计算几何？学了不会？会了不对？

 1 #include<bits/stdc++.h>
 2 using namespace std;
 3 const double EPS=1e-6;
 4 inline int dcmp(double x){
 5     if(fabs(x)<EPS)return 0;
 6     else if(x<0)return -1;
 7     else return 1;
 8 }
 9 struct Vector{
10     double x,y;
11     Vector(double X=0.0,double Y=0.0):x(X),y(Y){}
12     Vector operator+(const Vector &A)const{
13         return Vector(x+A.x,y+A.y);
14     }
15     Vector operator-(const Vector &A)const{
16         return Vector(x-A.x,y-A.y);
17     }
18     double operator*(const Vector &A)const{//点乘· 
19         return x*A.x+y*A.y;
20     }
21     Vector operator*(const double &X)const{//数乘 
22         return Vector(x*X,y*X);
23     }
24     bool operator==(const Vector&A)const{
25         return ( !dcmp(x-A.x) && !dcmp(y-A.y) );
26     }
27 };
28 inline double polarangle(Vector &A){//向量极角
29     return atan2(A.y,A.x); //?什么函数? 
30 }
31 inline double cross(const Vector& A,const Vector& B){//叉乘* （只取数值大小） 
32     return A.x*B.y-A.y*B.x;
33 }
34 inline double length(const Vector &A){
35     return sqrt(A.x*A.x+A.y*A.y);
36 }
37 inline Vector(const Vector& A,const double a){
38 
39 int main(){
40     printf("%lf",length(Vector(1,1)));
41     return 0;
42 }