Why std::transform doesn't work this way:
std::string tmp = "WELCOME";
std::string out = "";
std::transform(tmp.begin(), tmp.end(), out.begin(), ::tolower);
out is empty!
But this works:
std::transform(tmp.begin(), tmp.end(), tmp.begin(), ::tolower);
I don't want the transformation to happen in-place.
You are writing in out-of-bounds memory, since the range of out is smaller than that of tmp. You can store the result in out by applying std::back_inserter.
As user17732522 pointed out, since it's not legal to take the adress of a standard libary function, it's better to pass over a lamda object that calls std::tolower on the character when needed.
std::transform(tmp.begin(), tmp.end(), std::back_inserter(out), [](auto c) {
return std::tolower(static_cast<unsigned char>(c));
});