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：

关键概念：名字查找与继承；理解c++中继承的关键在于理解如何确定函数调用。确定函数调用遵循以下四个步骤：

1. 首先确定进行函数调用的对象、引用或指针的静态类型。
2. 在该类中查找函数，如果找不到，就在直接基类中查找，如此循着类的继承链往上找，直到找到该函数或者查找完最后一个类。如果不能在类或其相关基类中找到该名字，则调用是错误的。
3. 一旦找到了该名字，就进行常规类型检查，查看如果给定找到的定义，该函数调用是否合法。
4. 假定函数调用合法，编译器就生产代码。如果函数是虚函数且通过引用或指针调用，则编译器生成代码以确定根据对象的动态类型运行哪个函数版本，否则，编译器生成代码直接调用函数。