如何制作泛型函数 C

Question

I have to create an HashMap. I created two structs. A node struct and a table struct . Second struct will "contain" my nodes. My trouble is the insert function: I have to insert in my table (*t) couple (key,value). The problem is that key and value type must be generic. So , I would like to insert int, double, char, ecc.. How can I make my function into a generic function ?

insert.c

struct node{
    int key;
    int val;
    struct node *next;
    struct node *prev;
};
struct table{
    int size;
    struct node **list;
};

int hashCode(int key){
    if(key>0)
        return key-1;
    return key+1;
}

void insert(struct table *t,int key,int val){
    int pos = hashCode(key);
    struct node *list = t->list[pos];
    struct node *newNode = (struct node*)malloc(sizeof(struct node));
    struct node *temp = list;
    while(temp){
        if(temp->key==key){
            printf("%s", "Key already created");
            return;
        }
        temp = temp->next;
    }
    newNode->next = list;
    newNode->key = key;
    newNode->val = val;
    if(list!=NULL){
        list->prev = newNode; 
    }
    t->list[pos] = newNode;
    newNode->prev = NULL;
}

main.c

int main(){
    struct table *t = /* I create the structure */ ;
    insert(t,2,3);
    insert(t,3,4);
    insert(t,2,3);
    // insert(t,'a','c');  function insert should execute also with char, int, float, double ecc...
    return 0;
}

Answer 1

You define the union like this :

union value {
    int i;
    float f;
    char c;
//and so on
};

And your node like this :

struct node{
    int key;
    union value val;
    struct node *next;
    struct node *prev;
};

You change your function argument like this :

void insert(struct table *t, int key, union value val) {}

And your main becomes (I use a macro to simplify the creation of unions) :

#define UVAL(v, k) (union value){ .k = v}

int main(){
    struct table * t = NULL; //Todo

    insert(t, 2, UVAL(3, i) );
    insert(t, 3, UVAL(4, i));
    insert(t, 2, UVAL(3, i));
    return 0;
}

With some floats instead :

int main(){
    struct table * t = NULL; //Todo

    insert(t, 2, UVAL(3.5f, f));
    insert(t, 3, UVAL(4.f,  f));
    insert(t, 2, UVAL(3.5f, f));
    return 0;
}

Answer 2

Since C11 you can use _Generic

#include <stdio.h>

#define hashCode(x) _Generic((x), \
    int: hash_int, \
    double: hash_double \
)(x)

int hash_int(int value)
{
    printf("Hashing %d\n", value);
    return 42;
}

int hash_double(double value)
{
    printf("Hashing %f\n", value);
    return 42;
}

int main(void)
{
    hashCode(3);
    hashCode(3.14);
    return (0);
}

From cppreference

Explanation

Provides a way to choose one of several expressions at compile time, based on a type of a controlling expression

First, the type of controlling-expression undergoes lvalue conversions. The conversion is performed in type domain only: it discards the top-level cvr-qualifiers and atomicity and applies array-to-pointer/function-to-pointer transformations to the type of the controlling expression, without initiating any side-effects or calculating any values.

The type after conversion is compared with type-names from the list of associations.

If the type is compatible with the type-name of one of the associations, then the type, value, and value category of the generic selection are the type, value, and value category of the expression that appears after the colon for that type-name.

If none of the type-names are compatible with the type of the controlling-expression, and the default association is provided, then the type, value, and value category of the generic selection are the type, value, and value category of the expression after the default : label.