Nested struct variable initialization

Question

How can I initialize this nested struct in C?

typedef struct _s0 {
   int size;
   double * elems;
}StructInner ;

typedef struct _s1 {
   StructInner a, b, c, d, e;
   long f;
   char[16] s;
}StructOuter;  StructOuter myvar = {/* what ? */ };

There's a similar post [here](http://stackoverflow.com/questions/629433/how-to-initialize-nested-structures-in-c) on SO for C++. — tranmq, Nov 19 '09 at 08:57

pmg · Accepted Answer · 2009-11-19T09:28:28.900

20

To initialize everything to 0 (of the right kind)

StructOuter myvar = {0};

To initialize the members to a specific value

StructOuter myvar = {{0, NULL}, {0, NULL}, {0, NULL},
                     {0, NULL}, {0, NULL}, 42.0, "foo"};
/* that's {a, b, c, d, e, f, s} */
/* where each of a, b, c, d, e is {size, elems} */

Edit

If you have a C99 compiler, you can also use "designated initializers", as in:

StructOuter myvar = {.c = {1000, NULL}, .f = 42.0, .s = "foo"};
/* c, f, and s initialized to specific values */
/* a, b, d, and e will be initialized to 0 (of the right kind) */

edited Nov 19 '09 at 09:28

answered Nov 19 '09 at 09:00

pmg

106,608
13
126
198

Just to clarify, `StructOuter myvar = { 0 };` will do the same (0-initialization) for all inner structs, so we don't need to explicitly set those to `{0, NULL}`, right? – domsson Apr 28 '20 at 15:44
1

Yes @domsson, the `{ 0 }` initializer will 0-initialize everything, recursively when needed. – pmg Apr 28 '20 at 20:52

score 5 · Answer 2 · answered Dec 14 '17 at 16:12

To highlight struct labels in particular:

StructInner a = {
    .size: 1,
    .elems: { 1.0, 2.0 }, /* optional comma */
};

StructOuter b = {
    .a = a, /* struct labels start with a dot */
    .b = a,
         a, /* they are optional and you can mix-and-match */
         a,
    .e = {  /* nested struct initialization */
        .size: 1,
        .elems: a.elems
    },
    .f = 1.0,
    .s = "Hello", /* optional comma */
};

score 2 · Answer 3 · answered Nov 19 '09 at 08:54

2

double a[] = { 1.0, 2.0 };
double b[] = { 1.0, 2.0, 3.0 };
StructOuter myvar = { { 2, a }, { 3, b }, { 2, a }, { 3, b }, { 2, a }, 1, "a" };

It seems a and b cannot be initialized in-place in plain C

answered Nov 19 '09 at 08:54

Kcats

884
1
15
26

score 1 · Answer 4 · answered Sep 11 '20 at 07:48

Following also works with GCC, C99. GCC doesnt complain about it. I am not sure if this is standard.

double arr[] = { 1.0, 2.0 };   // should be static or global
StructOuter myvar = 
{
    .f = 42,
    .s = "foo",
    .a.size = 2,
    .a.elems = &arr,
    .b.size = 0,  // you can explicitly show that it is zero
    // missing members will actually be initialized to zero
};

Nested struct variable initialization

4 Answers4

Linked