const-verload

//history = list<int>

int Attribute::getValue()
{
    if (changed > 0)
    {
        for (int idx = history.length() - changed; idx < history.length() - 1; ++idx)
        {
            value += history.at(idx);
        }
        changed = 0;
    }
    return value;
}

int Attribute::getValue() const
{
    if (changed > 0)
    {
        int cvalue = value;
        for (int idx = history.length() - changed; idx < history.length() - 1; ++idx)
        {
            cvalue += history.at(idx);
        }
        return cvalue;
    }
    return value;
}

void Attribute::setValue(int value)
{
    history.append(value);
    ++changed;
}