ForwardIterator -> ReadableIteratorConcept & ForwardTraversalConcept

[wanghan02]

The concept of ForwardIterator is flawed because it mixed 2 sets of concepts (value access and traversal) into 1 package.

http://www.boost.org/doc/libs/1_65_1/libs/iterator/doc/new-iter-concepts.html

It requires value_type (const)& as return type when dereference is applied, which is not mandatory in spirit parsing. A return type which is convertible to value_type is good enough. ReadableIteratorConcept and ForwardTraversalConcept should be what we need for the iterator check.

For example, the iterator of the range returned by boost::adaptors::transform(std::string, func) is normally not a ForwardIterator. But it fulfills ReadableIteratorConcept and ForwardTraversalConcept and should be able to be parsed by spirit.

Test cases are added for both qi and x3.

There is also a ticket for this but the solution suggested is not correct.
https://svn.boost.org/trac10/ticket/12817

[wanghan02]

@Kojoley pull request #230 was not a correct solution (same as in the ticket). But I cannot close that pull request.

[Kojoley]

Seems perfectly logical to me.
@djowel I'm going to merge this in a day or two if you have nothing against.

[djowel]

Hmmm, seems my previous comment did not get through? Or is this a different PR?

[Kojoley]

Who knows.

[djowel]

OK... anyway, what I wrote was: I like the new iterator traits, but it is not really "new". It was written in 2003 and so far, I don't see it assimilated by the standards (C++11 and onwards). Correct me if I am wrong. So, unless there's real activity towards that, this falls into the category of "don't fix it if it ain't broken".

[Kojoley]

The PR is not about simply switching to "new" iterator concepts, but to make spirit less paranoid and allow the things like boost::transformed_range. Currently it produces the compilation error because of asserts and will work fine if you remove them.

[djowel]

I see that now. OK, we need this alright. It fixes something that's broken. I'll merge this one.

[djowel]

And thanks @wanghan02 and @Kojoley