为什么有虚函数表？

主要还是为了实现C++的多态，即运行时绑定，而且虚函数表是在编译时期就已确认，后续链接、执行时都不会改变其地址

示例

#include <iostream>

using namespace std;

class Base
{
public:
    virtual void a();
    virtual void b();
    virtual void c();
};

void Base::a()
{
    cout << "base::a" << endl;
}

void Base::b()
{
    cout << "base::b" << endl;
}

void Base::c()
{
    cout << "base::c" << endl;
}

class Derived : public Base
{
public:
    void a();
};

void Derived::a()
{
    cout << "derived::a" << endl;
}

typedef void (*func)();

int main(int argc, char **argv)
{
    // 先看继承类的地址空间
    Derived derive;
    printf("derive::vptr %p\n", *(int *)&derive);

    // 获取虚函数表的地址
    int *v_ptr = (int *)(*(int *)&derive);

    printf("derive::vptr->a %p\n", *v_ptr);
    printf("derive::vptr->b %p\n", *(v_ptr+1));
    printf("derive::vptr->c %p\n", *(v_ptr+2));

    ((func)*v_ptr)();
    ((func)*(v_ptr + 1))();
    ((func)*(v_ptr + 2))();

    // 看看Base的地址空间
    Base base1;
    printf("base1::vptr %p\n", *(int *)&base1);

    // 获取虚函数表的地址
    v_ptr = (int *)(*(int *)&base1);

    printf("base1::vptr->a %p\n", *v_ptr);
    printf("base1::vptr->b %p\n", *(v_ptr + 1));
    printf("base1::vptr->c %p\n", *(v_ptr + 2));

    ((func)*v_ptr)();
    ((func)*(v_ptr + 1))();
    ((func)*(v_ptr + 2))();

    // 看看Base的地址空间
    Base base2;
    printf("base2::vptr %p\n", *(int *)&base2);

    // 获取虚函数表的地址
    v_ptr = (int *)(*(int *)&base2);

    printf("base2::vptr->a %p\n", *v_ptr);
    printf("base2::vptr->b %p\n", *(v_ptr + 1));
    printf("base2::vptr->c %p\n", *(v_ptr + 2));

    ((func)*v_ptr)();
    ((func)*(v_ptr + 1))();
    ((func)*(v_ptr + 2))();

    int test;
    cin >> test;

    return 0;
}

输出：

derive:vptr 00268B6C
derive:vptr->a 002610A0
derive:vptr->b 00261415
derive:vptr->c 002613D4
derived::a
base::b
base::c
base1:vptr 00268B34
base1:vptr->a 002610E6
base1:vptr->b 00261415
base1:vptr->c 002613D4
base::a
base::b
base::c
base2:vptr 00268B34
base2:vptr->a 002610E6
base2:vptr->b 00261415
base2:vptr->c 002613D4
base::a
base::b
base::c

结论：

1. 虚函数和类相关，和对象无关，从base1和base2的虚函数的地址完全一样可知
2. 继承关系时，derived重写的虚函数，会将对应虚函数表中的函数地址改写成子类的函数的地址，以实现动态binding