const_cast<type-id>(expression)

//**********************//
类类型
class B{
public:
int m_num;
B():m_num(50){}
};
void foo(void) {

const B* b1 = new B();
B* b2 = const_cast<B*>(b1);
b2->m_num = 200;
cout <<"b1:" << b1->m_num << endl;//200
cout <<"b2:" << b2->m_num << endl;//200

const B b3;
B b4 = const_cast<B&>(b3);
b4.m_num = 300;
cout << "b3:" << b3.m_num << endl;//50
cout << "b4:" << b4.m_num << endl;//300
}
//************************//

//************************//
基本类型
void foo(){
const int a = 100;
int* p1 = const_cast<int*>(&a);
*p1 = 200;
cout << *p1 << endl;//200
cout << a << endl;//100

const int* p2 = new int(100);
int* p3 = const_cast<int*>(p2);
*p3 = 200;
cout << *p2 << endl;//200
cout << *p3 << endl;//200
}
//************************//

你会发现:
A:可以为基本类型或者类类型;

const A a;随便怎么修改a都不会变化
const A* p = new A();去掉p的const属性后,*p就变化了.

//*****************//
class A{
public:
A(){
m_num=1;
}
int m_num;
};
void foo (void){
A a;
const A &r = a;
A a1 = const_cast<A&>(a);
a1.m_num = 200;
cout << a1.m_num << endl;//200
cout << a.m_num << endl;//1
}
//****************//

const_cast<type-id>(expression)中,type-id只能为指针或引用，其他的都错,这个表达式即可以去除
expression中的const属性或volatil属性，还能增加const属性或者volatil属性
const int i = 10;
int i1 = const_cast<int>(i) //错误

增加const属性与volatil属性相反.