A while ago, someone with high reputation here on Stack Overflow wrote in a comment that it is necessary to cast a char-argument to unsigned char before calling std::toupper and std::tolower (and similar functions).

On the other hand, Bjarne Stroustrup does not mention the need to do so in the C++ Programming Language. He just uses toupper like

string name = "Niels Stroustrup";

void m3() {
  string s = name.substr(6,10);  // s = "Stroustr up"
  name.replace(0,5,"nicholas");  // name becomes "nicholas Stroustrup"
  name[0] = toupper(name[0]);   // name becomes "Nicholas Stroustrup"
}

(Quoted from said book, 4th edition.)

The reference says that the input needs to be representable as unsigned char. For me this sounds like it holds for every char since char and unsigned char have the same size.

So is this cast unnecessary or was Stroustrup careless?

Edit: The libstdc++ manual mentions that the input character must be from the basic source character set, but does not cast. I guess this is covered by @Keith Thompson's reply, they all have a positive representation as signed char and unsigned char?