C语言中的多态应用问题

Question

这是我在stackowerflow 中的第一个问题。所以我很兴奋:) 我正在尝试在 C 中练习多态性。我读过很多文章。我用我所理解的编写了一个小示例代码。我的目标是创建一个形状类。此类包括字段和长度方法。派生自该类的矩形和圆形类对字段和长度方法进行自己的计算。

但是编译时出现问题： enter image description here 在 OOP_in_C_example_3_polymorphism_3.exe 中的 0x000000000000000A 处引发异常：0xC0000005：访问冲突执行位置 0x000000000000000A。

当我删除其中一行时它会变得更好。但这一次它根据最后调用的构造函数进行操作： enter image description here

我哪里有问题？

形状.h：

#include <stdint.h>

struct Vtable;

typedef struct{
    struct Vtable* vptr;
    uint16_t x;
    uint16_t y;

}Shape;

struct Vtable {
    void(*area)(Shape* me);
    void(*lenght)(Shape* me);
};

inline void area(Shape* me) {
    (*me->vptr->area)(me);
}

inline void lenght(Shape* me) {
    (*me->vptr->lenght)(me);
}

void shape_ctor(Shape* me, int x, int y);

shape.c:

#include "shape.h"

void shape_ctor(Shape* me,int x, int y) {   
    struct Vtable vtbl = { &area,&lenght };
    me->vptr = &vtbl;
    me->x   = x;
    me->y   = y;
}

rect.h:

#include "shape.h"

typedef struct  {
    Shape super;
    uint16_t en;
    uint16_t boy;
}rect;

void rect_ctor(rect* me, uint16_t x, uint16_t y, uint16_t en, uint16_t boy);

rect.c:

#include "rect.h"
#include <stdio.h>

void rect_lenght(void* me);
void rect_area(void* me);


void rect_ctor(rect* me,uint16_t x, uint16_t y, uint16_t en, uint16_t boy) {
    struct Vtable vtbl = { &rect_area, &rect_lenght };
    
    shape_ctor(&me->super, x, y);
    me->super.vptr = &vtbl;                 //override vptr
    me->en  = en;
    me->boy = boy;
}


void rect_lenght(void* me) {
    rect* me_ = (rect*)me;      //downcast işlemi
    printf("rectangle lenght: %d\n",(me_->en + me_->boy) * 2);
}
void rect_area(void* me) {
    rect* me_ = (rect*)me;      //downcast işlemi
    printf("rectangle area: %d\n", (me_->en)*(me_->boy));
}

circle.h:

#include "shape.h"

typedef struct {
    Shape super;
    uint16_t radius;
}circle;


void circle_ctor(circle* me, uint16_t x, uint16_t y, uint16_t r);

circle.c:

#include "circle.h"
#include <stdio.h>

void circle_lenght(void* me);
void circle_area(void* me );


void circle_ctor(circle* me, uint16_t x, uint16_t y, uint16_t r) {
    struct Vtable vtbl = {&circle_area,&circle_lenght};

    shape_ctor(&me->super, x, y);
    me->super.vptr = &vtbl;
    me->radius = r;
}


void circle_lenght(void* me) {
    circle* me_ = (circle*)me;
    printf("circle lenght: %f\n", (me_->radius) * (2) * (3.14));
}
void circle_area(void* me){
    circle* me_ = (circle*)me;
    printf("circle area: %f\n", (me_->radius) * (me_->radius) * (3.14));
}

main.c:

#include <stdio.h>
#include "rect.h"
#include "circle.h"


int main() {
    rect  k1;
    circle d1;

    rect_ctor(&k1, 10, 10, 15, 20);
    circle_ctor(&d1, 10, 10, 5);
    
    
    k1.super.vptr->lenght(&k1);
    d1.super.vptr->lenght(&d1);
}

Answer 1

但是编译时出现问题：在此处输入图片描述

这是在执行期间，而不是在编译期间

你有未定义的行为，因为你离开函数后使用局部变量的地址，你需要替换

void shape_ctor(Shape* me,int x, int y) {   
    struct Vtable vtbl = { &area,&lenght };

通过

void shape_ctor(Shape* me,int x, int y) {   
    static struct Vtable vtbl = { &area,&lenght };

和

void circle_ctor(circle* me, uint16_t x, uint16_t y, uint16_t r) {
    struct Vtable vtbl = {&circle_area,&circle_lenght};

