How to return reference

Cartridge At(int i) const;

Cartridge CDataChain::At(int i) const
{
        return m_CarChain1.at(0);
}

               ↓變成這樣寫

Cartridge& At(int i);

Cartridge& CDataChain::At(int i)
{
        return m_CarChain1.at(0);
}