Why can't arrays of same type and size be assigned?

Question

If I declare two arrays - arr1 and arr2 - of, say, type int of size 10 each, and initialize first array, and I wish to create a copy of arr1 in arr2; why can't I just give the instruction arr2 = arr1 ?

I know two structures of same type can be assigned. Why is that not the case with arrays?

Answer 1

The problem with arrays is that in all expressions (except when passed to the sizeof and the unary & operators) they convert to a pointer to their first element.

So, supposing you have:

int arr1[10];
int arr2[10];
...

Then if you write something like

arr1 = arr2;

you are actually attempting to do this:

arr1 = &arr2[0];

or this:

&arr1[0] = &arr2[0];

In both cases you have a problem preventing your code from compiling. In the former case you're attempting to assign between two incompatible types (array vs pointer), while in the latter case you're attempting to modify a constant pointer (&arr1[0]).

Answer 2

Actually they can, but in indirect way:

#include <stdio.h>

int main(){
  int arr1[3] = {0};
  int arr2[3] = {1, 2, 3};

  struct tmp{
      int arr[3];
  };

  printf("%d %d %d\n", arr1[0], arr1[1], arr1[2]);

  // casting array to address of struct
  // before dereferencing & asigning to it
  *(struct tmp*)arr1 = *(struct tmp*)arr2;

  printf("%d %d %d\n", arr1[0], arr1[1], arr1[2]);

  return 0;
}

You can even make it in a little-bit more generic way:

#include <stdio.h>

#define STRUCT_TO(type, size) struct temp##__LINE__{type arr[size];}; *(struct temp##__LINE__*)
#define STRUCT_FROM *(struct temp##__LINE__*)

int main(){
  int arr1[3] = {0};
  int arr2[3] = {1, 2, 3};

  printf("%d %d %d\n", arr1[0], arr1[1], arr1[2]);

  STRUCT_TO(int,3) arr1 = STRUCT_FROM arr2;

  printf("%d %d %d\n", arr1[0], arr1[1], arr1[2]);

  return 0;
}

Or ... if we can sacrifice some portability, then we can make syntax a little-bit more symmetrical:

#include <stdio.h>

#define ARRAY_LENGTH(arr) (sizeof(arr)/sizeof(arr[0]))
#define TEMP_STRUCT struct temp##__LINE__
#define AS_STRUCT_DESTINATION(arr) TEMP_STRUCT{typeof(arr[0]) arrTmp[ARRAY_LENGTH(arr)];}; *(TEMP_STRUCT*) arr
#define AS_STRUCT_SOURCE(arr) *(TEMP_STRUCT*) arr

int main(){
  int arr1[3] = {0};
  int arr2[3] = {1, 2, 3};

  printf("%d %d %d\n", arr1[0], arr1[1], arr1[2]);

  AS_STRUCT_DESTINATION(arr1) = AS_STRUCT_SOURCE(arr2);

  printf("%d %d %d\n", arr1[0], arr1[1], arr1[2]);

  return 0;
}