How to set arraylist as empty after free?

Question

I'm testing my array_list with some test: I need to check if my array list is empty after the de-allocation, but I have some problems

...
typedef struct array_list{
    void** array;
    size_t size;
    size_t capacity;
}array_list_t;

array_list_t* array_list_new(size_t capacity) {
    array_list_t* result = (array_list_t*) malloc(sizeof(array_list_t));
    result->array = (void**) malloc(sizeof(void*)*capacity);
    result->size = 0;
    result->capacity = capacity;
    return result;
}

void array_list_free(array_list_t* array) {
    free(array->array);
    free(array);
}

int array_list_is_empty(array_list_t* list){
    if(list->size == 0){
        return 1;
    }else{
        return 0;
    }
}

#include "unity.h"
#include "array_list.h"
...

int main () {
    array_list_t* array = array_list_new(10);
    TEST_ASSERT_EQUAL_INT(1, array_list_is_empty(array)); // 1 == 1 OK
    array_list_insert(array,new_int(1));
    TEST_ASSERT_EQUAL_INT(0, array_list_is_empty(array)); // 0 == 0 OK
    array_list_free(array);
    TEST_ASSERT_EQUAL_INT(1, array_list_is_empty(array)); // 1 == 0 NOT_EQUAL
}

I thought to solve this problem setting the size as 0 after free, for example:

(... free(array); array->size = 0; array->capacity = 0; array = NULL; ...)

How do I solve this problem?

Answer 1

Once you do free(array) in array_list_free(), it's no longer valid to dereference array. So you need to set the array variable to NULL in main():

array_list_free(array);
array = NULL;

Then array_list_is_empty() can check whether its argument is NULL before testing the size:

int array_list_is_empty(array_list_t *list) {
    return list == NULL || list->size == 0;
}

A better design would be for array_list_free() to just free array->array, and allow the caller to do free(array) when it's done with that array list. This is the usual approach: whichever component allocates an object is responsible for freeing it.