Reflecting inner struct in outer struct initialization

Question

I have a code of structs stated as below:

typedef struct A {
  B numberOfB[3];
} A;

typedef struct B {
  int number1;
  int number2;
  boolean bool1;
  boolean bool2;
} B;

In the source code I have an initialization which looks like this:

A* pointerToA = (A[3]) {

  {5, 1, TRUE, TRUE,
   6, 1, TRUE, FALSE,
   7, 1, TRUE, FALSE},

  {5, 1, TRUE, TRUE,
   6, 1, TRUE, FALSE,
   7, 1, TRUE, FALSE},

  {5, 1, TRUE, TRUE,
   6, 1, TRUE, FALSE,
   7, 1, TRUE, FALSE},
}

Such construction in source code is updating fields in struct B but I do not understand how are fields in B are updated as the values are just listed as it was a array of 12 values. Could someone explain that in details?

Answer 1

Reflecting the substructure in an initializer is optional. So here having 12 value is sufficient to initialize an A.

This is really bad style though, I think, since it makes the code quite difficult to read. Much better would even be to use designated initializers such that the initialization becomes robust versus changes of the structure. Something like

[0] = {.number1 = 5, .number2 = 1, .bool1 = true, .bool2 = true, },

Also observe that C has a proper Boolean type and constants that you best use through the header <stdbool.h> as bool, false and true.

Answer 2

Compound literal actually introduced from the C99:

A compound literal it looks like doing a cast containing an initializer. Its value is an object of the type specified in the cast, containing the elements specified in the initializer; it is an lvalue.

In C, a compound literal designates an unnamed object with static or automatic storage duration.

Semantics for simple compound literal. Below is just the sample struct initialization using compound literal.

 struct sample {int a; int b;} struct_obj;

This is how you construct the compound literal for struct_obj.

 struct_obj = ((struct sample) {1, 2});

This is equivalent to

 {
   struct foo temp = {1, 2};
   structure = temp;
 }

In your case, struct A is initialized with array values.

A* pointerToA = (A[3]) {..... } // here A[3] is initialized with the values in your array.