Self-Referencing C Struct [duplicate]

This question already has answers here: Why can't we initialize members inside a structure? (6 answers) Closed 7 years ago.

Can you have a structure in C that has elements of that sa开发者_如何学运维me structure? My first attempt at implementing a binary search tree in C is the following:

#include <stdio.h>

struct binary_tree_node {

    int value;
    struct binary_tree_node *left = null;
    struct binary_tree_node *right = null;

};

main() {

    struct binary_tree_node t;
    t.value = 12;

    struct binary_tree_node y;
    y.value = 44;
    t.left = &y;
}

I can't figure out what's wrong with this code, any help would be appreciated. I realize there are other questions on binary search implementations in C, but I'm trying to figure this out from scratch with my own code (and some guidance of course). Thanks!

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Remove the = null in your struct declaration. You can declare the self-reference, but you cannot set it.

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This is the error message on gcc 4:

test.c:6: error: expected ‘:’, ‘,’, ‘;’, ‘}’ or ‘__attribute__’ before ‘=’ token
test.c: In function ‘main’:
test.c:18: error: ‘struct binary_tree_node’ has no member named ‘left’

Firstly, you null is NULL in C. Secondly, you cannot set a value to an element in a struct inside the struct definition.

So, it would look something like this:

#include <stdio.h>

struct binary_tree_node {

    int value;
    struct binary_tree_node *left;
    struct binary_tree_node *right;

};

main() {

    struct binary_tree_node t;
    t.left = NULL;
    t.right = NULL;
    t.value = 12;

    struct binary_tree_node y;
    y.left = NULL;
    t.right = NULL;
    y.value = 44;
    t.left = &y;
}

Or, you can create a function to make left and right NULL,

#include <stdio.h>

struct binary_tree_node {

    int value;
    struct binary_tree_node *left;
    struct binary_tree_node *right;

};

void make_null(struct binary_tree_node *x) {
    x->left = NULL;
    x->right = NULL;
}

main() {

    struct binary_tree_node t;
    make_null(&t)
    t.value = 12;

    struct binary_tree_node y;
    make_null(&y);
    y.value = 44;
    t.left = &y;
}

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You cannot define the values inside the struct when defining the struct. This code snippet may benefit your project:

typedef struct binary_tree_node
{
    int value;
    binary_tree left;
    binary_tree right;
} binary_tree_node, *binary_tree;

#define DATA(T) ((T)->value)
#define LEFT(T) ((T)->left)
#define RIGHT(T) ((T)->right)