View Compilation Error

0_0_19894195_19163.cpp: In function 'int main()':
0_0_19894195_19163.cpp:69:37: error: invalid initialization of non-const reference of type 'point&' from an rvalue of type 'point'
    double d = double(cross(p[ch[i]] - p[ch[i - 1]], p[cur] - p[ch[i]])) / len;
                                     ^
0_0_19894195_19163.cpp:31:5: note: in passing argument 1 of 'int cross(point&, point&)'
 int cross(point &a, point &b){
     ^
0_0_19894195_19163.cpp:70:40: error: invalid initialization of non-const reference of type 'point&' from an rvalue of type 'point'
    while ((tmp = double(cross(p[ch[i]] - p[ch[i - 1]], p[(cur + 1) % tbnum] - p[ch[i]])) / len) >= d){
                                        ^
0_0_19894195_19163.cpp:31:5: note: in passing argument 1 of 'int cross(point&, point&)'
 int cross(point &a, point &b){
     ^