激情与梦想

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http://zongtongyi.blogbus.com/logs/48515553.html

There days, I joined a written examination, and I found a intereting topic about C++'s polymiorphism, so I wrote some extend codes after I came back, here it is:

#include <iostream>
using namespace std;

class Base
{
public:

    int b;
    virtual void f(void){cout << "Base::f(int) " << endl;}
};

class Derive:public Base
{
public:

    int d;
    virtual void f(void){cout << "Derive::f(int) " << endl;}
};

int main()
{
    Base b;
    Derive d;
    Base *pb = &d;
    Base &rb = d;
    b.f();
    d.f();
    pb->f();
    rb.f();

    cout <<sizeof(int)<<endl
        <<sizeof(Base) << endl
        <<sizeof(*pb) << endl
        <<sizeof(Derive)<<endl;

    return true;
}

Compiled on Ubuntu 8.04 、 g++ .
the f(void) with or without "virtual" will change the output

1、without virtual, the output will be :

Base::f(int)
Derive::f(int)
Base::f(int)
Base::f(int)
4
4
4
8

because the Derive overlap Base's f(void), no matter pointer or refer

sizeof(Base) is "int b" 's size, it's 4; sizeof(*pb) is the same to sizeof(Base); sizeof(Derive) is "int b" plus "int d", so it's 8.

2、with virtual, the output will be :

Base::f(int)
Derive::f(int)
Derive::f(int)
Derive::f(int)
4
8
8
12

because the Derive overload Base's f(void), no matter pointer or refer

sizeof(Base) is "int b" plus "virtual f(void)" 's pointer, pointer's size is 4; sizeof(*pb) is the same to sizeof(Base); sizeof(Derive) is "int b" plus "int d" plus "virtual f(void)" 's pointer, just one pointer for "virtual f(void)".