Question

我正在阅读用C语言实现接口，我真的想试一试。我以为我知道怎么做但是我被困在最后一块拼图上。我不能在不破坏封装的情况下继续。我有两个＆＃34;对象＆＃34;，＆＃34; Fruit＆＃34;和＃34; Apple＆＃34;。我希望用户处理Fruit对象，但永远不知道底层类型（在这种情况下为Apple）。

我似乎无法做到，因为我无法找到Apple知道如何使用Fruit创建自己的方法。我不想让Main.c看到constructor，但Apple.c需要看到它。我不知道如何继续。

我有以下项目结构：

Fruit.h

// fruit is too general to be created here
// fruit objects can only be created in other files
typedef struct Fruit Fruit;
void grow(Fruit *f);

Fruit.c

#include "Fruit.h"
// It's very important user cannot see 'struct Fruit'
struct Fruit 
{ 
    void *fruit_obj;    // Fruit object of unknown type
    void(*grow)(void*); // grow function for that type of fruit 
};

// User should not see this!
// Fruit objects can only be made from other files!
Fruit* constructor(void *fruit_obj, void(*grow)(void*))
{
    Fruit *f = malloc(sizeof(Fruit));
    f->fruit_obj = fruit_obj;
    f->grow = grow;
    return f;
}

void grow(Fruit *f)
{
    f->grow(f->fruit_obj);
}

Apple.h

#include "Fruit.h"
Fruit* createApple();

Apple.c

#include "Apple.h"
// The user should never ever see this struct.
struct Apple
{
    int size;
}

void apple_grow(void *a)
{
    struct Apple *apple = (struct Apple*)a;
    apple->size++;
    printf("Your apple has grown to %d size!\n", apple->size);
}

// My attempt to make me able to create fruit from this file.
extern Fruit* constructor(void *fruit_obj, void(*grow)(void*));
Fruit* createApple()
{
    struct Apple *a = malloc(sizeof(struct Apple));
    a->size = 1;
    Fruit *f = constructor(a, &apple_grow);
    // LINKER ERROR: 'undefined reference to 'constructor''
    return f;
}

MAIN.C

#include "Apple.h"
int main()
{
    Fruit *f = createApple();
    grow(f);
}

尝试在C中实现接口时未定义的引用

0 个答案: