class QByteArray' has no member named 'back'， about qt-aes HOT 4 CLOSED

QByteArray ret(rawText);
switch (padding)
{
case Padding::ZERO:
    //Works only if the last byte of the decoded array is not zero
    while (ret.at(ret.length()-1) == 0x00)
        ret.remove(ret.length()-1, 1);
    break;
case Padding::PKCS7:
    ret.remove(ret.length() - ret.back(), ret.back());//what is the fuction--"back()"?
    
    break;
case Padding::ISO:
{
    // Find the last byte which is not zero
    int marker_index = ret.length() - 1;
    for (; marker_index >= 0; --marker_index)
    {
        if (ret.at(marker_index) != 0x00)
        {
            break;
        }
    }

    // And check if it's the byte for marking padding
    if (ret.at(marker_index) == static_cast<char>(0x80))
    {
        ret.truncate(marker_index);
    }
    break;
}
default:
    //do nothing
    break;
}
return ret;