c++虚函数表
C++中的虚函数(Virtual Function)的实现通过一张虚函数表来实现的,简称为V-Table。在这个表中,主要是一个类的虚函数的地址表,这张表解决了继承、覆盖的问题,保证其真实反应实际的函数。这样,在有虚函数的类的实例中分配了指向这个表的指针的内存,所以,当用父类的指针来操作一个子类的时候,这张虚函数表就显得尤为重要了,它就像一个地图一样,指明了实际所应该调用的函数。
编译器应该是保证虚函数表的指针存在于对象实例中最前面的位置(这是为了保证取到虚函数表的有最高的性能——如果有多层继承或是多重继承的情况下)。这意味着可以通过对象实例的地址得到这张虚函数表,然后就可以遍历其中函数指针,并调用相应的函数。
下面讨论虚函数表的各种情况,我用的是VS 2013,因此都是在VS 2013编译器下讨论各种情况的:
一、单继承(无虚函数覆盖)
1、例子
#include<iostream> using namespace std; class Base { public: virtual void f() { cout << "Base::f()" << endl; } virtual void g() { cout << "Base::g()" << endl; } virtual void h() { cout << "Base::h()" << endl; } private: virtual void i() { cout << "Base::i()" << endl; } }; class Derive:public Base{ public: virtual void f1(){ cout << "Derive::f1()" << endl; } virtual void g1(){ cout << "Derive::g1()" << endl; } virtual void h1(){ cout << "Derive::h1()" << endl; } }; int main(){ typedef void(*pFun)(); //定义一个函数指针,参数列表空,返回值void Base base; int** pVtab = (int**)&base; for (int i = 0; pVtab[0][i] != NULL; ++i){ cout << "虚函数表存放第"<<i+1<<"个函数地址的地址:" << pVtab[0]+i << endl; //pVtab[0]+i等同于(int*)*(int*)(&base) + i,参考文章那边有错误 cout << "虚函数表第" << i + 1 << "个函数地址:" << (int*)pVtab[0][i] << endl;//(int*)pVtab[0][i]等同于(int*)*((int*)*(int*)(&base) + 1) ((pFun)pVtab[0][i])(); } cout << "------------------------------------------------------------------" << endl; Derive derive; pVtab = (int**)&derive; for (int i = 0; pVtab[0][i] != NULL; ++i){ cout << "虚函数表存放第" << i + 1 << "个函数地址的地址:" << pVtab[0] + i << endl; //pVtab[0]+i等同于(int*)*(int*)(&derive) + i cout << "虚函数表第" << i + 1 << "个函数地址:" << (int*)pVtab[0][i] << endl; //(int*)pVtab[0][i]等同于(int*)*((int*)*(int*)(&derive) + 1) ((pFun)pVtab[0][i])(); } system("pause"); return 0; }
2、运行结果:
4、基类虚表指针及虚表布局:
5、继承类虚表指针及虚表布局:
6、说明
1)VS 2013中虚表指针放在对象的起始位置。
2)在同一个访问区段中虚函数按照其声明顺序放于表中,但各个访问区段的先后顺序并不一定。
3)父类的虚函数在子类的虚函数前面。
4)基类的虚函数i()是private的,但用虚表指针能访问到。
二、单继承(有虚函数覆盖)
1、例子
#include<iostream> using namespace std; class Base { public: virtual void f() { cout << "Base::f()" << endl; } virtual void g() { cout << "Base::g()" << endl; } virtual void h() { cout << "Base::h()" << endl; } }; class Derive :public Base{ public: virtual void f(){ cout << "Derive::f()" << endl; } virtual void g1(){ cout << "Derive::g1()" << endl; } virtual void h1(){ cout << "Derive::h1()" << endl; } }; int main(){ typedef void(*pFun)(); //定义一个函数指针,参数列表空,返回值void Base base; int** pVtab = (int**)&base; for (int i = 0; pVtab[0][i] != NULL; ++i){ cout << "虚函数表存放第" << i + 1 << "个函数地址的地址:" << pVtab[0] + i << endl; //pVtab[0]+i等同于(int*)*(int*)(&base) + i,参考文章那边有错误 cout << "虚函数表第" << i + 1 << "个函数地址:" << (int*)pVtab[0][i] << endl;//(int*)pVtab[0][i]等同于(int*)*((int*)*(int*)(&base) + 1) ((pFun)pVtab[0][i])(); } cout << "------------------------------------------------------------------" << endl; Derive derive; pVtab = (int**)&derive; for (int i = 0; pVtab[0][i] != NULL; ++i){ cout << "虚函数表存放第" << i + 1 << "个函数地址的地址:" << pVtab[0] + i << endl; //pVtab[0]+i等同于(int*)*(int*)(&derive) + i cout << "虚函数表第" << i + 1 << "个函数地址:" << (int*)pVtab[0][i] << endl; //(int*)pVtab[0][i]等同于(int*)*((int*)*(int*)(&derive) + 1) ((pFun)pVtab[0][i])(); } system("pause"); return 0; }2、运行结果:
