虚函数备忘

把关于虚函数不清楚的地方小测试了下，记录下来，备忘

测试编译器：mingw32-gcc-3.4.5

struct A{    virtual void f1(){        std::cout<<"A::f1"<<std::endl;    }    virtual void f2(){        std::cout<<"A::f2"<<std::endl;    }    void f3(){        std::cout<<"A::f3"<<std::endl;    }};struct B:A{    void f1(){        std::cout<<"B::f1"<<std::endl;    }    virtual void f2(){        std::cout<<"B::f2"<<std::endl;    }    void f3(int){        std::cout<<"B::f3"<<std::endl;    }};void vf_test(){    A a;    B b;    //now, p1 point to a B instance, "*p1" object's vpointer point to B's vtable    A *p1=new B;    //ok, p1 is A's pointer, so it calls none virtual function A::f3()    p1->f3();    //compile time error, notice that the member functions have the same name as the derived-class's will be rewrite.    //so, B::f3(int) hides A::f3()    //b.f3();    //calls A::f1()    (*p1).A::f1();    //calls B::f1()    (*p1).f1();    //calls B::f2()    (*p1).f2();    //(A(*p1)).f1() and (A(*p1)).f2() symbols will be linked to A's member function.    (A(*p1)).f1();    (A(*p1)).f2();    //but, notice that "*p1" object's vpointer point to B's vtable    //so, after type casting, "*p1" object's vpointer looks like already point to A's vtable at runtime    //let's take a test about it    //A's and B's object only have a vpointer of 4 bytes    std::cout<<"size of A is "<<sizeof(A)<<std::endl;    std::cout<<"size of B is "<<sizeof(B)<<std::endl;    std::cout<<"virtual pointer's value of type A is "<<*(unsigned long*)(&a)<<std::endl;    std::cout<<"virtual pointer's value of type B is "<<*(unsigned long*)(&b)<<std::endl;    std::cout<<"virtual pointer's value of p1 is "<<*(unsigned long*)p1<<std::endl;    std::cout<<"virtual pointer's value of &A(*p1) is "<<*(unsigned long*)(&(A(*p1)))<<std::endl;    delete p1;}

output:

A::f3

A::f1

B::f1

B::f2

A::f1

A::f2

size of A is 4

size of B is 4

virtual pointer's value of type A is 4508528

virtual pointer's value of type B is 4508544

virtual pointer's value of p1 is 4508544

virtual pointer's value of &A(*p1) is 4508528

这个测试主要是说明虚表指针的运行时切换问题，关于虚函数与类的作用域的较详细解释可参见《C++ Primer 4th Edition》15.5.4， 特别要注意派生类成员名字屏蔽基类成员名字的问题，这里只摘录其中一段对于继承体系中名字查找的小结供大家参考：

原文链接: https://www.cnblogs.com/tnt_vampire/archive/2010/05/15/1736240.html

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