Why can I not assign interchangeably with two structs that have identical contents?

Question

I'm trying to learn C and I've come across something weird:

struct
{
  int i;
  double j;
} x, y;

struct
{
  int i;
  double j;
} z;

Here, you can see I created two structs that are identical in their elements.

Why is it that when I try to assign x = z it will generate a compile error but x = y does not? They have the same contents, so why can't I assign them back and forth with each other, regardless?

Is there any way I can make this so I can assign x = z? Or do they simply have to be the same struct.

Can any C gurus point me in the right direction?

Answer 1

They have the same content, but not the same type. If they are intended to be of the same type, simply typedef x z;. If they aren't the same thing, but just happen to contain the same fields, it's better to create a separate function that will assign the fields properly.

---

My usual style for declaring structs in C includes the typedef, so I forgot to mention it (sorry!). Here's the syntax:

typedef struct
{
  int foo;
  double bar;
} x;

/* Further down, if needed */
typedef x z;

Answer 2

Making identically structured types the same is called "duck typing". This is done in some languages, but not in C.