虚函数表是个什么鬼?
在多重继承里的虚函数表可以在vs里面看到,如下
有一个基类就有一张表,可以通过
int** pVtab = (int**)&d;
pFun = (Fun)pVtab[0][0];
来访问每一个虚函数,如下代码:
// pvtable1.cpp : 定义控制台应用程序的入口点。 // #include "stdafx.h" #include <iostream> using namespace std; class Base1 { public: Base1(){ cout << "Base1::Base1()" << endl; } ~Base1(){ cout << "Base1::~Base1()" << endl; } virtual void f() { cout << "Base1::f" << endl; } virtual void g() { cout << "Base1::g" << endl; } virtual void h() { cout << "Base1::h" << endl; } }; class Base2 { public: Base2(){ cout << "Base2::Base2()" << endl; } ~Base2(){ cout << "Base2::~Base2()" << endl; } virtual void f() { cout << "Base2::f" << endl; } virtual void g() { cout << "Base2::g" << endl; } virtual void h() { cout << "Base2::h" << endl; } }; class Base3 { public: Base3(){ cout << "Base3::Base3()" << endl; } ~Base3(){ cout << "Base3::~Base3()" << endl; } virtual void f() { cout << "Base3::f" << endl; } virtual void g() { cout << "Base3::g" << endl; } virtual void h() { cout << "Base3::h" << endl; } }; class Derive : public Base1, public Base2, public Base3 { public: Derive(){ cout << "Derive::Derive()" << endl; } ~Derive(){ cout << "Derive::~Derive()" << endl; } virtual void f() { cout << "Derive::f" << endl; } virtual void g1() { cout << "Derive::g1" << endl; } }; typedef void(*Fun)(void); int doIt() { Fun pFun = NULL; Derive d; int** pVtab = (int**)&d; //Base1's vtable //pFun = (Fun)*((int*)*(int*)((int*)&d+0)+0); pFun = (Fun)pVtab[0][0]; pFun(); //pFun = (Fun)*((int*)*(int*)((int*)&d+0)+1); pFun = (Fun)pVtab[0][1]; pFun(); //pFun = (Fun)*((int*)*(int*)((int*)&d+0)+2); pFun = (Fun)pVtab[0][2]; pFun(); //Derive's vtable //pFun = (Fun)*((int*)*(int*)((int*)&d+0)+3); pFun = (Fun)pVtab[0][3]; pFun(); //The tail of the vtable pFun = (Fun)pVtab[0][4]; cout<<pFun<<endl; //Base2's vtable //pFun = (Fun)*((int*)*(int*)((int*)&d+1)+0); pFun = (Fun)pVtab[1][0]; pFun(); //pFun = (Fun)*((int*)*(int*)((int*)&d+1)+1); pFun = (Fun)pVtab[1][1]; pFun(); pFun = (Fun)pVtab[1][2]; pFun(); //The tail of the vtable pFun = (Fun)pVtab[1][3]; cout<<pFun<<endl; //Base3's vtable //pFun = (Fun)*((int*)*(int*)((int*)&d+1)+0); pFun = (Fun)pVtab[2][0]; pFun(); //pFun = (Fun)*((int*)*(int*)((int*)&d+1)+1); pFun = (Fun)pVtab[2][1]; pFun(); pFun = (Fun)pVtab[2][2]; pFun(); //The tail of the vtable pFun = (Fun)pVtab[2][3]; cout<<pFun<<endl; cout<<sizeof(d)<<endl; return 0; } int _tmain(int argc, _TCHAR* argv[]) { doIt(); return 0; }
运行结果如下:
最后用sizeof获取对象的大小等于成员变量的大小加上虚函数表指针的大小
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