Eric Niebler wrote:
Erik Wien wrote:
The iterators used are bidirectional, not random access (impossible on UTF-8 and UTF-16)
No. Andrei Alexandrescu explained a scheme to me whereby a UTF-16 encoded string can have a random-access iterator, and I think it should. The basic idea is you keep a plain array of 16-bit integers which are the 16-bit characters and the first 16 bits of surrogate pairs. Then you have a data structure which maps from string offsets to the second 16 bits of surrogate pairs. Random access involves a simple index and a map look-up. Sequential access requires no map look-up. And since surrogate pairs are very rare, the map will almost always be empty and the look-up is skipped.
Nice! But this seems to make c_str O(N) operation. If I need to speak to a library in the common extern "C" language of interoperability, and that library happens to need UTF-16 encoded wchar_t const [], which by coincidence has the same representation as char16_t const [], I won't be very happy if The C++ string seems to ignore this common scenario.