iterator_traits<vehicle_part_iterator<T>> is missing reference

[StephanTLavavej]

Describe the bug

I work on MSVC's C++ Standard Library implementation, and we regularly build popular open-source projects, including yours, with our development toolsets in order to prevent regressions from breaking your code. This also allows us to provide advance notice of breaking changes - in this case, we've found an issue in your code that will cause compiler errors with an upcoming version of the Visual C++ toolset.

Specifically, we merged microsoft/STL#2447 implementing a new optimization in the minmax_element family of algorithms. This optimization involves inspecting iterator types to see if they're eligible for the optimization, and as part of that, we need to inspect iterator_traits<UserDefinedIterator>::reference at compile-time.

There's an iterator_traits<vehicle_part_iterator<T>> in your codebase that does not provide a reference typedef (or pointer):

Cataclysm-DDA/src/vpart_range.h

Lines 84 to 90 in bcd8b3b

	template<class T> struct iterator_traits<vehicle_part_iterator<T>> {
	using difference_type = size_t;
	using value_type = vpart_reference;
	// TODO: maybe change into random access iterator? This requires adding
	// more operators to the iterator, which may not be efficient.
	using iterator_category = std::forward_iterator_tag;
	};

When compiled with the upcoming VS 2022 17.4 Preview 1 toolset or later, this will emit an error of the form:

error C2794: 'reference': is not a member of any direct or indirect base class of 'std::iterator_traits<_FwdIt>'
        with
        [
            _FwdIt=vehicle_part_iterator<vehicle_part_range>
        ]

The fix should be straightforward (and backwards-compatible with all previous toolsets, and other compilers). According to the C++ Working Paper N4910 25.3.2.3 [iterator.traits]/1, the reference and pointer typedefs should match what operator*() and operator->() return, respectively:

Cataclysm-DDA/src/vpart_range.h

Lines 53 to 57 in bcd8b3b

	const vpart_reference &operator*() const {
	cata_assert( vp_ );
	return *vp_;
	}
	const vpart_reference *operator->() const {

Therefore, this should be added to iterator_traits<vehicle_part_iterator<T>>:

using reference = const vpart_reference &;
using pointer = const vpart_reference *;

---

Unrelated aside: I saw the comment "This requires adding more operators to the iterator, which may not be efficient." It is certainly tedious to provide the large number of operators required for random-access iterators (including the obscure n + iter). However, this will have a minimal impact on compile-time throughput (build speed), and will typically improve run-time performance. This is because many STL algorithms and data structures will detect random-access iterators automatically, and perform constant-time operations. If they only have forward-iterator operations to work with, they have to fall back to linear-time operations. A simple example is calculating the distance between two iterators (which is used by vector range construction and many other places). std::distance() will detect random-access iterators and call last - first which performs a single subtraction, but for forward iterators it must repeatedly increment a copy of first until it's equal to last.

Steps to reproduce

See bug description (requires VS 2022 17.4 Preview 1 or later to reproduce, which has not yet shipped).

Expected behavior

Cataclysm-DDA should compile without errors.

Screenshots

No response

Versions and configuration

Compile-time issue, not running the game. Again, requires VS 2022 17.4 Preview 1 (or the latest build of https://github.com/microsoft/STL ).

Additional context

No response