C++ 对象模型学习记录（2） 第3章 data语义学

1. 关于sizeof的运行结果

#include <iostream>
//在GCC 中
using namespace std;
class X{};
class Y :public X{};
class Z : public X{};
class W :public Y,public Z{};
int main()
{
    cout << sizeof(X) << endl;//1
    cout << sizeof(Y) << endl;//1
    cout << sizeof(Z) << endl;//1
    cout << sizeof(W) << endl;//2
    return 0;
}

　　而在vs 2010中是默认的编译器中

#include <iostream>

using namespace std;
class X{};
class Y :public X{};
class Z : public X{};
class W :public Y,public Z{};
int main()
{
    cout << sizeof(X) << endl;//1
    cout << sizeof(Y) << endl;//1
    cout << sizeof(Z) << endl;//1
    cout << sizeof(W) << endl;//1
    return 0;
}

　　加了虚继承后，GCC和vs的竟然相同了，总的来说就是有了 virtual 继承，加了一个vptr指针和指向的是一个 要么是virtual base table，要么是offset

#include <iostream>

using namespace std;
class X{};
class Y :public virtual X{};
class Z : public virtual X{};
class W :public Y,public Z{};
int main()
{
    cout << sizeof(X) << endl;//1
    cout << sizeof(Y) << endl;;//4
    cout << sizeof(Z) << endl;//4
    cout << sizeof(W) << endl;//8
    return 0;
}

　2. 关于指向data member 数据成员的指针

#include <iostream>
#include <stdio.h>
using namespace std;
struct  Base1{int val1; };
struct  Base2{int val2; };
struct Derived : Base1,Base2 {};

//Derived2为了和Derived比较

struct Derived2 : Base1,Base2{int val3;};
void func1(int Derived::*dmp,Derived *pd)
{
    //期望传进来的是一个"指向 derived class 的member "的指针
    //如果传进来的是“指向base class 的member”的指针，
    //结果如何？
    cout << pd->*dmp << endl;
}
void func2(Derived *pd)
{
    //bmp将成为1
    int Base2::* bmp = &Base2::val2;//Base2::* 型的变量
     cout << bmp << endl;
     printf("~~~~~~~~~~~~~~~~~~~%d\n",bmp);

    func1(bmp,pd);
    //这里必须由编译器内部转换
   // func1(,pd);
}
int main()
{
     Base1 base1;
     base1.val1 = 1;
     Base2 base2;
     base2.val2 = 2;
     Derived d;
     Derived *p = &d;
     func2(p);
     cout << "-------------------" << endl;
     //以下结果都是0
     printf("&Base1::val1 = %p\n",&Base1::val1);
     printf("&Base2::val2 = %p\n",&Base2::val2);
     printf("&Derived::val1 = %p\n",&Derived::val1);
     printf("&Devrived::val2 = %p\n",&Derived::val2);

     cout << &Base1::val1 << endl;
     cout << &Base2::val2 << endl;
     cout << &Derived::val1 << endl;
     cout << &Derived::val2 << endl;

     cout << "--------------" << endl;
     printf("&Derived2::val1 = %p\n",&Derived2::val1);
     printf("&Devrived2::val2 = %p\n",&Derived2::val2);
     printf("&Derived2::val = %p\n", &Derived2::val3 );

      cout << &Derived2::val1 << endl;
     cout << &Derived2::val2 << endl;
     cout << &Derived2::val3 << endl;
    return 0;
}

　　运行结果如下：GCC和VC下都是同样的结果

可以看出的是 在c++中cout 和printf 输出的结果竟然还是不一样的 == ，应该是 C++重载了 <<运算符，使<<能够适应各种输出的原因吧

 int Base2::* bmp = &Base2::val2;//Base2::* 型的变量
     cout << bmp << endl;

通过 cout 可以看出这里输出的就是1，但是 在GCC下 不能转化为 int型的，所以才有了调用   func1(bmp,pd); 并在函数中 cout << pd->*dmp << endl; 时得到了 一个随机数 

