C programming. How to hide implementation of the struct in .c if I need to refer to its instance in header file (.h)

Question

So I am curious how to hide implementation of the struct in .c file if we need to refer to its instance in header file. For example I have the following struct in header fil

Answer 1

Something like this should work fine. Note that the definition of struct Node never leaves List.c.

list.h

#pragma once
#include <stdbool.h>

struct List {
    struct Node *front;
};

void list_init(struct List **list);
void list_free(struct List *list);
void list_insert(struct List *list, void *value);
bool list_contains(struct List *list, void *value);

list.c

#include <stdbool.h>
#include <stdlib.h>
#include "list.h"

struct Node {
    void *value;
    struct Node *next;
};

void list_init(struct List **list) {
    *list = malloc(sizeof(**list));
}

void list_free(struct List *list) {
    struct Node *node = list->front;
    while (node != NULL) {
        struct Node *next = node->next;
        free(node);
        node = next;
    }
    free(list);
}

void list_insert(struct List *list, void *value) {
    struct Node *node = malloc(sizeof(*node));
    node->value = value;
    node->next = list->front;
    list->front = node;
}

bool list_contains(struct List *list, void *value) {
    struct Node *node;
    for (node = list->front; node != NULL; node = node->next)
        if (node->value == value)
            return true;
    return false;
}

main.c

#include <stdio.h>
#include "list.h"

int main() {
    struct List *l;
    list_init(&l);

    int *value_1 = malloc(sizeof(int));
    int *value_2 = malloc(sizeof(int));
    int *value_3 = malloc(sizeof(int));

    list_insert(l, value_1);
    list_insert(l, value_2);
    list_insert(l, value_3);

    printf("Does the list contain value_1: %d\n", list_contains(l, value_1));
    printf("Does the list contain null:    %d\n", list_contains(l, NULL));

    list_free(l);
    return 0;
}

It's very possible that I have some errors in this code. If you see any, feel free to fix them.

Answer 2

You have all the right to say:

typedef struct Node *NodePtr;
struct List
{
    NodePtr front;    
};

this typedef struct Node *NodePtr; is a forward declaration. You can use it for as long as you use pointers to types. Pointers need no knowledge about the type's (classes) structure. And the compiler is happy.