C++ static member variables and static member functions

1. Static member variables

1) Define static member variables

 The keyword static can be used to describe the members of a class, and static members provide a sharing mechanism for objects of the same
type
 A static member is local to the class, it is not an object member

例如：
#include<iostream>
using namespace std;
class  counter
{     
static  int  num ; //声明与定义静态数据成员
  public :
      void  setnum ( int i ) { num = i ; }   //成员函数访问静态数据成员
      void  shownum() { cout << num << '\t' ; }
} ;
int  counter :: num = 0 ;//声明与定义静态数据成员
void main ()
{   counter  a , b ;
    a.shownum() ; //调用成员函数访问私有静态数据成员
    b.shownum() ;
    a.setnum(10) ;
    a.shownum() ;
    b.shownum() ;
}

2) Use static member variables

// 例  使用公有静态数据成员 
#include<iostream.h>
class  counter
{ public :
      counter (int a) { mem = a; }
      int mem;      //公有数据成员
      static  int  Smem ;   //公有静态数据成员
} ;
int  counter :: Smem = 1 ;  //初始值为1 
void main()
{   counter c(5);
    int i ;
    for( i = 0 ; i < 5 ; i ++ )
      { counter::Smem += i ;
         cout << counter::Smem << '\t' ;  //访问静态成员变量方法2
      }
    cout<<endl;
    cout<<"c.Smem = "<<c.Smem<<endl; //访问静态成员变量方法1
    cout<<"c.mem = "<<c.mem<<endl;
}

2. Static member functions

1) Concept

 The number of static member functions is prefixed with the keyword static 
The static member function provides common operations that do not depend on the class data structure, it does not have this pointer
object call

2) Case

#include <iostream>
using namespace std;

class X{
int DataMem;
public:
    static void StaFun(int i,X *p);
};
void X::StaFun(int i,X *p)
{
    p->DataMem=i;
}
void g()
{
    X x;
    X::StaFun(1,&x);
    x.StaFun(1,&x);
}

3) Difficult problems: In static member functions, ordinary variables cannot be used.

//静态成员变量属于整个类的，分不清楚，是那个具体对象的属性。