3、类继承关系:
4、基类虚表指针及虚表布局:
5、继承类虚表指针及虚表布局:
6、说明
1)继承类覆盖的f()函数被放到了虚表中原来父类虚函数的位置。
三、多继承(无虚函数覆盖)
1、例子
#include<iostream> using namespace std; class Base1 { public: virtual void f() { cout << "Base1::f()" << endl; } virtual void g() { cout << "Base1::g()" << endl; } virtual void h() { cout << "Base1::h()" << endl; } }; class Base2 { public: virtual void f() { cout << "Base2::f()" << endl; } virtual void g() { cout << "Base2::g()" << endl; } virtual void h() { cout << "Base2::h()" << endl; } }; class Base3 { public: 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: virtual void f1(){ cout << "Derive::f1()" << endl; } virtual void g1(){ cout << "Derive::g1()" << endl; } virtual void h1(){ cout << "Derive::h1()" << endl; } }; int main(){ typedef void(*pFun)(); //定义一个函数指针,参数列表空,返回值void Derive derive; int** pVtab = (int**)&derive; for (int s = 0; s < 3; ++s){ for (int i = 0; pVtab[s][i] != NULL; ++i){ cout << "第" << s + 1 << "个虚函数表存放第" << i + 1 << "个函数地址的地址:" << pVtab[s] + i << endl; //pVtab[s]+i等同于 (int*)*((int*)(&derive) + s) + i cout << "第" << s + 1 << "个虚函数表存放第" << i + 1 << "个函数地址:" << (int*)pVtab[s][i] << endl; //(int*)pVtab[s][i]等同于(int*)*((int*)*((int*)(&derive) + s) + i) ((pFun)pVtab[s][i])(); } cout << "------------------------------------------------------------------" << endl; } system("pause"); return 0; }2、运行结果:
3、类继承关系:
4、继承类虚表指针及虚表布局:
5、说明
1)每个父类都有自己的虚表。
2)子类的虚拟成员函数指针被放到了第一个父类的表中(第一个父类是按照声明顺序来判断的)。
四、多继承(有虚函数覆盖)
1、例子:
#include<iostream> using namespace std; class Base1 { public: virtual void f() { cout << "Base1::f()" << endl; } virtual void g() { cout << "Base1::g()" << endl; } virtual void h() { cout << "Base1::h()" << endl; } }; class Base2 { public: virtual void f() { cout << "Base2::f()" << endl; } virtual void g2() { cout << "Base2::g2()" << endl; } virtual void h() { cout << "Base2::h()" << endl; } }; class Base3 { public: 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: virtual void f(){ cout << "Derive::f()" << endl; } virtual void g2(){ cout << "Derive::g2()" << endl; } virtual void h1(){ cout << "Derive::h1()" << endl; } }; int main(){ typedef void(*pFun)(); //定义一个函数指针,参数列表空,返回值void Derive derive; int** pVtab = (int**)&derive; for (int s = 0; s < 3; ++s){ for (int i = 0; pVtab[s][i] != NULL; ++i){ cout << "第" << s + 1 << "个虚函数表存放第" << i + 1 << "个函数地址的地址:" << pVtab[s] + i << endl; //pVtab[s]+i等同于 (int*)*((int*)(&derive) + s) + i cout << "第" << s + 1 << "个虚函数表存放第" << i + 1 << "个函数地址:" << (int*)pVtab[s][i] << endl; //(int*)pVtab[s][i]等同于(int*)*((int*)*((int*)(&derive) + s) + i) ((pFun)pVtab[s][i])(); } cout << "------------------------------------------------------------------" << endl; } Base1* base1 = &derive; base1->f();//Derive::f() Base2* base2 = &derive; base2->g2();//derive::g2() Base3* base3 = &derive; base3->h();//Base3::h() system("pause"); return 0; }2、运行结果:
3、类继承关系:
4、继承类虚表指针及虚表布局:
5、说明
1)三个父类虚函数表中的f()的位置被替换成了子类的函数指针。
2)Base2中的虚函数g2()的位置被替换成了子类的函数指针
posted on 2015-06-07 09:57 ruan875417 阅读(162) 评论(0) 编辑 收藏 举报