Initialise first n members only of struct or array

Question

I'm using the clang compiler (c++ 11 I think) that comes with RAD studio 10.2. By mistake I discovered today that the first n members of a struct or array can be assigned using the usual curly brackets e.g.

int a[500]={1};
struct {int a,b,c;} st={2,3};

The above compiles and works fine but I've never come across this or seen it used before and I can find no mention of it online (maybe I'm searching using the wrong type of wording). Is this c++ documented?

Answer 1

For aggregate initialization,

(emphasis mine)

If the number of initializer clauses is less than the number of members and bases (since C++17) or initializer list is completely empty, the remaining members and bases (since C++17) are initialized by their default member initializers, if provided in the class definition, and otherwise (since C++14) by empty lists, in accordance with the usual list-initialization rules (which performs value-initialization for non-class types and non-aggregate classes with default constructors, and aggregate initialization for aggregates).

That means, for int a[500]={1};, the 1st element is initialized to 1 and the remaining 499 elements of the array are value-initialized to 0. For struct {int a,b,c;} st={2,3};, the member a is initialized to 2 and b is initialized to 3, the last member c is value-initialized to 0 too.