&c 和 c 请看红字部分

class Complex
{
private :
    float Real, Image;
public:
    Complex(float x = 0, float y = 0)
    {
        Real = x;
        Image = y;
    }
    void Show(int i)
    {
        cout << "c" << i << "=" << Real << "+" << Image << "i" << endl;
    }
    Complex operator + (Complex c);//////////////////////////请问这里书籍上给的是&c 但是这里用c结果一样
    Complex operator - (Complex &c);/////////////////////////为什么要用&c
    void  operator += (Complex &c);
    void operator = (Complex &c);
};



Complex Complex :: operator + (Complex c)///////////////////
{
    Complex t;
    t.Real =  Real + c.Real;
    t.Image = Image + c.Image;
    return  t;
}


Complex Complex :: operator -(Complex &c)/////////////////////////
{
    Complex t;
    t.Real = Real - c.Real;
    t.Image = Image - c.Image;
    return t;
}

void Complex :: operator += (Complex &c)
{
    Real = Real + c.Real;
    Image = Image + c.Image;
}

void Complex :: operator = (Complex &c)
{
  Real  = c.Real;
  Image = c.Image;
}

void main()
{
    Complex c1(15,34), c2(30,57),c3,c4;
    c1.Show(1);
    c2.Show(2);
    c3 = c1 + c2;
    c4 = c1 - c2;
    c3.Show(3);
    c4.Show(4);
    c2 += c1;
    c2.Show(5);


}