2D Array and double pointer

[sedawk]

Hi,

This is a simple question:

I have a function accepts double pointer, the prototype is:
// code begin
void foo(int **m);
{
// do something with m
}
// code end

It works fine. The problem now is that in my main function the user only provide 2D array type data, e.g.,
// code begin
main()
{
a[3][3]= ... // an array;
foo(a); // <== warning and the program not working
}
// code end

I changed the line &quot;foo(a)&quot; to &quot;foo(&a)&quot; but not helpful. Still giving me the same warning:

'int ** ' differs in levels of indirection from 'int (*)[3][3]'

The compiler is VC 6++ on Windows XP. Now I have to transfer the input 2D array into double pointer array. I think there must be something can do this. I checked some posts here on passing 2D array to a function but not work in this case.

Thanks

 

[Salem]

> a[3][3]= ... // an array;
The prototype would be
void foo ( int a[3][3] );

And the call would be
foo ( a );

--

 

[sedawk]

Hi Salem,

Thanks for your reply. I think I didn't describe the question clearly. Yes, I know the prototype:

void foo(int a[3][3]);

will work fine. But the problem is I can't change the prototype but only input data, ie., a[3][3] or **a in main() function. Now what I am doing is to converse all data form to double pointer array. It works fine.

 

[teriviret]

Be careful about double-pointers. They could mean two different things:

1. A pointer to an array of pointers to arrays.
2. A pointer to a pointer to a flattened single-dimensional array.

When you define a[3][3] you are actually defining a flattened single-dimensional array, because that's how it's organized in memory. It does not make sense to use a double-pointer here, at least in your example.

A double-pointer would be more appropriate if you defined something like this which is a multi-level 2D-array:

typedef int row[3];
int row *a[3];

Here the array &quot;a&quot; contains three pointers to three objects of type &quot;row&quot;, which are themselves arrays of three integers each. Then it makes sense to pass &a which is a double-pointer.