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Make collection iterators fulfill LegacyInputIterator and input_iterator concept #626

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@m-fila m-fila commented Jun 11, 2024

BEGINRELEASENOTES

  • Added missing operations and type aliases so collection iterators fulfill LegacyInputIterator and std::input_iterator concept

ENDRELEASENOTES

Fixes: #150, fixes #272
Conflicts: #273

Implement missing features so CollectionIterators and MutableCollectionIterators fulfill the requirements imposed on the C++ iterators (#598). It's expected than not all requirements can be fulfilled as some are in direct conflict with podio design:

  • LegacyForwardIterator: dereference should return a reference, but in podio iterators are 'proxy iterators' returning user layer objects as a proxy for reference
  • LegacyForwardItertor: dereferencing two iterators that are equal should give two references bound to the same object, but in podio the returned 'proxies' are different objects
  • std::forward_iterator: pointers and references obtained from an iterator should be valid as range the range is valid, but in podio the iterators behave like 'stashing iterators' -> pointers point to iterator member variable limiting their validity to the lifetime of the iterator.

As a consequence the iterators can the podio iterators can be at most LegacyInputIterator and std::input_iterator. This PR implements missing features to achieve this.

List of changes:

Named requirements:

  • LegacyIterator:
    • copy constructor and assignment
    • type aliases
  • LegacyInputIterator:
    • postfix increment
  • LegacForwardIterator:
    • value-initialization
    • dereference return reference not proxy
    • dereferencing equal iterators returns references to the same object
  • OutputIterator:
    • assignment should modify collection
  • iterator_category is std::input_iterator_tag
  • check selected algorithms operating on LegacyInputIterators

Concepts:

  • semantics checks for concepts
  • std::input_or_output_iterator
    • same as LegacyIterator but copyability not required
  • std::input_iterator
    • std::indirectly_readable and std::readable (can dereference both iterator and const iterator)
  • std::forward_iterator
    • syntax: constrains (except iterator_concept to be derived fromstd::forward_iterator_tag)
    • semantic: pointers obtained from iterator should remain valid as long as range is valid (no-stashing)
  • iterator_concept is std::input_iterator_tag
  • check selected range algorithms constrained to std::input_iterator

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@tmadlener tmadlener left a comment

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I know this is not fully done yet, but I think it would be nice to also document which (classes of) algorithms will be enabled by this (if any).

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@Zehvogel
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can you also take a look at podio::RelationRange? :)

@m-fila
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m-fila commented Sep 25, 2024

Thanks for pointing out, I'll check it out later

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