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#include <stdio.h>
class A
{
 private:
  int id;
  A() {}//°Ñ¹¹Ô캯Êý·ÅÔÚprivate£ºÏÂÄ¿µÄÊÇÔÚÀàÍâ²»ÄÜÔÚÕ»ÉÏÖ±½Ó·ÖÅä¿Õ¼ä¶¨Òå¶ÔÏó¡£
 public:
  static A *pt;
  static A *instance()
  {
   if (pt == NULL)//×¢ÒâÊÇË«µÈºÅ£¬»¹ÓкÃÏñÔÚC++ÖÐÓÃnull ²»ºÃʹ¡£
   {
    pt=new A;
    return pt;
   }
   else
    return pt;
  }
     
  ~A()
  {
   delete pt;//ÒòΪ¿ª±ÙÁ˶ѿռ䣬ҪÓÃdelete ÊÍ·Å¡£
  }
  void id_set(int x)
  {
   id=x;
  }
  void id_get()
  {
   printf("%d",id);
  }
 
};
A* A::pt=NULL;//¾²Ì¬³ÉÔ±±äÁ¿Ö»ÄÜÔÚÀàÍâ³õʼ»¯¡£
int main()
{
//´ËÀ಻ÄÜÔÚÕ»ÉÏ·ÖÅä¶ÔÏóÖ»ÄÜͨ¹ýÀàÖÐÌṩµÄº¯Êý½Ó¿ÚÓÃnewÔÚ¶ÑÉÏ·ÖÅä¶ÔÏó¡£
 A *ppt=A::instance();
 ppt->id_set(10);
 ppt->id_get();
 A *ppt1=A::instance();
 ppt1->id_get();
    ppt1->id_set(5);
 ppt->id_get();
 return 0;//ÔÚº¯Êý½áÊøÊ±×Ô¶¯µ÷ÓÃÎö¹¹º¯Êý~A£¨£©£»
}
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[6.3.2.3-3] An integer constant expression with the value 0, or such an expression cast to type void *, is called a null pointer constant.
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    int (*compar)(const void *, const void *))
{
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    void* max_data = base;
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