C++ convert wchar_t to char16_t and back

Question

I have a program that is supposed to run on all platfotms. So I need to get rid of wchar_t and use char16_t instead. How can I convert wchar_t <-> char16_t For example how would I get to use wsclen() here?

wchar_t WT = L'Hello World! of WCHART';
wchar_t* buf = new wchar_t[234];
int a = wcslen(buf);
int aa = wcslen(&WT);

char16_t W16[] = u"Hello World! of CHAR16";
int b = wcslen( W16);                           <--- ERROR HERE