Re: [PR c++/78572] handle array self references in intializers

[Nathan Sidwell <nathan@acm.org>]

On 12/20/2016 11:25 AM, Aldy Hernandez wrote:
> The problem in this PR is that we're trying to initialize an array with
> members of itself:

> Jakub has even gone further to show that for the following:
>
>     ... = { array[3]=5, array[7]=3, array[7]=8, array[7] = 9 };
>
> things get even worse, because we generate code to write twice into [3]:
>
>     {[3]=5, [0]=5, [7]=9, [1]=3, [2]=8, [3]=9}

I looked at this a couple of weeks ago, and got confused.  It wasn't 
very clear to me how the side-effect assignments interact with any 
implict zero assignments.  (Especially if one then tries to support 
C-style designated initializers).

IIUC the side effects of an initializer are evaluated before the storage 
of the initializer itself (separated by a sequence point).  further it 
seems that default zero initialization of non-explicitly initialized 
elements happens after any side-effected store to that element (and 
hence zaps the sideeffect).  The rules for non-aggregates appear to be 
as-if one builds a temporary object and then copy-constructs it into the 
object -- clearly zapping any and all side-effects.  However for 
aggregate initializations it looked like elements were initialized in 
order -- so the side effect on a later element could overwrite the 
initialization of an earlier element.

It wasn't entirely clear.

nathan
-- 
Nathan Sidwell