When Compilers Disagree About UTF‑8

In the summer 2006 I started an open-source C++ library for handling UTF-8 strings. I wanted it to be portable and to work well with STL but did not go crazy with optimizations at the time. Recently, as I started writing about UTF-8 decoding, I spent more time revisiting the internals of the library. The function I decided to optimize decodes a UTF-8 encoded code point: template <typename octet_iterator> utf_error validate_next(octet_iterator& it, octet_iterator end, utfchar32_t& code_point) { if (it == end) return NOT_ENOUGH_ROOM; // Save the original value of it so we can go back in case of failure // Of course, it does not make much sense with i.e. stream iterators octet_iterator original_it = it; utfchar32_t cp = 0; // Determine the sequence length based on the lead octet const int length = utf8::internal::sequence_length(it); // Get trail octets and calculate the code point utf_error err = UTF8_OK; switch (length) { case 0: return INVALID_LEAD; case 1: err =…