C++对象内存布局⑤GCC编译器单个虚拟继承

C++对象内存布局--⑤GCC编译器--单个虚拟继承

　　测试GNU的GCC编译器在处理虚拟继承上跟VS不同的地方。派生类的虚函数表跟虚基类表合并。

//GCC编译器--单个虚拟继承.cpp
//2010.8.18
//虚基类表到底是那个？，还是说派生类的虚函数表的上边和下边都是？
//GCC编译器
#include <iostream>
using   namespace std;
//////////////////////////////////////////////////////////////////
class Base
{
    public:
        Base(int a = 10):a(a)
        {
            cout << "Base::Base()" << endl;
        }
        virtual void show1()
        {
            cout << "Base::show1()" << endl;
            cout << a << endl;
        }
    private:
        int a;
};
//////////////////////////////////////////////////////////////////
class Derived : virtual public Base
{
    public:
        Derived(int b = 100):b(b)
        {
            cout << "Derived::derived()" << endl;
        }
        virtual void show2()
        {
            cout << "Derived::show2()" << endl;
            cout << b << endl;
        }
    private:
        int b;
};
//////////////////////////////////////////////////////////////////
int main()
{
    Derived obj;
    int** p = (int**)&obj;
    cout << "虚拟继承了基类的派生类的对象内存布局：" <<endl;
    for (int i = 0; i != sizeof(obj)/4; ++i)
    {
        cout << p[i] << endl;
    }
     typedef void (*fun)(void*pThis);//this指针非常重要
    cout << endl << "第一虚函数表第一项，虚函数Derived::show2()地址：" << (int*)p[0][0] << endl;
    ((fun)(p[0][0]))(p);
    cout << "第二虚函数表第一项，虚函数Base::show1()地址   ：" << (int*)p[2][0] << endl;
    ((fun)(p[2][0]))(p+2);
    cout << endl << "派生类虚函数表指针往低地址方向寻址：" << endl;
    cout << "p[0][-1] = " << (int*)p[0][-1] << endl;
    cout << "p[0][-2] = " << (int*)p[0][-2] << endl;
    cout << "p[0][-3] = " << (int*)p[0][-3] << endl;

    cout << endl << "派生类虚函数表指针往高地址方向寻址：" << endl;
    cout << "p[0][0] = " << (int*)p[0][0] << "(这个是show2()函数地址)" << endl;
    cout << "p[0][1] = " << (int*)p[0][1] << endl;
    cout << "p[0][2] = " << (int*)p[0][2] << endl;
    cout << "p[0][3] = " << (int*)p[0][3] << endl;

    cout << endl << "是否往高方向寻找和往地方向寻找所找到的都是虚基类表呢？" << endl;
    //system("pause");
    return 0;
}
/*
Base::Base()
Derived::derived()
虚拟继承了基类的派生类的对象内存布局：
0x472d4c
0x64
0x472d5c
0xa

第一虚函数表第一项，虚函数Derived::show2()地址：0x41cd50
Derived::show2()
100
第二虚函数表第一项，虚函数Base::show1()地址   ：0x41ccbc
Base::show1()
10

派生类虚函数表指针往低地址方向寻址：
p[0][-1] = 0x471388
p[0][-2] = 0
p[0][-3] = 0x8

派生类虚函数表指针往高地址方向寻址：
p[0][0] = 0x41cd50(这个是show2()函数地址)
p[0][1] = 0
p[0][2] = 0xfffffff8
p[0][3] = 0x471388

是否往高方向寻找和往地方向寻找所找到的都是虚基类表呢？
*/