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看了lighttpd中的fdevent的实现过程有些巧妙，写个例子模拟它的的实现：
虚基类是fdos（os操作系统，fd是随便加的没什么意义），它有三个派生类分别是：windows, linux, mac，
它们的都有的public方法是：open, close, playgame。
/// fdos.h#ifndef FDOS_H_INCLUDED
#define FDOS_H_INCLUDEDtypedef enum
{OS_UNSET = -1,OS_WINDOWS,OS_LINUX,OS_MAC
}fd_type;typedef struct fdos
{fd_type type;int maxfds;int (*open)(struct fdos *os);void (*close)(struct fdos *os);void (*playgame)(struct fdos *os, char *name);
}fdos;// public
fdos* fdos_init(fd_type type, int maxfds);
void fdos_free(fdos *os);int fdos_open(fdos *os);
void fdos_close(fdos *os);
void fdos_playgame(fdos *os, char *name);// private
int fdos_windows_init(fdos *os);
int fdos_linux_init(fdos *os);
int fdos_mac_init(fdos *os);#endif // FDOS_H_INCLUDED/// fdos.cpp#include "fdos.h"
#include <stdio.h>
#include <stdlib.h>fdos* fdos_init(fd_type type, int maxfds)
{fdos *os;os = (fdos*)malloc(sizeof(*os));os->maxfds = maxfds;switch (type){case OS_WINDOWS:if (0 != fdos_windows_init(os)){printf("windows init fail\n");return NULL;}return os;case OS_LINUX:if (0 != fdos_linux_init(os)){printf("linux init fail\n");return NULL;}return os;case OS_MAC:if (0 != fdos_mac_init(os)){printf("mac init fail\n");return NULL;}return os;case OS_UNSET:break;}printf("fdos init fail\n");return NULL;
}void fdos_free(fdos *os)
{if (os){free(os);}
}int fdos_open(fdos *os)
{return os->open(os);
}void fdos_close(fdos *os)
{return os->close(os);
}void fdos_playgame(fdos *os, char *name)
{return os->playgame(os, name);
}
/// fdwindows.cpp#include "fdos.h"
#include <stdio.h>static int fdos_windows_open(fdos *os)
{printf("windows open\n");return 0;
}static void fdos_windows_close(fdos *os)
{printf("windows close\n");
}static void fdos_windows_playgame(fdos *os, char *name)
{printf("windows playgame: %s\n", name);
}int fdos_windows_init(fdos *os)
{os->type = OS_WINDOWS;// x作为宏参数，##x则表示连接宏参数// 如果是#x, 则表示是一个字符串，如：// #define MY_CONST(x) #x// printf("%s", MY_CONST(hello));// 输出:hello
#define SET(x) \os->x = fdos_windows_##x;// 相当于：fdos_windows_openSET(open);// 相当于：fdos_windows_closeSET(close);// 相当于：fdos_windows_playgameSET(playgame);return 0;
}
/// fdlinux.cpp#include "fdos.h"
#include <stdio.h>static int fdos_linux_open(fdos *os)
{printf("linux open\n");return 0;
}static void fdos_linux_close(fdos *os)
{printf("linux close\n");
}static void fdos_linux_playgame(fdos *os, char *name)
{printf("linux playgame: %s\n", name);
}int fdos_linux_init(fdos *os)
{os->type = OS_LINUX;
#define SET(x) \os->x = fdos_linux_##x;SET(open);SET(close);SET(playgame);return 0;
}
/// fdmac.cpp#include "fdos.h"
#include <stdio.h>static int fdos_mac_open(fdos *os)
{printf("mac open\n");return 0;
}static void fdos_mac_close(fdos *os)
{printf("mac close\n");
}static void fdos_mac_playgame(fdos *os, char *name)
{printf("mac playgame: %s\n", name);
}int fdos_mac_init(fdos *os)
{os->type = OS_MAC;
#define SET(x) \os->x = fdos_mac_##x;SET(open);SET(close);SET(playgame);return 0;
}
/// main.cpp#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include "fdos.h"/// test
int main(void)
{fdos *os = fdos_init(OS_WINDOWS, 3);fdos_open(os);fdos_playgame(os, "wow");fdos_close(os);fdos_free(os);os = fdos_init(OS_LINUX, 3);fdos_open(os);fdos_free(os);os = fdos_init(OS_MAC, 3);fdos_playgame(os, "zx");fdos_free(os);return 0;
}
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