Write to double const pointer - c

Question

I want to implement my own string implementation for education. For that I defined a struct named string as follows:
struct string {
const char *const data;
const int length;
};
I use functions to create these string structs and then I assign them to variables.
In order to override the const int length I use the following trick:
*(int *) &result.length = // a int
Now I also want to write to the const char *const data.
As far as I know the first const makes sure that you cant edit the items at which the pointer points, and the second const is that you can't point the pointer to a different memory location. These are properties of an immutable string. So my question is: How can I assign something to the const char *const data like I did to the const int length?
Edit: result as shown above is an instance of the struct string

Answer 1

Form the struct string at its declaration and initialize it.
Also recommend to store the size and not the length and use size_t.
#include <stdio.h>
#include <stdlib.h>
struct string {
const char * const data;
const size_t size;
};
struct string string_copy(const char *src) {
size_t size = strlen(src) + 1;
char *copy = malloc(size);
if (copy) {
memcpy(copy, src, size);
} else {
size = 0;
}
struct string retval = {copy, size}; // ****
return retval;
// or return a compound literal (C99)
return (struct string){ copy, size};
}
void string_free(struct string s) {
free((void*)s.data);
}
int main(void) {
struct string a = string_copy("Hello");
printf("%zu <%s>\n", a.size, a.data);
string_free(a);
// do not use `a` subsequently
return 0;
}
I do not recommend to initialize with a string literal like struct string retval = {"World", 6}; as that limits the usefulness of struct string.
Using a opaque struct has many advantages #Jonathan Leffler that exceed this approach - mainly to keep other code from messing with the struct string.