fatal error LNK1120: 2 unresolved externals

#include <iostream>
using namespace std;
#include <string>

class Stud
{
private:
int no; //学号
string name; //姓名
double deg; //成绩
static double sum; //总分
static int num; //人数

public:
Stud(int no=0,string name="noName",double deg=0.0)
{

this->no=no;
this->name=name;
this->deg=deg;
this->num+=1;
this->sum+=deg;
}

void setdata(int no,string name,double deg)
{
this->no=no;
this->name=name;
this->deg=deg;
this->sum=(sum-this->deg+deg);
//如果在构造函数中给了成绩，这里又要重新设置，先把原来给的减掉再加上这里新设置的，避免了成绩重复累加


}
void disp()
{
cout<<this->no<<"\t" <<this->name<<"\t" <<this->deg<<"\t"<<endl;
}
static double avg()
{

double average=Stud::sum/Stud::num;

return average;

}


};

int main(int argc, char *argv[])
{


Stud student[5];

int no;
string name=NULL;
double deg;
for(int i=0;i<5;i++)

{

cout<<"请输入学号："<<endl;
cin>>no;
cout<<"请输入姓名："<<endl;
cin>>name;
cout<<"请输入成绩："<<endl;
cin>>deg;

student[0].setdata(no,name,deg);

}


//显示每个同学的成绩
cout<<"学号"<<"\t" <<"姓名"<<"\t" <<"成绩"<<"\t"<<endl;
for(int j=0;j<5;j++)
{

student[0].disp();
}


//输出平均成绩
cout<<endl<<"平均成绩为"<<Stud::avg();




return 0;


}