> Even:
> In fact the nature of the object which is returned depends on the iterator, 
> and as documented in the API, for the sake of implementation simplicity, 
> those iterators do not necessarily respect the whole semantics of iterators.

I don't think that is necessary, at least in the case of Layers you can conform 
to the InputIterator concept without much extra work

[snip]

> So basically you should only use them in range based loops, where you don't 
> really manipulate the iterator and cannot do stuff like the above.

That is also the limitation of InputIterator. 

> Improvements (if doable and not adding (too much) overhead to the underlying 
> objects) are welcome of course.

I would recommend that Layers::iterator is updated to conform to the C++ 
iterator concepts (to the very least to the InputIterator concept). 
It still requires the following: 
- five typedefs (value_type, reference, pointer, iterator_category, 
difference_type) 
- three constructors (default, copy, move)
- two assignment operators (copy, move)
- post-increment operator (operator++(int))

See also http://en.cppreference.com/w/cpp/concept/InputIterator.

*************************************************
Regarding the for(auto&& layer : layers) discussion: 


The return type of Layers::Iterator::operator*() is OGRLayer*. 

Therefore, the following:

    for(auto& layer : layers)

is incorrect and should not compile, because it binds a temporary to a 
reference.
 
Furthermore, the following: 
  
    for(const auto& layer: layers)

is only correct because an assignment of a temporary to a const reference 
extends its lifetime, and it is therefore equivalent to

   for(const auto layer : layers) 

Considering the potential confusion, you can just avoid the issue completely 
and write:

   for(const OGRLayer* layer : layers)


Kind regards, Alex
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