gistfile1.cpp

inline bool is_ascii_upper(char ch) { return 'A' <= ch && ch <= 'Z'; }

void lowercase(std::istream& istr)
{
    std::string s;
    while (std::getline(istr, s, '\n'))
    {
        for (char& ch : s)
        {
            if (is_ascii_upper(ch))
            {
                ch = ch - 'A' + 'a';
            }
        }
        std::cout << s << "\n";
    }
}