C Initialization Designators

I have been working on a C parser over the last few days, and because of this I have been reading the section of C99 relating to syntax much more carefully than I have in the past. In particular, §6.7.8 Initialization had a construct I had never seen before.

initializer:
    assignment-expression
    { initializer-list }
    { initializer-list , }

initializer-list:
    designation_opt initializer
    initializer-list , designation_opt initializer

designation:
    designator-list =

designator-list:
    designator
    designator-list designator

designator:
    [ constant-expression ]
    . identifier

It starts very conventionally. If we are initializing a scalar (or copying an aggregate or union), we use an assignment expression. This is the familiar

int x = 5;
struct aggregate_t s = some_function_returning_aggregate();

We can have a list of initializers enclosed in braces. This is the well known construct for initializing arrays:

int arr[] = {1,2,3,4,5};

We can initialize based on the members of a structure (this was added in C99, so anyone using compilers that are based on 25+ year old standards won't have this):

struct {
    int x;
    int y;
} point = { .x=4, .y=7 };

This is all basic stuff, that I see in common use. What caught my eye, largely because I couldn't see how the EBNF translated to C syntax, was the concept of designator-lists and the [ constant-expression ] form of designators. This lets us initialize particular elements of an array, and do precise initialization of nested aggregates.

int arr[4] = { [0]=0*0, [2]=2*2 }

struct {
    struct Point points[4];
    struct Point otherPoints[4];
} s = { .point[2].x=3, .otherPoints[3].y=1 };

Since this could leave a minefield of partially initialized structures, it is easy to understand why the are so infrequently used. I am not sure I have ever seen this syntax in a codebase, and I have certainly never seen it in teaching materials. It also wasn't carried over into C++, so it could prevent a program from being compiled by both C and C++ compilers. Nevertheless, if you want to pick up on interesting (and potentially useful) language features, reading the standard (or your compiler's manual) is invaluable.