How do subobjects work in C?

Question

What is the meaning of standalone square brackets inside of a C array of a custom type?

typedef enum {
    BAR_1 = 0,
    BAR_2,
    BAR_3,
} bar_types;

typedef struct {
    int is_interesting;
    int age;
} foo_s;

static foo_s bars[] = {
    [BAR_1] = {1, 2},  /* What is being done here? */
    [BAR_2] = {1, 41},
    [BAR_3] = {0, 33},
};

In the above code, what is the meaning of [BAR_1] = {1, 2}? When is it possible to use standalone square brackets?

I've noticed that if I add duplicate value in brackets, clang gives an warning about subobject initialisation.

static foo_s bars[] = {
    [BAR_1] = {1, 2},
    [BAR_2] = {1, 41},
    [BAR_3] = {0, 33},
    [BAR_3] = {0, 33},
};

-----

$clang example.c
example.c:17:19: warning: subobject initialization 
  overrides initialization of other fields within its
  enclosing subobject [-Winitializer-overrides]
    [BAR_3] = {0, 33},
              ^~~~~~~

What is a C subobject?

Those just designate specific array elements to be initialised. The enum values act like the corresponding `int` values, so `[BAR_1] = {1, 2}` becomes `[0] = {1, 2}`, meaning the first struct in the array (index 0) gets initialised with `is_interesting` as 1 and `age` as 2. — Dmitri, Feb 20 '17 at 03:25
For what a subobject is, see http://stackoverflow.com/questions/18451683/c-disambiguation-subobject-and-subclass-object — racraman, Feb 20 '17 at 03:29

score 2 · Answer 1 · edited May 23 '17 at 12:17

2

this is struct initialization, the "subobject" is the instance of the struct that is getting initialized.

Your warning comes from trying to initialize the same array position twice.

also see How to initialize a struct in accordance with C programming language standards

edited May 23 '17 at 12:17

Community

1
1

answered Feb 20 '17 at 03:26

Keith Nicholas

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2

Warning, not error. It is valid to initialize it twice. – 2501 Feb 20 '17 at 08:46
Of course, warnings shouldn't be ignored: -Wno-initializer-overrides. :-) – 2501 Feb 21 '17 at 01:54

How do subobjects work in C?

1 Answers1