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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.

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  • It's legit in C. Array members are initialized from the beginning. So you even can: int i[] = { [1] = func(i[0]), [0] = 5 }; Commented Aug 13, 2018 at 9:09
  • 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 stackoverflow.com/questions/629017/… Commented Aug 13, 2018 at 9:14
  • @Goodies you could also initialize the whole array using a single value 42 Would you mind to show how? Commented Aug 13, 2018 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..) Commented Aug 14, 2018 at 14:17
  • @Goodies in this case all the rest elements will be zeroes. Commented Aug 14, 2018 at 14:21

1 Answer 1

Reset to default
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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

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