通过

void circle_ctor(circle* me, uint16_t x, uint16_t y, uint16_t r) {
    static struct Vtable vtbl = {&circle_area,&circle_lenght};

我不得不将area和length的定义从shape.h移到shape.c，我为他们删除了无用的 inline。

我还必须保护标题免受多重包含，例如：

#ifndef SHAPE_H
#define SHAPE_H

#include <stdint.h>

struct Vtable;

typedef struct{
    struct Vtable* vptr;
    uint16_t x;
    uint16_t y;
}Shape;

struct Vtable {
    void(*area)(Shape* me);
    void(*lenght)(Shape* me);
};

void shape_ctor(Shape* me, int x, int y);

#endif

之后，编译执行：

/tmp % gcc -Wall -g *.c
circle.c: In function 'circle_ctor':
circle.c:9:19: warning: initialization from incompatible pointer type [enabled by default]
     static struct Vtable vtbl = {&circle_area,&circle_lenght};
                   ^
circle.c:9:19: warning: (near initialization for 'vtbl.area') [enabled by default]
circle.c:9:19: warning: initialization from incompatible pointer type [enabled by default]
circle.c:9:19: warning: (near initialization for 'vtbl.lenght') [enabled by default]
main.c: In function 'main':
main.c:14:5: warning: passing argument 1 of 'k1.super.vptr->lenght' from incompatible pointer type [enabled by default]
     k1.super.vptr->lenght(&k1);
     ^
main.c:14:5: note: expected 'struct Shape *' but argument is of type 'struct rect *'
main.c:15:5: warning: passing argument 1 of 'd1.super.vptr->lenght' from incompatible pointer type [enabled by default]
     d1.super.vptr->lenght(&d1);
     ^
main.c:15:5: note: expected 'struct Shape *' but argument is of type 'struct circle *'
rect.c: In function 'rect_ctor':
rect.c:9:19: warning: initialization from incompatible pointer type [enabled by default]
     static struct Vtable vtbl = { &rect_area, &rect_lenght };
                   ^
rect.c:9:19: warning: (near initialization for 'vtbl.area') [enabled by default]
rect.c:9:19: warning: initialization from incompatible pointer type [enabled by default]
rect.c:9:19: warning: (near initialization for 'vtbl.lenght') [enabled by default]
/tmp % ./a.out
rectangle lenght: 70
circle lenght: 31.400000
/tmp % 

修改 main 以分配堆中的元素以允许 valgrind 知道它们的大小以检测可能的访问：

#include <stdlib.h>

#include "rect.h"
#include "circle.h"

int main() {
    rect  * k1 = malloc(sizeof(*k1));
    circle * d1 = malloc(sizeof(*d1));

    rect_ctor(k1, 10, 10, 15, 20);
    circle_ctor(d1, 10, 10, 5);
    
    
    k1->super.vptr->lenght(k1);
    d1->super.vptr->lenght(d1);
    
    free(k1);
    free(d1);
    
    return 0;
}

valgrind下执行：

/tmp % valgrind ./a.out
==53229== Memcheck, a memory error detector
==53229== Copyright (C) 2002-2015, and GNU GPL'd, by Julian Seward et al.
==53229== Using Valgrind-3.12.0 and LibVEX; rerun with -h for copyright info
==53229== Command: ./a.out
==53229== 
rectangle lenght: 70
circle lenght: 31.400000
==53229== 
==53229== HEAP SUMMARY:
==53229==     in use at exit: 0 bytes in 0 blocks
==53229==   total heap usage: 2 allocs, 2 frees, 48 bytes allocated
==53229== 
==53229== All heap blocks were freed -- no leaks are possible
==53229== 
==53229== For counts of detected and suppressed errors, rerun with: -v
==53229== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)
/tmp % 

---

为了避免在编译时出现所有这些警告，我鼓励您添加所需的演员表。

---

注意定义vtbl也比较好，你不想让它们被错误破坏，所以

void shape_ctor(Shape* me,int x, int y) {   
    static const struct Vtable vtbl = { &area,&lenght };

void circle_ctor(circle* me, uint16_t x, uint16_t y, uint16_t r) {
    static const struct Vtable vtbl = {&circle_area,&circle_lenght};

在 shape.h 中：

typedef struct{
    const struct Vtable* vptr;

您可以将成员/变量/参数const 声明得越多，它在编译时检测错误的能力就越好。