[英]C Dereference void* pointer
I am new to C and for my first project I need to implement an array based queue.我是 C 新手,对于我的第一个项目,我需要实现一个基于数组的队列。
I want my queue to be able to hold any kind of object so I created a QueueElement structure to hold a void pointer to an object of any type.我希望我的队列能够保存任何类型的对象,因此我创建了一个 QueueElement 结构来保存指向任何类型对象的 void 指针。 I think all is working except I am unable to read the 'position' and 'value' fields from my QueueElement struct.
我认为除了我无法从我的 QueueElement 结构中读取“位置”和“值”字段之外,一切都在工作。 I get the following error when I try to compile.
当我尝试编译时出现以下错误。
Error:错误:
Runnable.c: In function `main':
Runnable.c:10: error: dereferencing pointer to incomplete type
Runnable.c:11: error: dereferencing pointer to incomplete type
I'm pretty sure I'm just not casting properly.我很确定我只是没有正确投射。 Any help is appreciated.
任何帮助表示赞赏。
Thanks again, Pooch再次感谢,波奇
Runnable.c可运行文件
#include <stdio.h>
#include "Queue.h"
int main(void) {
int i = 9;
Queue q = CreateQueue();
QueueElement e = CreateQueueElement(&i);
Enqueue(q, e);
QueueElement f = Dequeue(q);
/* PROBLEM IS HERE */
printf("position: %d", f->position);
printf("value: %d", (int *)(f->value));
DestroyQueue(q);
return 0;
}
Queue.h队列.h
#ifndef QUEUE_H
#define QUEUE_H
#include "QueueElement.h"
typedef struct QueueStruct *Queue;
Queue CreateQueue(void);
void DestroyQueue(Queue q);
void Enqueue(Queue q, QueueElement e);
QueueElement Dequeue(Queue q);
#endif
Queue.c队列文件
#include "QueueElement.h"
#include "Queue.h"
#define QUEUE_SIZE 10
struct QueueStruct {
QueueElement contents[QUEUE_SIZE];
int size;
};
Queue CreateQueue(void) {
Queue q = malloc(sizeof(struct QueueStruct));
q->size = 0;
return q;
}
void DestroyQueue(Queue q) {
int i;
for(i = 0; i < q->size; i++) {
free(q->contents[i]);
}
free(q);
}
void Enqueue(Queue q, QueueElement e) {
if (q->size < QUEUE_SIZE) {
q->contents[q->size++] = e;
}
}
QueueElement Dequeue(Queue q) {
if (q->size > 0) {
return q->contents[--q->size];
}
return;
}
QueueElement.h队列元素.h
#ifndef QUEUE_ELEMENT_H
#define QUEUE_ELEMENT_H
typedef struct QueueElementStruct *QueueElement;
QueueElement CreateQueueElement(void *v);
void DestroyQueueElement(QueueElement e);
int GetPosition(QueueElement e);
#endif
QueueElement.c队列元素
#include <stdio.h>
#include "QueueElement.h"
struct QueueElementStruct {
int position;
void *value;
};
QueueElement CreateQueueElement(void *v) {
QueueElement e = malloc(sizeof(struct QueueElementStruct));
e->position = 0;
e->value = v;
return e;
}
void DestroyQueueElement(QueueElement e) {
free(e);
}
int GetPosition(QueueElement e) {
return e->position;
}
The definition of QueueElementStruct
has to be visible in Runnable.c to be able to access fields of it. QueueElementStruct
的定义必须在 Runnable.c 中可见才能访问它的字段。 You can put QueueElementStruct
into a header that you can include in Runnable.c
and QueueElement.c
.您可以将
QueueElementStruct
放入可以包含在Runnable.c
和QueueElement.c
的标头中。 Alternatively, you can use your GetPosition
function and add a GetValue
function and use those from Runnable.c
instead of direct field access.或者,您可以使用
GetPosition
函数并添加GetValue
函数并使用Runnable.c
函数而不是直接字段访问。
You have to cast the void *
back to point at the "real" type before you can dereference it.在取消引用它之前,您必须将
void *
回指向“真实”类型。 eg, if you start with an int
, you can take its address, and put it in the queue.例如,如果您以
int
开头,则可以获取其地址,并将其放入队列中。 To look at the int
, you'll have to cast it back to int *
.要查看
int
,您必须将其转换回int *
。 Keeping track of the real type can be (usually is) non-trivial (eg, creating an enumeration of all types you want to be able to put in the collection and associating one of those with each item in the collection).跟踪真实类型可能(通常是)非常重要(例如,创建您希望能够放入集合中的所有类型的枚举,并将这些类型之一与集合中的每个项目相关联)。
There's a reason that C++ (for one example) opts for only putting one type of object into any given collection. C++(例如)选择只将一种类型的对象放入任何给定集合是有原因的。
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