In C, compile time constant structs and array with deep self-reference.

self_referencing_construct_with_deep_self_references.c

/* Summary: In C, compile time constant structs and array with deep self-references.

** Context: const and compile-time constants

Constructs fully determined at compile time have some benefits.
But const in C is weaker than constexpr that C++ has.

As prog-fh summarizes on
https://stackoverflow.com/questions/66144082/why-does-gcc-clang-handle-code-slightly-differently-example-given

> "The const means « I swear that I won't change the value of this variable » (or the compiler will remind me!). But it does not
> mean that it could not change by another mean I don't see in this compilation unit; thus this is not exactly a constant."

One example of invalid C:

const int mysize = 2;
const int myarray[mysize];

gcc: error: variably modified ‘myarray’ at file scope

clang: warning: variable length array folded to constant array as an extension [-Wgnu-folding-constant]
const int myarray[mysize];

** Good news: C can do compile time constant structs and array with deep self-references.

Yes, in C you can define and fully declare complex data structures that are accepted as compile-time constants, *including
pointers to parts of itself*.

See "self-contained, statically allocated, totally const data structure with backward and forward references (pointers)?" for a
previous example at
https://stackoverflow.com/questions/47037701/can-c-syntax-define-a-self-contained-statically-allocated-totally-const-data-s


The code below compiles without warning.

gcc -S -W -Wall -Wextra self_referencing_construct_with_deep_self_references.c

 */

#include <stdio.h> // for NULL

/* Example too trivial to be actually useful. */

const int mysize = 2;
const int myarray[mysize];


struct selfref
{
    const struct selfref *const pointer;
};

const struct selfref permutation[2] = { {&permutation[1]}, {&permutation[0]} };

void permutation_check(void)
{
  printf("Adresses:\n%p\n%p\n", &(permutation[0]), &(permutation[1]));
  printf("Pointed addresses:\n%p\n%p\n", permutation[0].pointer, permutation[1].pointer);
}

/* Example hinting at the existence of real-life uses. */

struct question_answers
{
    const char *const question;
    const char *const answer1;
    const struct question_answers *const next1;
    const char *const answer2;
    const struct question_answers *const next2;
};

const struct question_answers dialog_as_array[] = {
  { "Which pill?",
    "Red pill", &dialog_as_array[2],
    "Blue pill", &dialog_as_array[1] },
  { "You wake up in your bed. The story ends. ",
    "Believe it was all a dream.", NULL,
    "Realize matrix has won.", NULL },
  { "Welcome to the Real World. What next?",
    "Become Neo.", &dialog_as_array[4],
    "Give up.", &dialog_as_array[3] },
  { "Mister Anderson!",
    "Billions of people living out their lives, oblivious.", NULL,
    "Evolution, like the dinosaur.", NULL },
  { "Where we go from there is a choice I leave to you.",
    "Go crazy.", NULL, "This is fine.", NULL },
};

struct dialog
{
    const struct question_answers entry;
    const struct question_answers blue_pill_taken;
    const struct question_answers red_pill_taken;
    const struct question_answers red_pill_taken_give_up;
    const struct question_answers red_pill_taken_become_neo;
};

const struct dialog dialog_as_struct = {
    .entry = { "Which pill?",
	       "Red pill", &dialog_as_struct.red_pill_taken,
	       "Blue pill", &dialog_as_struct.blue_pill_taken },
    .blue_pill_taken = { "You wake up in your bed. The story ends. ",
			 "Believe it was all a dream.", NULL,
			 "Matrix has won.", NULL },
    .red_pill_taken = { "Welcome Neo to the Real World. What next?",
			"Become Neo.", &dialog_as_struct.red_pill_taken_become_neo,
			"Give up.", &dialog_as_struct.red_pill_taken_give_up },
    .red_pill_taken_give_up = { "Mister Anderson!",
				"Billions of people living out their lives, oblivious.", NULL,
				"Evolution, like the dinosaur.", NULL },
    .red_pill_taken_become_neo = { "Where we go from there is a choice I leave to you.",
				   "Go crazy.", NULL, "This is fine.", NULL },
};

int main(void)
{
  permutation_check();
}