《Cracking the Coding Interview》——第13章：C和C++——题目3

2014-04-25 19:42

题目：C++中虚函数的工作原理？

解法：虚函数表？细节呢？要是懂汇编我就能钻的再深点了。我试着写了点测大小、打印指针地址之类的代码，能起点管中窥豹的作用，从编译器的外部感受下虚函数表、虚函数指针的存在。

代码：

 1 // 13.3 How does virtual function works in C++?

 2 #include <cstring>

 3 #include <iostream>

 4 using namespace std;

 5 

 6 class A {

 7 };

 8 

 9 class B {

10 public:

11     void f() {cout << "class B" << endl;};

12 };

13 

14 class C {

15 public:

16     C();

17 };

18 

19 class D {

20 public:

21     virtual void f() {

22         cout << "class D" << endl;

23     };

24 };

25 

26 class E: public D {

27 public:

28     void f() {

29         cout << "class E" << endl;

30     };

31 };

32 

33 int main()

34 {

35     D *ptr = new E();

36     D d;

37     E e;

38     unsigned ui;

39     

40     cout << sizeof(A) << endl;

41     cout << sizeof(B) << endl;

42     cout << sizeof(C) << endl;

43     cout << sizeof(D) << endl;

44     cout << sizeof(E) << endl;

45     // The result is 1 1 1 4 4 on Visual Studio 2012.

46     ptr->f();

47     // class with no data member have to occupy 1 byte, so as to have an address.

48     // class with virtual functions need a pointer to the virtual function table.

49     printf("The address of d is %p.\n", &d);

50     printf("The address of e is %p.\n", &e);

51     printf("The address of d.f is %p.\n", (&D::f));

52     printf("The address of e.f is %p.\n", (&E::f));

53 

54     memcpy(&ui, &d, 4);

55     printf("d = %X\n", ui);

56     memcpy(&ui, &e, 4);

57     printf("e = %X\n", ui);

58     memcpy(&ui, ptr, 4);

59     printf("*ptr = %X\n", ui);

60 

61     return 0;

62 }