foreach vs. iterators

[Walter Bright]

Georg Wrede wrote:
> Walter Bright wrote:
>> On the other hand, C++ iterators aren't a general substitute for 
>> opApply. For example, it is not at all easy to turn a recursive data 
>> structure (i.e. binary tree) into a linearized iterator.
> 
> If I'm not missing the essence here, there are several kinds of 
> iterators, and not every data structure should try to provide all of 
> these. For example, an iterator into a tree structure might be 
> breadth-first or depth-first, but one wouldn't expect a linearized (if I 
> understand the term in this context at all) iterator to exist. 
> Conversely, an array is hardly thinkable without a linear iterator.

Given the structure:

class Node
{	Node left;
	Node right;
	...data...
}

how do you iterate that with an iterator? This is what I mean by 
'linearizing' it. The STL sidesteps the problem by avoiding providing 
such a container.