对const的引用，但是普通引用失败

Question

#include <iostream>
using namespace std;

class point
{
public:
    int x,y;

    inline int getX() { return x; } 
    inline int getY() { return y; }
    inline void setX(int v) { x=v; }
    inline void setY(int v) { y=v; }
};

point operator+ (point& p1, point& p2) 
{
    point sum = {p1.x+p2.x, p1.y+p2.y};
    return sum;
}

ostream& operator<< (ostream& out, point& p)
{
    out<<"("<<p.x<<","<<p.y<<")";
    return out;
}

int main()
{
    point a={3,5},b={6,7};
    cout<<"Sum="<<a+b;
}

以上代码因编译错误而失败：

  

startingClasses.C:33:17：错误：从'point'类型的右值开始无效初始化'point＆amp;'类型的非const引用     COUT＆LT;＆LT; “总和=” ＆LT;

但是下面的代码可以运行：

#include <iostream>
using namespace std;

class point
{
public:
    int x,y;

    inline int getX() { return x; } 
    inline int getY() { return y; }
    inline void setX(int v) { x=v; }
    inline void setY(int v) { y=v; }
};

point operator+ (point& p1, point& p2) 
{
    point sum = {p1.x+p2.x, p1.y+p2.y};
    return sum;
}

ostream& operator<< (ostream& out, **const** point& p)
{
    out<<"("<<p.x<<","<<p.y<<")";
    return out;
}

int main()
{
    point a={3,5},b={6,7};
    cout<<"Sum="<<a+b;
}

在代码中添加const使其正常工作......仍然不明白。