c语言杂谈系列：模拟虚函数

从整体来看，笔者的做法与之前的模拟多态十分相似，毕竟c++多态的实现与虚函数密切相关

废话少说，see my code：

kernel.c#include "kernel.h"
#include <stdio.h>void shape_draw(struct shape_t* obj) {/* Call draw of the real Instance */obj->vtable->draw();}

kernel.h:#ifndef UNTITLED_KERNEL_H
#define UNTITLED_KERNEL_Hstruct shape_t {/*Virtual Method Table */const struct shape_interface* const vtable;
};struct shape_interface {void (*draw)();};void shape_draw(struct shape_t* obj);//obj->vtable->draw();#endif //UNTITLED_KERNEL_H

try.c:#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "try.h"void draw() {printf("error is try!\n");
}void draw1() {printf("error is try2!\n");
}struct shape_t* shape_create_rectangle() {//直接赋值,这里有个命名错误，跟gcc有关：//static const struct shape_interface_t vtable = { draw1 } ;//static struct shape_t base = { &vtable };//笔者认为给结构体成员赋值，下面的写法更妥当static const struct shape_interface vtable = { .draw = draw1} ;static struct shape_t base = { .vtable = &vtable};//推荐上面这种写法，因为某些编译器很有趣struct rectangle_t* rectangle = malloc(sizeof(*rectangle));memcpy(&rectangle->base, &base, sizeof(base));return (struct shape_t*)(&rectangle->base);
}

顺便一提，clion的编译器相当有趣

笔者在之前曾经写错了shape_interface (_t)结构体名称,但是笔者发现：

//static const struct shape_interface_t *vtable = { draw1 } ;
//static struct shape_t base = { &vtable };

改成这样也能运行

这是为什么呢？笔者推测，gcc应该是无法找到对于结构体，就把vtable当成了数组，加上*就成为了数组。然后&vtable就成为了二级指针，由于draw1本身就是一个指针，把它转成空指针什么的可以随便赋值。gcc在找不到对应结构体后，索性为base里的vtable开辟了一段空间，由于&vatble是二级指针，但是找不到对应地址指向，可能它在编译过程中被转为了一级空指针，且等于draw1本身，这样就能解释通了。（如果有c语言高手可以留言解答一下，笔者对c语言和编译器的处理所知甚少）

try.h:#ifndef UNTITLED_TRY_H
#define UNTITLED_TRY_H#include "kernel.h"struct rectangle_t {struct shape_t* base; /* Reference to Base Class *//* Rectangle specific Members */int x;int y;
};struct shape_t* shape_create_rectangle();#endif //UNTITLED_TRY_H

在主函数中这样调用即可：

main.c:#include "try.h"
#include "kernel.h"int main() {struct shape_t* rectangle = shape_create_rectangle();shape_draw(rectangle);return 0;
}

接下来是重点，虚函数表的实现，可以适当改动try.c文件：

try.c://
// Created by el on 2024/8/16.
//#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "try.h"void draw() {printf("error is try!\n");
}void draw1() {printf("error is try2!\n");
}struct shape_t* shape_create_rectangle() {//static const struct shape_interface *vtable[] = { draw , draw1 } ;static const struct shape_interface_t *vtable[] = { .vtable = draw} ;static struct shape_t base = { vtable + 1};struct rectangle_t* rectangle = malloc(sizeof(*rectangle));memcpy(&rectangle->base, &base, sizeof(base));return (struct shape_t*)(&rectangle->base);
}

使用函数指针数组，就可以模拟出比较相近的虚函数表。

整个c程序的UML图如下：

其实这张图跟笔者前一篇模拟多态的文章思想是一样的。