结构体动态数组的初始化

Question

This is my struct:

typedef struct ElementToInsert {
    char *key;
    char *value;
} element;

This is my function:

void init_hash(int size, element**arr) {
    *arr = malloc(size * sizeof(element*));
    if (arr == NULL)
    {
        printf("Out of memory\n");
        return 0;
    }
    for (int i = 0; i < size; i++) {

        (*arr)[i].key = NULL;
        (*arr)[i].value = NULL;
    }

}

And the main() :

void main() {
    element *hash_table = NULL;
    int hash_table_size = 10;
    init_hash(hash_table_size, &hash_table);
}

The problem is that when I run the debugger (in visual studio 2015) it doesn't show me all the array values. I expect to see something like: hash_table[0] = {key = NULL, value = NULL}, hash_table[1] = {key = NULL, value = NULL} but all I see is this:

Answer 1

Passing a double pointer to init_hash is unnecessarily complicated. While there are plenty of valid uses for double pointers, this isn't one of them. Normally double pointers are used for multiple output variables or to swap out existing memory, but since you have no return value and you're allocating the memory anyway, it just invites errors. Keep things simple and return a pointer to your list of elements.

element *init_hash(size_t size) {
    element *hash = calloc(size, sizeof(element));

    if (hash == NULL) {
        fprintf(stderr, "Out of memory\n");
        exit(1);
    }

    return hash;
}

A few things of note. First is that to allocate an array of structures, you allocate size * sizeof(element) . That allocates memory for size structs. If you use size * sizeof(element*) that allocates memory for size pointers to the struct . Both are valid, but you have to choose which one you're working with. You appear to be working with a list of structs.

Unlike malloc , calloc will zero the memory for you. This avoids the need to manually zero each struct saving some code and time.

If you run out of memory, and you don't have a plan to deal with it, it's probably best to just save yourself a whole lot of subsequent error messages and exit .

Finally, sizes are best stored as size_t . It's the type that C uses for array sizes and it can catch some type issues.