why is type char arr[][] incompatible with char** arr? and how to fix it-CodePudding

I've built a function with the header void function(int d, char*** w) if i call on it like this:

int d;
char** arr;
//some memory allocate function here
function(d, &arr);

i have no problem, but like this:

int d;
char arr[d][d];
function(d,%arr);

i get this error message: passing argument 2 of (function) from incompatible pointer type what's the difference? i learned that they're supposed to be the same type the only difference is that the arr[d][d] declaration has a set continuous part of the memory to work with

CodePudding user response：

In most contexts, an array will decay to a pointer to its first member. However, this does not extend past the first dimension of a multidimensional array.

This means that an array of type char [d][d] decays to a pointer of type char (*)[d], not char **. These types are incompatible.

If you define your function like this:

void function(int d, char arr[][d])

Then it can accept an array of type char [d][d].

CodePudding user response：

If you have a one-dimensional array like

int a[] = {1, 2, 3};

it looks like this in memory:

     --- --- --- 
 a: | 1 | 2 | 3 |
     --- --- ---

And if you then have a pointer to the array's first element:

int *ip = &a[0];

you get something that, by the rules of pointer arithmetic in C, works just about exactly like the array:

     --- --- --- 
 a: | 1 | 2 | 3 |
     --- --- --- 
      ^
      |
     -|- 
ip: | * |
     ---

But a two-dimensional array looks like this:

int a2[2][3] = { { 1, 2, 3 } , { 4, 5, 6 } };

     --- --- --- --- --- --- 
a2: | 1 | 2 | 3 | 4 | 5 | 6 |
     --- --- --- --- --- ---

Or we could draw it like this:

     --- --- --- 
a2: | 1 | 2 | 3 |
     --- --- --- 
    | 4 | 5 | 6 |
     --- --- ---

But the problem is that this layout is not compatible with a pointer-to-pointer. If we have

int **p2;

and if we initialize it to point to some pointers that point to some integers, it'll end up looking more like this:

     --- 
p2: | * |
     -|- 
      |
      v
     ---          --- --- --- 
    | *--------> |   |   |   |
     ---          --- --- --- 
    | *-----.
     ---     \        --- --- --- 
              `----> |   |   |   |
                      --- --- ---

And the problem is that there's nothing in a2 for our pointer p2 to point at that'll work. Since p2 is a pointer-to-a-pointer, it needs to point to a pointer in order to work. But there's no pointer anywhere in a2's representation that can fill this role.