C++RTTI小总结

标签：c++   rtti   vtable   vptr   运行时型别鉴定   

        RTTI算是C++的一大特性之一了，但也是一个不太好懂的特性。以前一直没有理解清楚RTTI。通过最近的学习，似乎掌握了一点皮毛，这里做一个小总结。首先是《C++编程思想》上的一个例子，由于缺头文件，我把容器改成了vector。

#include <iostream>
#include <time.h>
#include <typeinfo.h>
#include <vector>
#include <stdlib.h>  //for srand() and rand()
using namespace std;

class shape {
protected:
    static int count;
public:
    shape() { count++; }
    virtual ~shape() { count--; }
    virtual void draw() const = 0;
    static int quantity() {return count;}
};

int shape::count = 0;

class rectangle:public shape {
    void operator=(rectangle&);
protected:
    static int count;
public:
    rectangle() {count++;}
    ~rectangle() {count--;}
    rectangle(const rectangle&) {count++;}
    void draw() const {
        cout << "rectangle::draw()"<<endl;
    }
    static int quantity() {return count;}

};

int rectangle::count = 0;

class ellipse: public shape {
    void operator=(ellipse&);
protected:
    static int count;
public:
    ellipse() {count++;}
    ellipse(const ellipse&) {count++;}
    ~ellipse() { count--;}
    void draw() const{
        cout << "ellipse::draw()" <<endl;
    }
    static int quantity() {return count;}
};

int ellipse::count = 0;

class circle : public shape {
    void operator=(circle&);
protected:
    static int count;
public:
    circle() {count++;}
    circle(const circle&) {count++;}
    ~circle() {count--;}
    void draw() const {
        cout << "circle::draw()" << endl;
    }
    static int quanlity() {return count;}
};
int circle::count = 0;

int main()
{
    vector<shape*> shapes;
    time_t t;
    srand((unsigned)time(&t));
    const int mod = 12;
    for(int i = 0;i<rand()%mod; i++)
        shapes.push_back(new rectangle);
    for(int j = 0;j<rand() %mod; j++)
        shapes.push_back(new ellipse);
    for(int k = 0;k<rand() %mod; k++)
        shapes.push_back(new circle);
    int Ncircles = 0;
    int Nellipses = 0;
    int Nrects = 0;
    int Nshapes = 0;
    for(int u = 0;u<shapes.size();u++) {
        shapes[u]->draw();
        if(dynamic_cast<circle*>(shapes[u]))
                Ncircles++;
        if(dynamic_cast<ellipse*>(shapes[u]))
                Nellipses++;
        if(dynamic_cast<rectangle*>(shapes[u]))
                Nrects++;
        if(dynamic_cast<shape*>(shapes[u]))
                Nshapes++;

    }
    cout << endl <<endl
         << "circles = " << Ncircles <<endl
         << "ellipses = " << Nellipses <<endl
         << "rectangles = " << Nrects <<endl
         << "shapes = " << Nshapes <<endl
         << endl
         << "circle::quantity() = "
         << circle::quanlity() <<endl
         << "ellipses::quantity() = "
         << ellipse::quantity() << endl
         << "rectangle::quantity() = "
         << rectangle::quantity() << endl
         << "shape::quantity() = "
         << shape::quantity() << endl;
    shapes.clear();
}

class B {
public:
	virtual float f() {return 1.0;}
};
class D : public B {          
};


B* p = new D;
B& r = *p;

  多态类型指的是基类中没有虚函数的类型。典型的RTTI是通过在 VTABLE中放一个额外的指针来实现的。这个指针指向一个描述该特定类型的 typeinfo结构（每个新类只产生一个typeinfo的实例），所以typeid()表达式的作用实际上很简单。VPTR用来取typeinfo的指针，然后产生一个结果typeinfo结构的一个引用。dynamic_cast()的过程稍微复杂点，但也需要利用VTABLE中的信息来进行类型转换。

  我不知道怎么去访问到VTABLE的typeinfo信息，倒是在找到了一个访问VTABLE中函数信息的例子。这个例子告诉我们VTABLE不止在书上，感兴趣的可以运行一下，或者改改，有可能能找到typeinfo哟！

#include <iostream>

using namespace std;

class Base
{
private:
    int a;
public:
    virtual void fun1()
    {
        cout<<"Base::fun1()"<<endl;
    }

    virtual void fun2()
    {
        cout<<"Base::fun2()"<<endl;
    }

    virtual void fun3()
    {
        cout<<"Base::fun3()"<<endl;
    }

};

class A:public Base
{
private:
    int a;
public:
    void fun1() {
        cout<<"A::fun1()"<<endl;
    }
    void fun2() {
        cout<<"A::fun2()"<<endl;
    }

};
void foo(Base& obj)
{
    obj.fun1();
    obj.fun2();
    obj.fun3();
}

//int main()
//{
//    Base b;
//    A a;
//    foo(b);
//    foo(a);
//}
void *getp(void* p)
{
    return (void*)*(unsigned long *)p;
}
typedef void (*fun)();

fun getfun(Base* obj, unsigned long off)
{
    void *vptr = getp(obj);
    unsigned char *p = (unsigned char *)vptr;
    p += sizeof(void *) * off;
    cout<<(int)p<<endl;
    return (fun)getp(p);
}

int main()
{
    Base *p = new A;
    fun f = getfun(p,0);
    (*f)();
    f = getfun(p,1);
    (*f)();
    f = getfun(p,2);
    (*f)();
    delete p;
}

C++RTTI小总结

标签：c++   rtti   vtable   vptr   运行时型别鉴定   

原文地址：http://blog.csdn.net/cqu20093154/article/details/44044545