The libxml++ API takes, and gives, strings in the UTF-8 Unicode encoding, which can support all known languages and locales. This choice was made because, of the encodings that have this capability, UTF-8 is the most commonly accepted choice. UTF-8 is a multi-byte encoding, meaning that some characters use more than 1 byte. But for compatibility, old-fashioned 7-bit ASCII strings are unchanged when encoded as UTF-8, and UTF-8 strings do not contain null bytes which would cause old code to misjudge the number of bytes. For these reasons, you can store a UTF-8 string in a std::string object. However, the std::string API will operate on that string in terms of bytes, instead of characters.

The libxml++ API indicates when a string should be provided as UTF-8, or will be provided as UTF-8, by using the xmlpp::ustring type alias. However, this is really just a std::string, whose operator[] and size() consider bytes, not characters.