std::string str(buffer, buffer + length);
Or, if the string already exists:
str.assign(buffer, buffer + length);
Edit: I’m still not completely sure I understand the question. But if it’s something like what JoshG is suggesting, that you want up to length characters, or until a null terminator, whichever comes first, then you can use this:
std::string str(buffer, std::find(buffer, buffer + length, '\0'));