerz...@gmail.com> wrote:
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
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;
}
You'll get the same problems without the iterators. The most
obvious problem (and why the compiler is complaining) is that
you're returning a A::B, and trying to use it to initialize an
A1::C; you can't normally use a base type to initialize a
derived type. The obvious solution for this is to use covariant
return types, except that covariant returns only work with
pointers and references, not with values.
Which brings us to the more general problem: Once A1::begin has
returned an A::B, all you have is an A::B---slicing has occured.
More generally, polymorphism only works through references or
pointers (and polymorphic classes should not generally be copied
or assigned). If you want the returned iterator to behave
polymorphically, you'll have to use some variant of the
letter/envelope or the handle idiom; for STL semantics, you'll
need deep copy, which will make copying iterators expensive.
--
James Kanze