c++靜態成員變量使用前必須初始化，那麼下面這個例子爲什麼不用初始化？

有三個文件：

student.h文件

#include <string>
#include <iostream>
using namespace std;
const int MAX_NAME_SIZE = 30;
class Student{
 public:
   Student(const char * pszName);
  ~Student();
 public:
  static void PrintfAllStudents();
 private:
  char m_name[MAX_NAME_SIZE];
  Student *next;
  Student *prev;
  static Student *m_head;  //這裏爲什麼後面沒看到它的初始化語句？？
 };

function.cc文件

#include <iostream>
#include <string>
#include <string.h>
#include "student.h"
using namespace std;
Student::Student(const char *pszName)
  {
    strcpy(this->m_name,pszName);
    this -> next = m_head;
    this -> prev =NULL;
    if(m_head != NULL)
      m_head -> prev = this;
    m_head = this;
  }

Student::~Student(){
  if(this == m_head)
    {
      m_head = this-> next;
    }
  else{
    this -> prev->next = this->next;
    this -> next -> prev = this -> prev;
    
  }
}
void Student::PrintfAllStudents()
{
  for(Student *p =m_head;p!= NULL;p=p->next)
    printf("%s\n",p->m_name);
 }
Student *Student::m_head = NULL;


  

main.cc文件

#include <iostream>
#include <string>
#include "student.h"
using namespace  std;

int  main()
{
  Student studentA("AAA");
  Student studentB("BBB");
  Student studentC("CCC");
  Student studentD("DDD");
  Student student("MoreWindows");
  cout << "begin to print : " << endl;
  Student::PrintfAllStudents();
  return 0;
}

爲什麼沒有看到 Student的靜態類型成員Student *m_head的初始化語句？但是依然便以正確