C++-static members and constant objects of a class

The static members and constant objects of the class in C++
1.Xh file :

#ifndef X_H_INCLUDED
#define X_H_INCLUDED
class CX
{
    
    
    public:
        static int CXCount;
        CX();
        ~CX();
};
#endif // X_H_INCLUDED

2. CX.cpp (implementation file of the class)

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

int CX::CXCount=0;
CX::CX()
{
    
    
    CXCount++;
    cout<<"新建一个CX对象"<<endl;
    cout<<"当前CX对象个数："<<CXCount<<endl;
};

CX::~CX()
{
    
    
    CXCount--;
    cout<<"释放一个CX对象"<<endl;
    cout<<"当前CX对象个数："<<CXCount<<endl;
}

3.main.cpp

#include "X.h"
int main()
{
    
    
    CX a,b,c;
    CX*p1=new CX();
    CX*p2=new CX();
    delete p1;
    delete p2;

    return 0;
}

operation result:

新建一个CX对象
当前CX对象个数：1
新建一个CX对象
当前CX对象个数：2
新建一个CX对象
当前CX对象个数：3
新建一个CX对象
当前CX对象个数：4
新建一个CX对象
当前CX对象个数：5
释放一个CX对象
当前CX对象个数：4
释放一个CX对象
当前CX对象个数：3
释放一个CX对象
当前CX对象个数：2
释放一个CX对象
当前CX对象个数：1
释放一个CX对象
当前CX对象个数：0

Note: Static members are not attached to an object!
5. Constant object: use const to limit the changes of data members!

#include<iostream>

using namespace std;

class A
{
    
    
    int m_a;
public:
    A(int a):m_a(a){
    
    }
    void setA(int a){
    
    m_a=a;}
    int getA()const{
    
    return m_a;}
};

int main()
{
    
    
    const A a(100);
    a.setA(20);//注意！此处编译未通过！数据成员一旦赋值便不能再修改！

    return 0;
}