My Solution

std::string atbash(std::string str) {
    std::string eString{};
    std::vector<char> letters {'a','b','c','d','e','f','g','h','i','j','k','l','m',
    'n','o','p','q','r','s','t','u','v','w','x','y','z'};
    std::vector<char> uLetters {'A','B','C','D','E','F','G','H','I','J','K','L','M',
    'N','O','P','Q','R','S','T','U','V','W','X','Y','Z'};

    for(auto& c : str)
    {
        if(!isalpha(c) || c == ' ')
        {
            eString += c;
            continue;
        }
        if(islower(c))
        {
            auto it = std::find(letters.begin(), letters.end(), c);
            int index = std::distance(letters.begin(), it);
            eString += letters.at(letters.size()-1 - index);
        }
        else if(isupper(c))
        {
            auto it = std::find(uLetters.begin(), uLetters.end(), c);
            int index = std::distance(uLetters.begin(), it);
            eString += uLetters.at(uLetters.size()-1 - index);
        }
    }
    return eString;
}