CODEWITHSUNDEEP
carrayspointers
developer
developer

Reputation: 678

How to pass "pointer to a pointer" to a function that expects "pointer to array"

Consider this piece of code:

#define MAX 4
............
............ 
int** ptr = (int**)malloc(sizeof(int*)*MAX);
*ptr = (int*)malloc(sizeof(int)*MAX);
// Assigned values to the pointer successfully

How foo() can be invoked with ptr as parameter ? foo()'s prototype has been declared as below:

void foo(int arr[][MAX]);

Upvotes: 1

Views: 151

Answers (2)

Joe Z
Joe Z

Reputation: 17956

I am going to assume the function foo in your example actually wants a 2-D array of int, with MAX columns and an unspecified number of rows. This works in C and C++ because of how the rows lay out in memory. All the elements in row N+1 appear contiguously after all the elements in row N.

The syntax int arr[][MAX] asks for a pointer to the first element of such a 2-D array, not an array of pointers to rows. I'll assume you want the 2-D array, and not an array of pointers.

First, you need to correctly allocate your elements. You haven't specified what the leftmost dimension of arr[][MAX] is, or where it comes from. I'll assume it's in the variable dim here:

int (*ptr)[MAX];  /* pointer first element of an int[][MAX] array */

/* Allocate a 2-D array of dim * MAX ints */
ptr = (int (*)[MAX]) malloc( dim * MAX * sizeof(int) );

Then, to call your function, just do foo( ptr ) and it'll work without errors or warnings.

To make your code cleaner (especially if you're using many of these 2-D arrays), you might consider wrapping the pointer type in a typedef, and writing a small function to allocate these arrays.

typedef int (*int_array_2d)[MAX];

int_array_2d alloc_int_array_2d( int dim1 )
{
    return (int_array_2d) malloc( dim1 * MAX * sizeof(int) );
}

That way, elsewhere in your code, you can say something much simpler and cleaner:

int_array_2d ptr = alloc_int_array_2d( dim );

Use the type system to your advantage. The C and C++ syntax for the type and the typecast are ugly, and unfamiliar to most people. They look strange due to the precedence of * vs. []. If you hide it in a typedef though, it can help keep you focused on what you're trying to do, rather than understanding C/C++'s weird precedence issues that arise when you mix arrays and pointers.

Upvotes: 3

Sander De Dycker
Sander De Dycker

Reputation: 16243

You can't pass ptr as parameter to that foo function.

The memory layout of a 2-dimensional array (array of arrays) is quite different from that of an array of pointers to arrays.

Either change the function signature to accept a int** (and probably also size information), or define ptr to be a 2-dimensional array of the appropriate size.

Upvotes: 6

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