c来实现虚函数

#include <stdio.h> 

typedef struct
{
   void (*draw)(void*); //虚函数
   void (*rotate)(void*);
}vtable;

typedef struct
{
   vtable *vptr;   //虚表指针
}Shape;

typedef struct
{
   vtable *vptr;
   int x,y,w,h;
}Rect;


typedef struct
{
   vtable *vptr;
   int x,y,r;
}Cycle;

void DrawRect( void *p )
{
  Rect *me = (Rect *)p;
  //............
  printf( "DrawRect\n" );
}
void RotateRect( void *p )
{
  Rect *me = (Rect *)p;
  //............
  printf( "RotateRect\n" );
}
void DrawCycle( void *p )
{
  Cycle *me = (Cycle *)p;
  //............
  printf( "DrawCycle\n" );
}
void RotateCycle( void *p )
{
  Cycle *me = (Cycle *)p;
  //............
  printf( "RotateCycle\n" );
}

vtable RectOpers = { &DrawRect, &RotateRect };   //虚函数表定义
vtable CycleOpers = { &DrawCycle, &RotateCycle };

int main() 
{ 
	
    Rect rt1 = { &RectOpers, 0, 0, 10, 10 }; //实体类的实现
    Cycle cy1 = { &CycleOpers, 10, 10, 20 };
    
    Shape *sh = (Shape *)&rt1;
    sh->vptr->draw(sh);
    sh->vptr->rotate(sh);

	sh = (Shape *)&cy1;
	sh->vptr->draw(sh);
	sh->vptr->rotate(sh);
	

	return 0;
} 

vtable 实现的是一个虚函数表。三个struct实现每一个第一个位置放的都是vtable，Shape为基类，只有一个vtable *vptr。

强制类型转换后，取每一个子类的第一个指针，也就是vtable所指的指针。

而vtable RectOpers = { &DrawRect, &RotateRect };   //虚函数表定义
vtable CycleOpers = { &DrawCycle, &RotateCycle };‘

已经初始化了指针的值，故而调用sharp 的时候，实质上是调用的已经赋值的指针。实现了虚函数的作用。