[Boost-users] Re: Converting from const to non-const withiterator_adaptor

Your patch solves my immediate problem, thanks. Fortunately, both the classes that I need iterators for only require bidirectional semantics, so the random access issue does not arise, but thanks for pointing it out. Keith MacDonald "David Abrahams" wrote in message news:uoetfa7th.fsf_-_@boost-consulting.com...

"Keith MacDonald" writes:

...

I've done that, and attach the new source. However, I hereby relinquish all rights over baz2.cpp, so feel free to do whatever you like with it.

When I attempted to integrate the iterators with my actual code, I hit a problem that I don't know how to resolve with iterator_facade. To illustrate it, I've modified the example code and attach it as baz3.cpp. Now, Node is an abstract base class, and DerivedNode is instantiated. Compiling it results in the following error:

\CVS\boost\boost\iterator\iterator_facade.hpp(389) : error C2259: 'Node' : cannot instantiate abstract class due to following members: 'int Node::getInfo(void) const' : pure virtual function was not defined

Can your technique be used in such contexts?

Very interesting case. The problem is that, when the iterator is not writable, operator[] returns an object of the iterator's value_type. In this case the value_type is abstract, so it can't be returned.

This raises several problems that I can see.

1. Obviously, the library doesn't work for you. You can get around that by applying the enclosed patch to iterator_facade.hpp. We'll figure out the right long-term fix soon; this one might be it.

2. We have to re-think the readable iterator requirements if we want iterators over abstract types to be legal. My feeling is that the requirement should be that *a is convertible to value_type const&.

3. Your iterator isn't a legal random access iterator, because the random access iterator requirements say that a[n] must be convertible to value_type. You can't convert anything to value_type in this case, since it can't be constructed. I think this is a defect in the current standard, and will submit a DR.

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-- Dave Abrahams Boost Consulting www.boost-consulting.com

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