问题的提出
现状：n个同类的对象，每一个对象都分别有自己的数据成员，各自有值，互不相干。
期望：希望有某一个或几个数据成员为某类所有对象所共有，以实现数据共享。
方案：用全局变量

#include<iostream>
using namespace std;
int N = 0;
class Class
{
private:
   int a;
public:
   Class(){N++;a=0;}
   void add(){N++;}
};


int main( )
{
    Class c1, c2;
    N=300;
    c1.add();
    cout<<N<<endl;
    return 0;
}

例 引用静态数据成员

#include <iostream>
using namespace std;
class Student
{
public:
    Student(int n,string nam, int a):
            num(n),name(nam),age(a) { ++count; }
    ~Student() { --count; }
    int getCount()   { return count; }
private:
    static int count;
    int num;
    string name;
    int age;
};


int Student::count=0;
int main( )
{
    Student stu1(1001,"He",40);
    cout<<stu1.getCount()<<endl;
    Student *pt=new Student(1001,"You",20);
    cout<<pt->getCount()<<endl;
    cout<<stu1.getCount()<<endl;
    delete pt;
    cout<<stu1.getCount()<<endl;
    return 0;
}

用类名访问静态数据成员

#include <iostream>
using namespace std;
class Student
{
public:
    Student(int n,string nam, int a):
            num(n),name(nam),age(a) { ++count; }
    ~Student() { --count; }
    int getCount()   { return count; }
    static int count;
private:
    int num;
    string name;
    int age;
};


int Student::count=0;
int main( )
{
    cout<<Student::count<<endl;
    Student *pt=new Student(1001,"You",20);
    cout<<pt->getCount()<<endl;
    delete pt;
    cout<<Student::count<<endl;
    return 0;
}

为什么出错？

#include <iostream>
using namespace std;
class Student
{
public:
    Student(int n,string nam, int a):
            num(n),name(nam),age(a) { ++count; }
    ~Student() { --count; }
    int getCount()   { return count; }
private:
    static int count;
    int num;
    string name;
    int age;
};
int Student::count=0;
int main( )
{
    cout<<Student::getCount()<<endl; 
    cout<<Student::count<<endl;
    Student *pt=new Student(1001,"You",20);
    cout<<pt->getCount()<<endl;
    delete pt;
    return 0;
}

静态成员函数

#include <iostream>
using namespace std;
class Student
{
public:
    Student(int n,string nam, int a):
            num(n),name(nam),age(a) { ++count; }
    ~Student() { --count; }
    static int getCount()   { return count; }
private:
    static int count;
    int num;
    string name;
    int age;
};
int Student::count=0;
int main( )
{
    cout<<Student::getCount()<<endl; 
    Student *pt=new Student(1001,"You",20);
    cout<<pt->getCount()<<endl;
    delete pt;
    return 0;
}

静态成员函数不能处理非静态数据成员

#include <iostream>
using namespace std;
class Student
{
public:
    Student(int n,string nam, int a):
            num(n),name(nam),age(a) { ++count; }
    ~Student() { --count; }
    static int getCount()   
        { age++; return count; }
private:
    static int count;
    int num;
    string name;
    int age;
};
int Student::count=0;
int main( )
{
    cout<<Student::getCount()<<endl; 
    Student *pt=new Student(1001,"You",20);
    cout<<pt->getCount()<<endl;
    delete pt;
    return 0;
}