对于C++对象模型中所说的 &Base1::val1 中val 指的是 val在类的对象中的偏移量问题，只能通过printf("&Base1::val1 = %p\n",&Base1::val1);看到，cout 方式可能还是因为重载的缘故吧：所有的输出都是1（不知道原因，可能不是偏移量了吧）

printf("&Devrived::val2 = %p\n",&Derived::val2); 对于&Derived::val2实际的偏移量应该是4，可能由于编译器的优化，输出为0了。。（怎么优化的？ 想不明白）

 书中以   printf("&Derived2::val = %p\n", &Derived2::val3 ); 打印出了 val3的偏移量为8，可以看出

3. 那个point2d，3d问题

#include <iostream>
#include <stdio.h>
using namespace std;
class Point3d
{
    public:
    virtual ~Point3d();
    Point3d(float a,float b,float c)
    {
        x = a;
        y = b;
        z = c;
       // origin(x,y,z);
    }
    static Point3d origin;
    static void f()
    {
        //error: request for member 'x' in 'Point3d::origin',
        //which is of non-class type 'Point3d()'
        //printf("&origin.x = %p\n",&(origin.x));
        //printf("&origin.y = %p\n",&origin.y);
        //printf("&origin.z = %p\n",&origin.z);
       // printf("&origin = %p\n",&origin);
    }
    float x,y,z;
};

int main()
{
    float f = 1.0;
    cout << sizeof(f) << endl;
    cout << sizeof(float) << endl;
    cout << sizeof(double) << endl;
    printf("&Point3d::x = %p\n",&Point3d::x);//计算的是偏移量 4
    printf("&Point3d::y = %p\n",&Point3d::y);//4  ，结果是8
    printf("&Point3d::z = %p\n",&Point3d::z);//4 结果是C（12）

    cout << "&Point3d::x = " <<  &Point3d::x << endl; 
    cout << "&Point3d::y = " << &Point3d::y << endl;
    cout << "&Point3d::z = " << &Point3d::z << endl;
    //undefined reference to `Point3d::origin'
   // printf("&origin.x = %p\n",&(Point3d::origin.x));
   // printf("&origin.y = %p\n",&Point3d::origin.y);
   // printf("&origin.z = %p\n",&Point3d::origin.z);
    Point3d p();

    //Point3d::f();


    return 0;
}

木有打印出来 origin.x ，错误是 

（1） //undefined reference to `Point3d::origin' ，

（2）//error: request for member 'x' in 'Point3d::origin',

        //which is of non-class type 'Point3d()'

将 virtual ~Point3d(); 去掉虚函数～，变为 Point3d（）并完成初始化后：Point3d Point3d::origin(1,1,1);

#include <iostream>
#include <stdio.h>
using namespace std;
class Point3d
{
    public:
     ~Point3d(){};
    Point3d(float a,float b,float c)
    {
        x = a;
        y = b;
        z = c;
    }
    static Point3d origin;

    float x,y,z;
};
Point3d Point3d::origin(1,1,1);
int main()
{
    float f = 1.0;
    cout << sizeof(f) << endl;
    cout << sizeof(float) << endl;
    cout << sizeof(double) << endl;
    printf("&Point3d::x = %p\n",&Point3d::x);
    printf("&Point3d::y = %p\n",&Point3d::y);
    printf("&Point3d::z = %p\n",&Point3d::z);

    cout << "&Point3d::x = " <<  &Point3d::x << endl;
    cout << "&Point3d::y = " << &Point3d::y << endl;
    cout << "&Point3d::z = " << &Point3d::z << endl;
    //undefined reference to `Point3d::origin'
    printf("&origin.x = %p\n",&(Point3d::origin.x));
    printf("&origin.y = %p\n",&Point3d::origin.y);
    printf("&origin.z = %p\n",&Point3d::origin.z);

    //Point3d::f();


    return 0;
}

　　

结果如下：

对比与 两个图中的 &Point3d::x 值得出结论：

在GCC编译器中，如果加了 virtual ，它的对象布局中应该是 vptr在 数据成员的上面

而不是像 c++对象模型 cfont 编译器中在数据成员下面