vector 排序方法sort的使用

使用vector的sort方法，有两种形式，一种是在类内部使用操作符重载实现，一种是在类外面写一个比较函数。但是sort方法无法直接传入其它形参，在需要动态排序相对某个人的位置时，采取了间接通过静态变量的方法。为防止并发量巨大时，静态变量无法及时改变而产生错误数据，故用一循环去验证等待赋值，排完序后再还原为初始值。

#include <iostream>
#include <string>
#include <cmath>
#include <vector>

using namespace std;

double calc_distance(double _long1,double _lat1,double _long2,double _lat2)
{
    double rad = 6371.0;
    const double PI = 3.14159265358979323846;
    double long1 = _long1/180.0*PI;
    double lat1 = _lat1/180.0*PI;
    double long2 = _long2/180.0*PI;
    double lat2 = _lat2/180.0*PI;
    return rad*(acos(cos(lat2)*cos(lat1)*cos(long2-long1)+sin(lat2)*sin(lat1)));
}
class User
{
    private:
        int id;
        double longitude;
        double latitude;

    public:
        int getid(){return id;}
        double getlongitude(){return longitude;}
        double getlatitude(){return latitude;}
    User(int _id,double _longitude,double _latitude)
    {
        id = _id;
        longitude = _longitude;
        latitude = _latitude;
    }
    
    bool operator<(const User& obj) const
    {
        return id < obj.id;
    }

    bool operator>(const User& obj) const
    {
        return id > obj.id;
    }
    
    friend bool sort_by_distance(const User& obj1,const User& obj2);
    friend bool sort_by_id(const User& obj1,const User& obj2);
};

static double _longitude = 0.0;
static double _latitude = 0.0;

bool sort_by_distance(const User& obj1,const User& obj2) 
{
    return calc_distance(obj1.longitude,obj1.latitude,_longitude,_latitude)< calc_distance(obj2.longitude,obj2.latitude,_longitude,_latitude);
}

bool sort_by_id(const User& obj1,const User& obj2)
{
    return obj1.id < obj2.id;
}
int main()
{
    vector<User> vect;
    User test1(3,20.1,20.1); 
    User test2(2,30.1,30.1);
    User test3(1,40.1,40.1);
    User test4(4,50.1,50.1);
    User test5(5,60.1,60.1);
    vect.push_back(test1);
    vect.push_back(test2);
    vect.push_back(test3);
    vect.push_back(test4);
    vect.push_back(test5);
    while(true)
    {
        if(_longitude == 0.0 && _latitude == 0.0)
        {
            _longitude = 30.1;
            _latitude = 30.1;
            break;
        }
    }
    
    sort(vect.begin(),vect.end(),sort_by_distance);
    //sort(vect.begin(),vect.end(),sort_by_id);
    //sort(vect.begin(),vect.end(),less<User>());
    //sort(vect.begin(),vect.end(),greater<User>());

    _longitude = 0.0;
    _latitude = 0.0;

    vector<User>::iterator it = vect.begin();
    while(it != vect.end())
    {
        cout<<"ID:"<<it->getid()<<"\tLong:"<<it->getlongitude()<<"\tLat:"<<it->getlatitude()<<endl;
        it++;
    }

}