How to make nested aggregate initializer?

Question

I have code as below

struct A {int i; int j;}

int main()
{
    array<A, 2> a;
    a = {{1,2},{3,4}}; //compilation error: not take a right-hand operand of 
                       //type 'initializer list' (or no acceptable conversion)
}

I think it is a nested aggregate initialization, but why not work? And How to make a = {{1,2},{3,4}} work by changing the code?

Find way work

a = { {{1,2},{3,4}} };

Don't know why?

@Neil Butterworth: True, but formally the `{}` initializer will be used to *initialize* a temporary RHS object, which will then be passed to `=` operator. — AnT stands with Russia, Feb 25 '17 at 18:43
See also: http://stackoverflow.com/questions/31449898/initialisation-of-stdarray — AnT stands with Russia, Feb 25 '17 at 18:47

Nicol Bolas · Answer 1 · 2017-02-26T15:11:40.813

2

Aggregate initialization is initialization. a has already been initialized via default initialization. You can't initialize it again (well, you could, but let's not get into the deep magic).

If you want this to work, you must apply the braced-init-list to the declaration of a, not afterwards.

However, if you want to assign to a, you can always do this:

a = decltype(a){{{1,2},{3,4}}}

edited Feb 26 '17 at 15:11

answered Feb 25 '17 at 18:32

Nicol Bolas

449,505
63
781
982

score 0 · Answer 2 · answered Feb 25 '17 at 18:37

0

As already stated you're not initializing but rather assigning, consequently aggregate initialization is out of play here. This could work however, if you could give a little help to the compiler:

std::array<A, 2> a;
a = {A{1,2}, A{3,4}};

Live Demo

answered Feb 25 '17 at 18:37

101010

41,839
11
94
168

How to make nested aggregate initializer?

2 Answers2