The code in question is in loop that keeps on going until from reaches from_end or the conversion fails (due to insufficient input or otherwise), so both surrogates should be written in the same do_in invocation.
Take a look on my code, I don't update from_next till I write both surrogates, just sometimes I write them separately as usually uto + 1 == uto_end in MSVC implementation. So technically your code is wrong as you write into position of uto_end
Actually the mbstate_t is POD type that should be initialized to 0. I must make sure that sizeof(mbstate_t)>= 2, and then I use it as temporary storage for state.
I'm not talking about that, I meant the reinterpret casting between uchar and uint_type, but actually I suppose they're the same, maybe just different signedness, so that should be somewhat ok. It's still not allowed by the strict aliasing rules though.
I use uint16_t cast when sizeof(wchar_t) == 2 and uint32_t cast when sizeof of wchar_t == 4, I don't see any problem with this. So I always cast to pointer of virtually same binary type. Artyom