Why can't I assign arrays as &a = &b?

Question

I have a problem assigning an array like:

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

I know I could use pointers but I want to try it that way...

caf · Accepted Answer · 2014-11-17T11:50:57.517

21

You can't assign arrays in C. You can copy them with the memcpy() function, declared in <string.h>:

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

memcpy(&a, &b, sizeof a);

edited Nov 17 '14 at 11:50

answered Oct 24 '11 at 23:13

caf

2

@Jonathan Leffler: The `&array` is a deliberate choice - when copying a single object (here, a single array), I think the form `memcpy(&dest, &src, sizeof src)` should be preferred (or with `sizeof dest` - they should be the same). This is for consistency - the same `memcpy()` line would work if the type of `a` and `b` were a non-array type. You may not agree, but hopefully you can see that there is certainly "judgement" behind the choice. – caf Nov 17 '14 at 11:59
@weberc2: It is, and `sizeof a` is the size of the array. – caf Feb 14 '15 at 06:27
Why does it need a pointer to a pointer? Isn't it enough to have a pointer to the array? – weberc2 Feb 14 '15 at 13:37
@weberc2: There is no pointer to a pointer anywhere in that answer. `&a` *is* a pointer to the array, as you said yourself. – caf Feb 14 '15 at 22:43
@caf I think you misunderstood me, I said `a pointer to a pointer to an array`, to which you replied, `it is`. But thank you for clarifying, I understand now. – weberc2 Feb 14 '15 at 23:53
@weberc2: Ahh yes, you're right - I misread your comment. `&a` is just a pointer to the array. – caf Feb 15 '15 at 05:05

score 17 · Answer 2 · answered Oct 24 '11 at 22:33

17

That way doesn't work, as you have found. You cannot assign arrays in C.

Structs, however, are assignable. So you can do this:

typedef struct
{
    int x[3];
} T;

T a;
T b = { { 1, 2, 3 } };

a = b;

answered Oct 24 '11 at 22:33

Thanks this was straight, is there any function doing this job, so i dont have to rewrite lot of code. – Starfighter911 Oct 24 '11 at 22:47
Okay, i tried this: ´T a[]; T b[] = { { 1, 2, 3 } }; a = b;´ ,its still not working. Output: "Expression is no lvalue". – Starfighter911 Oct 24 '11 at 22:59
3

@Starfighter911: That's not the code I wrote! Simply `T a;`, not `T a[];`. – Oliver Charlesworth Oct 24 '11 at 23:01

2 Answers2