Is it allowed to initialize array recursively?

Question

I have the following code snippet:

int i[] = {42, i[0]};

Is such initialization allowed or leads to undefined behaviour?

Three major compilers (gcc, clang, msvc) give me 42 for i[1]. Hence looks legit, but I would like to see a cite from the standard for this case.

It's legit in C. Array members are initialized from the beginning. So you even can: `int i[] = { [1] = func(i[0]), [0] = 5 };` — KamilCuk, Aug 13 '18 at 09:09
Found some tips on this forum.. in this case a reference to the array itself is not needed, you could also initialize the whole array using a single value 42, see https://stackoverflow.com/questions/629017/how-does-array100-0-set-the-entire-array-to-0 — Goodies, Aug 13 '18 at 09:14
@Goodies `you could also initialize the whole array using a single value 42` Would you mind to show how? — Killzone Kid, Aug 13 '18 at 10:09
Read the topic I linked. This is valid C++ syntax: char array[100] = {42}; (note you will have to specify the array length in the declaration..) — Goodies, Aug 14 '18 at 14:17
Ah I've tested it, you're right.. I've only looked at the opening. — Goodies, Aug 15 '18 at 14:17

score 11 · Accepted Answer · edited Jun 20 '20 at 09:12

Yes, it is well-defined.

int i[] = {42, i[0]};

This is an aggregate¹ initialization². Aggregate initialization observes this rule:

[dcl.init.aggr]/6

The initializations of the elements of the aggregate are evaluated in the element order. That is, all value computations and side effects associated with a given element are sequenced before those of any element that follows it in order.

¹⁾ http://eel.is/c++draft/dcl.init.aggr#1

²⁾ http://eel.is/c++draft/dcl.init.aggr#3

Is it allowed to initialize array recursively?

1 Answers1

`[dcl.init.aggr]/6`