上篇我们尽量说一些宏观上的东西，数据成员与函数成员在类中的布局将在微观篇中具体讨论。

假设你对这些东西也感兴趣，那么好，我们一起将class的底层翻个底朝天。修炼好底层的内功，我想对于上层的提供。帮助可不止一点点吧？

typedef struct point3d{ //数据
      float x;
      float y;
      float z;
}Point3d;
void Point3d_print(const Point3d *pd{
      printf("%g,%g,%g",pd->x,pd->y,pd->z);
}

class Point3d{
      float _x;
      float _y;
      float _z;
public:
      void point3d_print(){
            printf("%g,%g,%g",_x,_y,_z);
      }
};

那么以下我们举样例来证明上述的讨论。

class A{};         // sizeof(A) = 1  有木有非常奇怪？稍后说明
class B{int x;};   //  sizeof(B) = 4
class C{
      int x;
public:
      int get(){return x;}
};                 //  sizeof(C) = 4; 是不是验证了我们上述的论述？

这样做使得用同一个空类定义两个对象的时候得以在内存中配置独一无二的地址。

A a,b;
if(&a == &b)cout<<"error"<<endl;

class A{
	char x;
	int y;
	char z;
};  // sizeof(A) == 12;
class B{
	char x;
	char y;
	int z;
};  // sizeof(B) = 8;
class C{
	int x;
	char y;
	char z;
};  // sizeof(C) = 8;
class D{
	long long x;
	char y;
	char z;
};  //sizeof(D) = 16; 因为longlong为8字节大小，此处以8字节对齐

static和nonstatic function members 则被放在对象之外。

class A{
	static int x;
};  //sizeof(A) = 1;
class B{
	int x;
public:
	int get(){
		return x;
	}
};  //sizeof(B) = 4
class C{
	int x;
public:
	virtual int get(){
		return x;
	}
};  //sizeof(C) = 8;

class A{
	void (*pf)(); //函数指针
};   //sizeof(A) = 4;
class B{
	int *p;   // 指针
};   //sizeof(B) = 4;

class X{
	int a;
	int b;
public:
	virtual void foo1(){cout<<"X::foo1"<<endl;}
	virtual void foo2(){cout<<"X::foo2"<<endl;}
};

#include<iostream>
using namespace std;
class X{
	int _a;
	int _b;
public:
	virtual void foo1(){cout<<"X::foo1"<<endl;}
	virtual void foo2(){cout<<"X::foo2"<<endl;}
};
typedef void (*pf)();
int main(){
	X a;
	int **tmp = (int **)&a;
	pf ptf;
	for(int i=0;i<2;i++){
		ptf = (pf)tmp[0][i];
		ptf();
	}
}

class A{
      int x;
};
class B:public A{
      int y;
};   //sizeof(B) = 8;

class C{
public:
	virtual void fooC(){
		cout<<"C::fooC()"<<endl;
	}
};  //sizeof(C) = 4;
class D:public C{
	int a;
public:
	virtual void fooD(){
		cout<<"D::fooD()"<<endl;
	}
};  //sizeof(D) = 8;

typedef void (*pf)();
int main(){
	C a;
	D b;
	int **tmpc = (int **)&a;
	int **tmpb = (int **)&b;
	pf ptf;
	ptf = (pf)tmpc[0][0];
	ptf();
	ptf = (pf)tmpb[0][0];
	ptf();
	ptf = (pf)tmpb[0][1];	
	ptf();
}

class A{
	int _a;
public:
	virtual void fooA(){
		cout<<"C::fooA()"<<endl;
	}
	virtual void poo(){
		cout<<"A::poo()"<<endl;
	}
};  //sizeof(A) = 8;
class B{
	int _b;
public:
	virtual void fooB(){
		cout<<"C::fooB()"<<endl;
	}
	virtual void poo(){
		cout<<"B::poo()"<<endl;
	}
};  ////sizeof(B) = 8;
class C:public A,public B{
	int _c;
public:
	void poo(){
		cout<<"C::poo()"<<endl;
	}
	virtual void hoo(){
        cout<<"C::hoo()"<<endl;
    }
};    //sizeof(C) = 20;

typedef void (*pf)();
int main(){
	C a;
	int **tmp = (int **)&a;
	pf ptf;
	for(int i=0;i<3;++i){
		ptf = (pf)tmp[0][i];
		ptf();
	}
	cout<<"-----------"<<endl;
	int s = sizeof(A)/4; //指针与int都占用4字节大小
	for(int i=0;i<2;i++){
		ptf = (pf)tmp[2][i];
		ptf();
	}
}

class A{
	int _a1;
	int _a2;
};    //sizeof(A) = 8;
class B:virtual public A{
	int b;
};    //sizeof(B) = 16;
class C:virtual public A{
	int c;
};    //sizeof(C) = 16;
class D:public B,public C{
	int d;
};    //sizeof(D) = 28;

int main(){
	D d;
	A *pta = &d;
	B *ptb = &d;
	C *ptc = &d;
	cout<<"D:  "<<&d<<endl;
	cout<<"B:  "<<ptb<<"   C:  "<<ptc<<endl;
	cout<<"A:  "<<pta<<endl;
}