ICU-23004: experiment with UTF-8/16 C++ iterators

[markusicu]

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Notice: the branch changed across the force-push!

• icu4c/source/common/unicode/utf16cppiter.h is different
• icu4c/source/common/unicode/uversion.h is no longer changed in the branch
• icu4c/source/test/intltest/intltest.vcxproj is different
• icu4c/source/test/intltest/intltest.vcxproj.filters is different
• icu4c/source/test/intltest/itutil.cpp is different
• icu4c/source/test/intltest/Makefile.in is different
• icu4c/source/test/intltest/utfcppitertest.cpp is different

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[markusicu]

Hi @eggrobin I think this is worth taking another look. I rebased on recent main, made changes from our discussions, and I think this looks roughly like a reasonable validating, forward-only (so far) Unicode 16-bit-string code point iterator. It no longer tries to be clever: It no longer reads & validates the code point while iterating, and no longer stores the result in the iterator.

Plenty of TODOs and questions left, but I would appreciate feedback on the shape of what I've got so far.

[markusicu]

I experimented with godbolt, and found that the compiler does its best fusing operator*() and operator++() when they both call the same implementation function. This makes operator++() look horribly inefficient, but the machine code for a regular range-based for loop from the optimizing clang 19 looks very concise.

I then also made the iterator bidirectional and added a special version for efficient rbegin() & rend() using the same principles.

The bidirectional iterator also exposes explicit but non-colloquial functions.

[eggrobin]

(As noted over email, I’ll take a look on Monday when I’m back from the holidays.)