Is it ok to have excess elements in array initializer?

Question

Usually when we initialize an array, we would probably do:

int arr[] = {1, 2, 3, 4, 5};

But is it ok to explicitly limit the length of the array as below? :

int arr[3] = {1, 2, 3, 4, 5};

Despite some warnings from the compiler, the executable doesn't seem to have any problems.

It is usually a good idea to take heed of warnings. But the compiler is gracious enough to ignore your ramblings on extra elements. However I feel this is sloppy programming. — Ed Heal, Sep 21 '13 at 05:32
What is the motivation for doing is? I don't see any practical use for it, why does it matter? — , Sep 21 '13 at 05:32
@aaronman, OP may want to limit the included items based on a macro or something — perreal, Sep 21 '13 at 05:32
@perreal the compiler already warns you and there is no reason to do it anyway, so why do it — aaronman, Sep 21 '13 at 05:34
If it is done is IDE like Turbo C++ or something like that it will show error — Anoop, Sep 21 '13 at 05:39
Worst case is you overwrite some memory with garbage values (which is bad and can break other stuff!) — David says Reinstate Monica, Sep 21 '13 at 05:39

score 10 · Accepted Answer · answered Sep 21 '13 at 05:37

10

No, it's not OK. From C99, 6.7.8.2:

Constraints

No initializer shall attempt to provide a value for an object not contained within the entity being initialized.

In my read, this means that providing excess initializer elements is a constraint violation, so it invokes undefined behavior.

answered Sep 21 '13 at 05:37

To clarify do you mean that in the example above arr[5] and above is defined behavior but arr[4] and below is undefined? – Pacerier Sep 21 '13 at 05:44
1

@Pacerier If I understand the Standard correctly, then yes, exactly. – Sep 21 '13 at 05:45

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