Boost :: algorithm :: to_upper / to_lower ok for utf8? boost :: locale not needed?

Question

I read in several places that speed up :: algorithm :: to_upper / to_lower are not suitable for utf8:

• http://www.boost.org/doc/libs/1_51_0/libs/locale/doc/html/conversions.html
• How to convert std :: string to lowercase?
• Work with UTF8

But on my system, ubuntu 12.4.1 is 32 bits with boost 1.46 and locale en_GB.UTF-8, everything looks fine if I pass the locale. For instance:

std::locale englishUTF8locale("en_GB.UTF-8") boost::algorithm::to_upper_copy(L"ü", englishUTF8locale) -> L"Ü" boost::algorithm::to_lower_copy(L"", englishUTF8locale) ->L"" 

It is worth noting that it works when using std :: wstring, but not when using std :: string

So, boost :: locale is not really needed? My problem is that I can only use boost 1.46, and the locale was created for 1.48