强制类型转换(const_cast)

 1 #include <iostream>
 2 using  namespace std;
 3 
 4 const int xx = 50;
 5 class A
 6 {
 7 public:
 8     int m_nNum;
 9 public:
10     A(int nValue = 100);
11 };
12 
13 A::A(int nValue):m_nNum(nValue)
14 {
15 }
16 
17 void TestFun()
18 {
19     // 第一种情况
20     const A *pA = new A(200);
21     // pA->m_nNum = 100; // compile error !
22     A* pAA = const_cast<A*>(pA);
23     pAA->m_nNum = 199;
24     cout<<pA->m_nNum<<endl;//199
25     // 第二种情况
26     A *pB = new A();
27     pA = pB; // 思考这个原因。为什么这样子可以呢？且看下面的分析：
28     A* const pC = new A(1);
29     A *pD = new A(2); 
30     // pC = pD; // compile error !
31     A*& pE = const_cast<A*>(pC);
32     pE = pD;
33     cout<<pC->m_nNum<<endl;  //2
34     // 第三种情况
35     const A a;
36     // a.m_nNum = 101; //compile error !
37     A b = const_cast<A&>(a);
38     b.m_nNum = 101;
39     cout<<a.m_nNum <<endl; //100
40     cout<<b.m_nNum <<endl; //101
41     // 第四种情况
42     const A c;
43     // c.m_nNum = 101; //compile error !
44     A& d = const_cast<A&>(c);
45     d.m_nNum = 102;
46     cout<<c.m_nNum <<endl; //102
47     cout<<d.m_nNum <<endl; //102
48     // 第五种情况
49     const A e;
50     // e.m_nNum = 103; //compile error !
51     A* pe = const_cast<A*>(&e);
52     pe->m_nNum = 103;
53     cout<<e.m_nNum <<endl; //103
54     cout<<pe->m_nNum <<endl; //103
55     // 第六种情况
56     const int xx = 50;
57     int* yy = const_cast<int *>(&xx);
58     *yy = 200;
59     cout <<xx<<endl; // 50
60     cout <<*yy<<endl; // 200
61     int aa = xx;
62     cout << aa << endl;
63     // 第七种情况
64     const int xxx = 50;
65     int yyy = const_cast<int&>(xxx);
66     yyy = 51;
67     cout << xxx << endl; // 50
68     cout << yyy << endl; // 51
69     // 第八种情况
70     const int xxx2 = 50;
71     int& yyy2 = const_cast<int&>(xxx2);
72     yyy2 = 52;
73     cout << xxx2 << endl; // 50
74     cout << yyy2 << endl; // 52
75 }
76 
77 void main()
78 {
79     TestFun();
80 }