《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, 4);
    printf("d = %X\n", ui);
    memcpy(&ui, &e, 4);
    printf("e = %X\n", ui);
    memcpy(&ui, ptr, 4);
    printf("*ptr = %X\n", ui);

    return 0;
}