怎么使类声明和成员函数分离

//student.h
class student
{
   public:
      student(int n):num(n){}
      void display();
   private:
      int num;
      string name;
      char sex;
};
//student.cpp
#include <iostream>
#include "student.h"
void student::display()
{
  cout<<"num"<<num;
}

为什么我写这个主函数会出现链接错误？
#include <iostream>
#include <string>
#include "student.h"
using namespace std;
int main()
{
  student s(7);
  s.display();
  system("pause");
  exit(1);
}
希望大家指教下小弟哈~！