空指针的动态数组 [英] Dynamic Array of Void Pointers

问题描述

我正在尝试基于动态数组创建动态集抽象数据类型.但是，当我尝试将数据添加到数组时，出现编译器警告和错误:

I'm trying to create a dynamic set abstract data type, based on a dynamic array. However, I get a compiler warning and an error when I try to add the data to the array, which are:

警告:取消引用"void *"指针(默认情况下启用)

warning: dereferencing 'void *' pointer [enabled by default]

错误:无效使用void表达式

error: invalid use of void expression

我的代码如下，我用注释标记了有问题的行

My code is as follows, I've marked the problematic line with a comment

struct SET 
{
//general dynamic array
void *data;
int elements; //number of elements
int allocated; // size of array
};

struct SET create()
{
//create a new empty set

struct SET s;
s.data = NULL;
s.elements = 0;
s.allocated = 0;  //allocations will be made when items are added to the set    
puts("Set created\n");
return s;
}

struct SET add(struct SET s, void *item)
{
//add item to set s

if(is_element_of(item, s) == 0) //only do this if element is not in set
{
    if(s.elements == s.allocated)  //check whether the array needs to be expanded
    {   
        s.allocated = 1 + (s.allocated * 2);  //if out of space, double allocations
        void *temp = realloc(s.data, (s.allocated * sizeof(s))); //reallocate memory according to size of the set

        if(!temp) //if temp is null 
        {
            fprintf(stderr, "ERROR: Couldn't realloc memory!\n");
            return s;
        }

        s.data = temp;
    }

    s.data[s.elements] = item;   //the error is here
    s.elements = s.elements + 1;
    puts("Item added to set\n");

    return s;
}

else
{
    fprintf(stdout, "Element is already in set, not added\n");
    return s;
}
}

我已经对void指针进行了研究，但显然我在这里缺少了一些东西.我将不胜感激.感谢您阅读并希望回答！

I've done research on void pointers, but clearly I'm missing something here. I'd appreciate any help I can get. Thanks for reading and hopefully answering!

推荐答案

首先，我认为您打算在结构中包含的是通用指针数组(void *数组)，因为您的项是void * ，并且您想将它们存储为数组.也就是说，您需要一个void *的动态数组，因此，您应该使用void **:

First, I think what you intend to have in your structure is an array of generic pointers (array of void *), because your items are void *, and you want to store them as an array. That is, you want a dynamic array of void *, thus, you should use void **:

struct SET {
    void **data;
    int elements;
    int allocated;
};

当然，您的add函数需要更新:

Of course, your add function needs to be updated:

struct SET add(struct SET s, void *item) {    
        if (is_element_of(item, s) == 0) {
            if (s.elements == s.allocated) {   
                s.allocated = 1 + (s.allocated * 2);
                void **temp = realloc(s.data, (s.allocated * sizeof(*s.data)));
                if (!temp) {
                    fprintf(stderr, "ERROR: Couldn't realloc memory!\n");
                    return s;
                }

            s.data = temp;
        }
        s.data[s.elements] = item;
        s.elements = s.elements + 1;
        puts("Item added to set\n");
        return s;
    }

    else {
        fprintf(stdout, "Element is already in set, not added\n");
        return s;
    }
}

请注意，realloc行已更改:您不想重新分配给s.allocated * sizeof(s)，而是想要s.allocated*sizeof(*s.data)，因为您将存储void *类型的元素(*s.data的类型为void *，我没有明确编写void *以便更容易适应将来的更改.

Note the realloc line was changed: you don't want to realloc to s.allocated * sizeof(s), you want s.allocated*sizeof(*s.data), since you'll be storing elements of type void * (the type of *s.data is void *, I didn't explicitly write void * to make it easier to accomodate possible future changes).

此外，我相信您应该更改函数以接收和返回指向struct SET的指针，否则，您将始终在结构周围进行复制(请记住，值是通过复制传递的.)

Also, I believe you should change your functions to receive and return pointers to struct SET, otherwise, you will always be copying around the structure (remember that values are passed by copy).

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