James Routley

June 25, 2025・7 minute read

I write type safe generic data structures in C using a technique that I haven’t seen elsewhere¹. It uses unions to associate type information with a generic data structure, but we’ll get to that. My approach works for any type of data structure: maps, arrays, binary trees… but for this article I illustrate the ideas by implementing a basic linked list. Since many people aren’t aware you can do C generics at all, I figured I’d start simple and build up to this:

typedef struct {
    int value;
} Foo;
    
List(int) int_list = {0};
list_prepend(&int_list, 3);

List(Foo) foo_list = {0};
list_prepend(&foo_list, (Foo){ 5 });
list_prepend(&foo_list, (Foo){ 3 });

// this won't compile, which is good!
// list_prepend(&foo_list, 7); 

list_for(item, &foo_list) {
    // `item` is of type `Foo *`
    printf("%i\n", item->value);
}

I hesitate to even mention this, because I do not like it², but its worth comparing to the technique at the end of this article. It works like this: you write your data structure in a header, using macros for your types, and then #include the header multiple times; once for each type the data structure will be used with.

Click to see the Code

list.h

#ifndef T
#error "T must be defined before including this header"
#endif

#define _CONCAT(a, b) a##b
#define CONCAT(a, b) _CONCAT(a, b)

#define NODE_TYPE CONCAT(T, ListNode)
#define PREPEND_FUNC CONCAT(T, _list_prepend)

typedef struct NODE_TYPE NODE_TYPE;
struct NODE_TYPE {
    NODE_TYPE *next;
    T data;
};

void PREPEND_FUNC(NODE_TYPE **head, T data) {
    NODE_TYPE *node = malloc(sizeof(*node));
    node->data = data;
    node->next = *head;
    *head = node;
}

#undef T
#undef _CONCAT
#undef CONCAT
#undef NODE_TYPE
#undef PREPEND_FUNC

main.c

typedef struct {
    int a;
} Foo;

typedef struct {
    char *str;
    double num;
} Bar;

#define T Foo
#include "list.h"

#define T Bar
#include "list.h"

FooListNode *foo_head = NULL;
Foo_list_prepend(&foo_head, (Foo){1})

BarListNode *bar_head = NULL;
Bar_list_prepend(&bar_head, (Bar){"hello", 5.4})

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I write type-safe generic data structures in C