I write a struct
struct Tree{
struct Node *root;
struct Node NIL_t;
struct Node * const NIL; //sentinel
}
I want
struct Node * const NIL = &NIL_t;
I can't initialize it inside the struct. I'm using msvs.
I use C, NOT C++. I know I can use initialization list in C++.
How to do so in C?
If you are using C99, you can used designated initializers to do this:
struct Tree t = { .root = NULL, .NIL = &t.NIL_t };
This only works in C99, though. I've tested this on gcc and it seems to work just fine.
#define TreeDecl(id, rt) struct Tree id = { .root = rt, .NIL = &id.NIL_t }
struct Tree t = { NULL, { 0 }, &t.NIL_t }; as long as 0 is a valid initializer for the first field in a struct Node.
NIL member came before the NIL_t member, then use struct Tree t = { NULL, &t.NIL_t };For those seeking a simple example, here it goes:
#include <stdio.h>
typedef struct {
const int a;
const int b;
} my_t;
int main() {
my_t s = { .a = 10, .b = 20 };
printf("{ a: %d, b: %d }", s.a, s.b);
}
Produces the following output:
{ a: 10, b: 20 }
A structure defines a data template but has no data itself. Since it has no data, there's no way to initialize it.
On the other hand, if you want to declare an instance, you can initialize that.
struct Tree t = { NULL, NULL, NULL };
NIL_t is a struct Node, not a struct Node *, so it probably won't be initialized as NULL. I'd also recommend making an struct Tree *init(...) function (or a void init(struct Tree *t, ...) function if you prefer, or a macro) to do this kind of work for you.
structs you could use struct Tree t = { NULL, { /* struct Node contents here */ }, NULL } (or &t.NIL_t for the last one.)struct Node inside a struct Tree. Not infinitely recursive in this case. (In the case that it is infinitely recursive, this would be a problem. But that's not the OP's case.)Maybe something like this will suffice?
struct {
struct Node * const NIL;
struct Node *root;
struct Node NIL_t;
} Tree = {&Tree.NIL_t};
