铸造无效**来为int的二维数组 - Ç [英] casting void** to 2D array of int - C

问题描述

我试图施放一个无效**指向int **二维数组用C

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

下面是我试图用劳动code（与所有的外来位去掉）：

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;
}

当我编译，它工作正常，但给了我一个警告变量矩阵设置，但没有使用。我真的不希望有使用临时矩阵变量，因为该功能只应该初始化数组，而不是把任何东西它;但是，如果我尝试直接投graph_data为int **当我assiging它graph->图如下所示：

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;

这让我从兼容的指针类型的警告的分配。

it gives me an assignment from incompatible pointer type warning.

我只是不投这正常吗？没有任何人有任何建议，我怎么可以让它不临时矩阵变量工作？如果还是不行，我可以用这个变量做，这样它不给我警告，它被设置但未使用？

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?

感谢

推荐答案

编译器是正确的，数组的数组（或指向数组）是不一样的一个指针的指针。试想想，他们将如何在内存中排列：

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:

大小的M×N的数组的数组形式的矩阵：

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和指针的形式指针相同的矩阵：

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] | ... |
  +--------------+--------------+-----+

这不要紧，如果你分配正确的尺寸，这两个变量根本不相容这就是你的编译器告诉你。

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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