lvalue : array and structure

Question

An lvalue is defined as an expression to which a value can be assigned. And it is illegal to assign and array with a array value. E.g.:

int x[2],y[2];
x = y;

Whereas structures can be treated as lvalues. Below structure assignment is valid.

typedef struct car {
    char color[20];
    int price;
} CAR;

CAR audi, bmw;
audi = bmw;

What is the difference?

What do you mean what is the difference? You already stated that. — Sourav Ghosh, May 17 '16 at 14:42
If you will, the problem is not sol much the LHS, which may indeed be an lvalue for all we care, but the RHS. The operand of assignment needs to undergo lvalue conversion, which is not available for arrays. — Kerrek SB, May 17 '16 at 14:44
This question could be salvaged if it would ask about **why** you can assign arrays in structs, bot not free-standing ones. i tried to answer this. — SergeyA, May 17 '16 at 14:49
Also, what will actually happen to the `color` field of `audi`? Is the array copied? — Eugene Sh., May 17 '16 at 14:59
@EugeneSh. Yes? `audi = bmw` is basically `memcpy(&audi, &bmw, sizeof(audi))` — uh oh somebody needs a pupper, May 17 '16 at 15:00
@sleeptightpupper Yes, I know that. But looking whether it is always the case (i.e. the equivalency of the assignment and `memcpy`) — Eugene Sh., May 17 '16 at 15:02
@EugeneSh.: Problem is not the array, but the fact an array decays to a pointer for the RHS. **Inside** a `struct` it is not converted, as you don't reference the array itself. — too honest for this site, May 17 '16 at 15:13

score 3 · Accepted Answer · answered May 17 '16 at 14:48

There are historic reasons why arrays are not assignable on themselves, but are assignable inside structs. There is really no technical reason for this discrepancy.

Anecdottal heresay is that when C was designed, it was based on a certain language (don't remember which one!) which didn't have array assingment, so this feature was exluded from C as well - to preserve compatibility. However, this language didn't have structs, so array assingment inside structs was OKayed.

lvalue : array and structure

1 Answers1