Other Solution

std::string atbash(std::string str) {
    for (std::string::iterator it = str.begin(); it != str.end(); ++it)
        if ('A' <= *it and *it <= 'Z' or 'a' <= *it and *it <= 'z')
            *it = 25 - *it + (('a' <= *it) ? 2 * 'a' : 2 * 'A');
    return str;
}