C: Initialize struct with constant sub-struct

Question

I have a single instantiation of a struct called single_instance. This instance contains constants objects of type struct example_type.

typedef struct {
  int n;
} example_type;

struct {
  int some_variables;
  // ...
  const example_type t1;
  const example_type t2;
  // ...
} single_instance;

Let's say that I want to init single_instance.t1 to {1} and single_instance.t2 to {2}.

What is the most elegant way to do so?

Inline initialization is not possible:

struct {
  int some_variables;
  // ...
  const example_type t1 = {1};
  const example_type t2 = {2};
  // ...
} single_instance;

This doesn't work too:

struct {
  int some_variables;
  // ...
  const example_type t1;
  const example_type t2;
  // ...
} single_instance {0, {0}, {1}};

I have read multiple other threads related to this topic, however it seems that they all refer to initialize structs with "a name". In this case I want just one instantiation of single_instance, as such the type should not have a name.

The last one works. You just forgot the =

klutt
– klutt

2019-07-24 13:59:34 +00:00
Commented Jul 24, 2019 at 13:59 — klutt
– klutt, Commented Jul 24, 2019 at 13:59

Ian Abbott · Accepted Answer · 2019-07-24 14:54:42Z

3

You can use designated initializers to explicitly initialize the members you are interested in. The remaining members will then be implicitly initialized to 0.

struct {
  int some_variables;
  // ...
  const example_type t1;
  const example_type t2;
  // ...
} single_instance = {.t1 = {1}, .t2 = {2}};

edited Jul 24, 2019 at 14:54

answered Jul 24, 2019 at 14:07

Ian Abbott

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Comments

Roman Hocke · Accepted Answer · 2019-07-24 14:02:52Z

1

This works for me (maybe You're just missing = in initialization):

#include<stdio.h>

typedef struct {
  int n;
} example_type;

struct {
  int some_variables;
  // ...
  const example_type t1;
  const example_type t2;
  // ...
} single_instance = {0, {1}, {2}};

int main() {

    printf("%d %d %d\n", single_instance.some_variables, single_instance.t1.n, single_instance.t2.n);
}

answered Jul 24, 2019 at 14:02

Roman Hocke

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