How to make an instance in runtime, C

Question

I need to create an instance of one of several data structures(for example: DynamicArray, BST, HashMap, Trie) on C language. The name of data structure read from file and then instance of data type must be create; In general, i need to develop polymorphism but i don't know how to create an instance of structure in runtime; I know there's a solving with void-pointer function but i don't know how...

ATTENTION: I need to create an application, that read file. There's a 6 variants of queues(binary heap, binomial heap, leftist heap, skew heap, fibonacci heap and Treap) and 4 variants(Vector, HashMap, Trie, BST) of containers for this queue. And if i don't develop a sort of polymorphism, then i need to work with 24 difference case because data in source file generate accidentally. Example test file:

LeftistHeap - queue
Trie - container
06:18:39 03/05/24 - time start
06:28:39 03/05/24 - time finish
42 - min time for 1 request 
53 - max time for 1 request 
14 - number of departments
15 25 44 47 48 39 38 32 37 28 11 45 27 11 - count for each department 

I try to use Generic for this problem, but it didn't solve my problem;

There's compilation error:

void error: ‘_Generic’ selector of type ‘void’ is not compatible with any association

void vec() {
    printf("Vector\n");
}

void bst() {
    printf("bst\n");
}

void ht() {
    printf("Ht\n");
}

#define TEST_FUNC(X) _Generic((X), \
    Vector: vec, \
    BST: bst, \
    Hash_Table: ht \
)    

int main()
{
    char* in = "Vector";

    void* data = NULL;
    
    if (strcmp(in, "Vector") == 0) {
 
        data = (Vector*)malloc(sizeof(Vector));
    } else if (strcmp(in, "BST") == 0) {
        data = (BST*)malloc(sizeof(BST));
    } 

    TEST_FUNC(*data); 
} 

Answer 1

_Generic((X) macro is used during compilation not at the run time. So you can't use it in your case. And remember: the cast when you assign to void * pointer, that cast doe not convert it to another type of pointer.

You need to do it runtime. Example:

typedef struct
{
    int dummy[5];
}BST;

typedef struct
{
    int dummy[15];
}VECTOR;

typedef struct
{
    int dummy[25];
}HT;

void vec(VECTOR *v) {
    printf("Vector\n");
}

void bst(BST *bst) {
    printf("bst\n");
}

void ht(HT *ht) {
    printf("Ht\n");
}

typedef enum
{
    BST_T,
    VECTOR_T,
    HT_T,
    UNKNOWN_T,
}type_t;

void TEST_FUNC(void *data, const type_t type)
{
    switch(type)
    {
        case VECTOR_T:
            vec(data);
            break;
        case BST_T:
            bst(data);
            break;
        case HT_T:
            ht(data);
            break;
        default:
            printf("Unknown type\n");
            break;
    }
}


int main()
{
    char* in = "Vector";
    size_t size = 0;
    type_t type = UNKNOWN_T;
    void *data = NULL;
    
    if (!strcmp(in, "Vector")) { size = sizeof(VECTOR); type = VECTOR_T;}
    else if (!strcmp(in, "HT")) { size = sizeof(HT); type = HT_T;}
    else if (!strcmp(in, "BST")) { size = sizeof(BST); type = BST_T;}
 
    if(size) data = malloc(size);

    TEST_FUNC(data, type); 
}