将 void** 转换为 int 的二维数组 - C [英] casting void** to 2D array of int - C

问题描述

我正在尝试将 void** 指针转换为 C 中的 int** 2D 数组

这是我尝试使用的代码(删除了所有无关位):

*假设我有一个名为 graph 的数据结构，其中包含一些*其中的元素void** graph"和一些元素int order"*/void initialise_graph_data(graph_t *graph){无效**graph_data = NULL;int (*matrix)[graph->order];size_t size = (graph->order * graph->order) * sizeof(int);graph_data = safe_malloc(size);/*安全的malloc工作正常*/矩阵 = (int(*)[graph->order])graph_data;图->图=图数据；}

当我编译它时，它工作正常，但警告我变量矩阵"已设置但未使用.我真的不想使用临时矩阵变量，因为该函数只是应该初始化数组，而不是在其中放置任何东西；但是，如果我尝试将 graph_data 直接转换为 int**，当我将其分配给 graph->graph 时，如下所示:

graph->graph = (int(*)[graph->order])graph_data;

它给了我一个来自不兼容指针类型警告的赋值.

我只是没有正确投射吗?有没有人对我如何在没有临时矩阵"变量的情况下使其工作有任何建议?或者如果没有，我可以用那个变量做什么，这样它就不会警告我它已设置但未使用?

谢谢

解决方案

编译器说的对，数组的数组(或指向数组的指针)和指向指针的指针是不一样的.想想它们在内存中的布局:

大小为 MxN 的矩阵，形式为数组数组:

<前>+--------------+--------------+-----+---------------+--------------+-----+-----+|矩阵[0][0] |矩阵[0][1] |... |矩阵[0][N-1] |矩阵[1][0] |... |矩阵[M-1][N-1] |+--------------+--------------+-----+---------------+--------------+-----+-----+

A 和同一个矩阵"的指针形式:

<前>+-----------+-----------+-----------+-----+|矩阵[0] |矩阵[1] |矩阵[2] |... |+-----------+-----------+-----------+-----+|||||伏||+--------------+--------------+-----+|||矩阵[2][0] |矩阵[2][1] |... |||+--------------+--------------+-----+|||伏|+--------------+--------------+-----+||矩阵[1][0] |矩阵[1][1] |... ||+--------------+--------------+-----+|伏+--------------+--------------+-----+|矩阵[0][0] |矩阵[0][1] |... |+--------------+--------------+-----+

分配大小是否正确无关紧要，这两个变量根本不兼容，这就是您的编译器告诉您的.

i am trying to cast a void** pointer to an int** 2D array in C

here is the code that i am trying to work with (with all the extraneous bits removed):

*assume that i have a data structure called graph with some 
 *element "void** graph" in it and some element "int order" */

void initialise_graph_data(graph_t *graph)
{
    void **graph_data = NULL;
    int (*matrix)[graph->order];
    size_t size = (graph->order * graph->order) * sizeof(int);

    graph_data = safe_malloc(size); /*safe malloc works fine*/
    matrix = (int(*)[graph->order])graph_data;
    graph->graph = graph_data;
}

when i compile that, it works fine, but gives me a warning that variable 'matrix' is set but not used. i dont really want to have to use the interim matrix variable because the function is just supposed to initialise the array, not put anything in it; but if i try to cast graph_data directly to an int** when i am assiging it to graph->graph like so:

graph->graph = (int(*)[graph->order])graph_data;

it gives me an assignment from incompatible pointer type warning.

am i just not casting it properly? does anyone have any suggestions as to how i can make it work without the interim "matrix" variable? or if not, what i can do with that variable so that it doesnt give me the warning that it is set but not used?

thanks

解决方案

The compiler is right, an array of arrays (or a pointer to an array) is not the same as a pointer to a pointer. Just think about how they would be laid out in memory:

A matrix of size MxN in the form of an array of arrays:

+--------------+--------------+-----+----------------+--------------+-----+------------------+
| matrix[0][0] | matrix[0][1] | ... | matrix[0][N-1] | matrix[1][0] | ... | matrix[M-1][N-1] |
+--------------+--------------+-----+----------------+--------------+-----+------------------+

A and the same "matrix" in the form of pointer to pointer:

+-----------+-----------+-----------+-----+
| matrix[0] | matrix[1] | matrix[2] | ... |
+-----------+-----------+-----------+-----+
  |           |           |
  |           |           V
  |           |           +--------------+--------------+-----+
  |           |           | matrix[2][0] | matrix[2][1] | ... |
  |           |           +--------------+--------------+-----+
  |           |
  |           V
  |           +--------------+--------------+-----+
  |           | matrix[1][0] | matrix[1][1] | ... |
  |           +--------------+--------------+-----+
  |
  V
  +--------------+--------------+-----+
  | matrix[0][0] | matrix[0][1] | ... |
  +--------------+--------------+-----+

It doesn't matter if you allocate the correct size, the two variables simply are incompatible which is what your compiler is telling you.

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