#include <algorithm>
#include <string> 

std::string source = "GNU Linux РуСсКиЙ ТеКсТ"; 
std::transform(source.begin(), source.end(), source.begin(), ::tolower);

std::string source = "GNU Linux РуСсКиЙ ТеКсТ";
wstring ws ( source.begin(), source.end() );
std::transform(ws.begin(), ws.end(), ws.begin(), ::tolower);
string sss( ws.begin(), ws.end() );
cout << sss;

setlocale(LC_ALL, "ru_RU.UTF-8");
string source = "GNU Linux РуСсКиЙ ТеКсТ";
wstring ws (source.begin(), source.end());
transform(ws.begin(), ws.end(), ws.begin(), ::tolower);
wcout << ws;

std::wstring ws(L"some Русский тексн");
std::transform(ws.begin(), ws.end(), ws.begin(), std::bind2nd(std::ptr_fun(&std::tolower<wchar_t>), std::locale("ru_RU.UTF-8")));
std::wcout << ws;

/usr/include/c++/4.2.1/bits/stl_function.h: In instantiation of 'std::binder2nd<std::pointer_to_binary_function<wchar_t, const std::locale&, wchar_t> >':
../untitled/main.cpp:10:   instantiated from here
/usr/include/c++/4.2.1/bits/stl_function.h:435: error: forming reference to reference type 'const std::locale&'
/usr/include/c++/4.2.1/bits/stl_function.h: In function 'std::binder2nd<_Operation> std::bind2nd(const _Operation&, const _Tp&) [with _Operation = std::pointer_to_binary_function<wchar_t, const std::locale&, wchar_t>, _Tp = std::locale]':
../untitled/main.cpp:10:   instantiated from here
/usr/include/c++/4.2.1/bits/stl_function.h:455: error: no matching function for call to 'std::binder2nd<std::pointer_to_binary_function<wchar_t, const std::locale&, wchar_t> >::binder2nd(const std::pointer_to_binary_function<wchar_t, const std::locale&, wchar_t>&, const std::locale&)'
/usr/include/c++/4.2.1/bits/stl_function.h:429: note: candidates are: std::binder2nd<std::pointer_to_binary_function<wchar_t, const std::locale&, wchar_t> >::binder2nd(const std::binder2nd<std::pointer_to_binary_function<wchar_t, const std::locale&, wchar_t> >&)
../untitled/main.cpp: At global scope:
../untitled/main.cpp: In instantiation of 'std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, const std::basic_string<_CharT, _Traits, _Alloc>&) [with _CharT = wchar_t, _Traits = std::char_traits<wchar_t>, _Alloc = std::allocator<wchar_t>]':
../untitled/main.cpp:11:   instantiated from here
../untitled/main.cpp:11: error: explicit instantiation of 'std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, const std::basic_string<_CharT, _Traits, _Alloc>&) [with _CharT = wchar_t, _Traits = std::char_traits<wchar_t>, _Alloc = std::allocator<wchar_t>]' but no definition available
../untitled/main.cpp: In instantiation of 'std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, const std::basic_string<_CharT, _Traits, _Alloc>&) [with _CharT = wchar_t, _Traits = std::char_traits<wchar_t>, _Alloc = std::allocator<wchar_t>]':
../untitled/main.cpp:11:   instantiated from here
../untitled/main.cpp:11: error: explicit instantiation of 'std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, const std::basic_string<_CharT, _Traits, _Alloc>&) [with _CharT = wchar_t, _Traits = std::char_traits<wchar_t>, _Alloc = std::allocator<wchar_t>]' but no definition available

#include <iostream>
#include <algorithm>
#include <locale>
#include <functional>

int main(){
    setlocale(LC_ALL, "ru_RU.UTF-8");
    std::wstring ws(L"some Русский тексн");
    std::transform(ws.begin(), ws.end(), ws.begin(), std::bind2nd(std::ptr_fun(&std::tolower<wchar_t>), std::locale("ru_RU.UTF-8")));
    std::wcout << ws;
    return 0;
}

some @CAA:89 B5:A=

some @CAA:89 B5:A=
some2 @CAA:89 B5:AB

string sss = "some Русский текст\n";
std::wstring ws (sss.begin(),sss.end());

QString QString::toLower() const
{
    const ushort *p = d->data;
    if (!p)
        return *this;
    if (!d->size)
        return *this;

    const ushort *e = d->data + d->size;

    // this avoids one out of bounds check in the loop
    if (QChar(*p).isLowSurrogate())
        ++p;

    while (p != e) {
        uint c = *p;
        if (QChar(c).isLowSurrogate() && QChar(*(p - 1)).isHighSurrogate())
            c = QChar::surrogateToUcs4(*(p - 1), c);
        const QUnicodeTables::Properties *prop = qGetProp(c);
        if (prop->lowerCaseDiff || prop->lowerCaseSpecial) {
            QString s(d->size, Qt::Uninitialized);
            memcpy(s.d->data, d->data, (p - d->data)*sizeof(ushort));
            ushort *pp = s.d->data + (p - d->data);
            while (p < e) {
                uint c = *p;
                if (QChar(c).isLowSurrogate() && QChar(*(p - 1)).isHighSurrogate())
                    c = QChar::surrogateToUcs4(*(p - 1), c);
                prop = qGetProp(c);
                if (prop->lowerCaseSpecial) {
                    int pos = pp - s.d->data;
                    s.resize(s.d->size + SPECIAL_CASE_MAX_LEN);
                    pp = s.d->data + pos;
                    const ushort *specialCase = specialCaseMap + prop->lowerCaseDiff;
                    while (*specialCase)
                        *pp++ = *specialCase++;
                } else {
                    *pp++ = *p + prop->lowerCaseDiff;
                }
                ++p;
            }
            s.truncate(pp - s.d->data);
            return s;
        }
        ++p;
    }
    return *this;
}

static inline uint surrogateToUcs4(ushort high, ushort low) {
        return (uint(high)<<10) + low - 0x35fdc00;
}

static inline uint surrogateToUcs4(QChar high, QChar low) {
        return (uint(high.ucs)<<10) + low.ucs - 0x35fdc00;
}

inline bool isHighSurrogate() const {
        return ((ucs & 0xfc00) == 0xd800);
}

inline bool isLowSurrogate() const {
        return ((ucs & 0xfc00) == 0xdc00);
}

#include <iostream>
#include <string>
#include <algorithm>

using namespace std;
int main (){    
    setlocale(LC_ALL,"ru_RU.UTF-8");
    wstring wstrTest = L"Это РуСсКИЙ теКст\n";    
    transform(wstrTest.begin(), wstrTest.end(), wstrTest.begin(),towlower);    
    wcout<<wstrTest;   
    return 0;
}