c - Designated initializers and omitted elements

Question

Can anybody please explain the following line about the designated initializers:

The initializer list can omit elements that are declared anywhere in the aggregate, rather than only at the end.

Answer 1

If you use a conventional initialiser list, the values for the elements are assigned in order, so if you have this struct:

typedef struct _foo {
  int a;
  int b;
} foo_t;

then this initialiser explicitly assigns a and not b:

foo_t value = { 7 };

without designated initialisers, the only elements which can be omitted are the ones declared at the end

using designated initialisers, you can omit elements that are declared anywhere:

foo_t value = { .b = 8 };

so the initialiser for value.a is omitted, despite being the first value in the struct.