如何正确使用无效**指针？ [英] how to use void ** pointer correctly?

问题描述

我想使用双无效指针，但我对使用一个有点困惑。
我有一个结构包含无效** 阵列。

I am trying to use a double void pointer but I am a little bit confused about the usage. I have a struct that contains a void ** array.

struct Thing{
    void ** array;
};

struct Thing * c = malloc (sizeof(struct Thing));
c->array = malloc( 10 * sizeof(void *) );

所以，如果我想在不同的对象分配给每个指针，并尝试检索值

So If I want to assign a different object to each pointer and try to retrieve the value

 // Option 1
 *(c->array + index) = (void *) some_object_ptr;

 // Option 2
 c->array[index] = (void *) some_object_ptr;

那么，我还有一个功能，让（无效*）的项目，它指向每一个细胞，而不是 some_object_ptr 。

then, I have another function that gives (void *) item that points to each cell, not the some_object_ptr.

如果我想找回它通过 some_object_ptr ，结果指着值
我应该做的。

If I want to retrieve the value which pointed to by some_object_ptr,
should I do

 function return type is 'void *' and takes argument 'void *'
 // Option 3 
 return (void**) item

 // Option 4
 return *((void**)item)?

奇怪的是，当我用数组的数组下标方法我不能使用选项4，唯一的选择3;当我用 *（C-GT＆;阵列+指数）我只能用opt.4。而不是opt.3。 ..

the weird thing is that when I used array the array subscript method I couldn't use option 4, only option 3; and when I used *(c->array + index) I could only use opt.4. and not opt.3. ..

谁能告诉我这件事？如果我做出任何无效的假设，那么请你指正？

Can anyone please tell me about this? If I am making any invalid assumptions, then could you please correct me?

推荐答案

A 无效** 只是一个指针指向内存的指针与未指定类型。您只能提领一次（因为你不能取消引用无效* ）。但是，除此之外，它基本上是像任何其他指针类型。如果它可以帮助你，想用同样的方式，你会与为int * 。

A void ** is just a pointer to a pointer to memory with an unspecified type. You can only dereference it once (since you can't dereference a void *). However, apart from that, it is basically like any other pointer type. If it helps you, think of it the same way as you would with int *.

所以，在你的具体情况，我们有：

So, in your specific situation, we have:

void** array;
int arrayLen = 10;
array = (void**)malloc(arrayLen * sizeof(void*));

some_type_t* some_object_ptr;    
// The following two assignment are equivalent since in C,
// array[index] <=> *(array + index)
array[index] = (void*)some_object_ptr;
*(array + index) = (void*)some_object_ptr;

然后阵列是指向整个阵列，而 *阵列是一个指向第一个元素，因为它相当于数组[0] 。

Then array is a pointer to the whole array, while *array is a pointer to the first element, since it is equivalent to array[0].

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