《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：

 // 13.3 How does virtual function works in C++?
 #include <cstring>
 #include <iostream>
 using namespace std;

 class A {
 };

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

 class C {
 public:
     C();
 };

 class D {
 public:
     virtual void f() {
         cout << "class D" << endl;
     };
 };

 class E: public D {
 public:
     void f() {
         cout << "class E" << endl;
     };
 };

 int main()
 {
     D *ptr = new E();
     D d;
     E e;
     unsigned ui;

     cout << sizeof(A) << endl;
     cout << sizeof(B) << endl;
     cout << sizeof(C) << endl;
     cout << sizeof(D) << endl;
     cout << sizeof(E) << endl;
     // The result is 1 1 1 4 4 on Visual Studio 2012.
     ptr->f();
     // class with no data member have to occupy 1 byte, so as to have an address.
     // class with virtual functions need a pointer to the virtual function table.
     printf("The address of d is %p.\n", &d);
     printf("The address of e is %p.\n", &e);
     printf("The address of d.f is %p.\n", (&D::f));
     printf("The address of e.f is %p.\n", (&E::f));

     memcpy(&ui, &d, );
     printf("d = %X\n", ui);
     memcpy(&ui, &e, );
     printf("e = %X\n", ui);
     memcpy(&ui, ptr, );
     printf("*ptr = %X\n", ui);

     return ;
 }