Re: [Boost-users] iteration from pure abstract classes

On Dec 6, 2005, at 12:43, David Abrahams wrote:

Hugh Hoover writes:

...

On Dec 3, 2005, at 15:44, David Abrahams wrote:

...

Hugh Hoover writes:

...

I have a need to provide STL compliant iterators from an abstract class A, that is, without knowing the runtime type of the actual "collection" being iterated.. As near as I can tell, these things

<snip>

...

Have you looked at boost::indirect_iterator? Just make a container of pointers to the abstract base and iterate over that :)

Thanks for the suggestion, but that won't really work.

Why?

mainly performance. Imagine a tree structure where the nodes are of different concrete types based on a common abstract interface. So far, no problem... But, the nodes have subtree arity ranging from 0 to >10,000 - to walk the structure means creating a new vector for each node visited with a size equal to the node's arity, and initializing from another structure (which may not be a vector). If I use some kind of abstract iterator interface, the >iterator< gets allocated, but can walk the concrete structure directly. Clearly, there's a tradeoff here - and I'm betting that the abstract iterator is cheaper than the vector - I haven't proven that. Hugh Hoover Enumclaw Software