Question

I’ m trying to implement a queue structure using C. My implementation is very simple; the queue can hold only ints and nothing else. I was wondering if I could simulate C++ templates in C(probably by using the preprocessor #define) so that my queue can hold any data type.

Note: I do not want to use void*. I think it is a bit risky and can easily cause bizarre runtime errors.

Answer 1

You can use subtle and ugly tricks in order to create that kind of templates. Here’s what I would do:

Creation of a templated list

Macro to define the list

I would first create a macro – let’s call it say define_list(type) – that would create all the functions for a list of a given type. I would then create a global structure containing function pointers to all the list’s functions and then have a pointer to that global structure in each instance of the list (note how similar it is to a virtual method table). This kind of thing:

#define define_list(type) \
\
    struct _list_##type; \
    \
    typedef struct \
    { \
        int (*is_empty)(const struct _list_##type*); \
        size_t (*size)(const struct _list_##type*); \
        const type (*front)(const struct _list_##type*); \
        void (*push_front)(struct _list_##type*, type); \
    } _list_functions_##type; \
    \
    typedef struct _list_elem_##type \
    { \
        type _data; \
        struct _list_elem_##type* _next; \
    } list_elem_##type; \
    \
    typedef struct _list_##type \
    { \
        size_t _size; \
        list_elem_##type* _first; \
        list_elem_##type* _last; \
        _list_functions_##type* _functions; \
    } List_##type; \
    \
    List_##type* new_list_##type(); \
    bool list_is_empty_##type(const List_##type* list); \
    size_t list_size_##type(const List_##type* list); \
    const type list_front_##type(const List_##type* list); \
    void list_push_front_##type(List_##type* list, type elem); \
    \
    bool list_is_empty_##type(const List_##type* list) \
    { \
        return list->_size == 0; \
    } \
    \
    size_t list_size_##type(const List_##type* list) \
    { \
        return list->_size; \
    } \
    \
    const type list_front_##type(const List_##type* list) \
    { \
        return list->_first->_data; \
    } \
    \
    void list_push_front_##type(List_##type* list, type elem) \
    { \
        ... \
    } \
    \
    _list_functions_##type _list_funcs_##type = { \
        &list_is_empty_##type, \
        &list_size_##type, \
        &list_front_##type, \
        &list_push_front_##type, \
    }; \
    \
    List_##type* new_list_##type() \
    { \
        List_##type* res = (List_##type*) malloc(sizeof(List_##type)); \
        res->_size = 0; \
        res->_first = NULL; \
        res->_functions = &_list_funcs_##type; \
        return res; \
    }

#define List(type) \
    List_##type

#define new_list(type) \
    new_list_##type()

Generic interface

Here are some macros that simply call the list’s functions via the stored function pointers:

#define is_empty(collection) \
    collection->_functions->is_empty(collection)

#define size(collection) \
    collection->_functions->size(collection)

#define front(collection) \
    collection->_functions->front(collection)

#define push_front(collection, elem) \
    collection->_functions->push_front(collection, elem)

Note that if you use the same structure to design other collections than lists, you’ll be able to use the last functions for any collections that stores the good pointers.

Example of use

And to conclude, a small example of how to use our new list template:

/* Define the data structures you need */
define_list(int)
define_list(float)

int main()
{
    List(int)* a = new_list(int);
    List(float)* b = new_list(float);

    push_front(a, 5);
    push_front(b, 5.2);
}

You can use that amount of tricks if you really want to have some kind of templates in C, but that’s rather ugly (just use C++, it’ll be simpler). The only overhead will be one more pointer per instance of data structure, and thus one more indirection whenever you call a function (no cast is done, you don’t have to store void* pointers, yeah \o/). Hope you won’t ever use that :p

Limitations

There are of course some limitations since we are using mere text replacement macros, and not real templates.

Define once

You can only define each type once per compile unit, otherwise, your program will fail to compile. This can be a major drawback for example if you write a library and some of your headers contain some define_ instructions.

Multi-word types

If you want to create a List whose template type is made of several words (signed char, unsigned long, const bar, struct foo…) or whose template type is a pointer (char*, void*…), you will have to typedef that type first.

define_list(int) /* OK */
define_list(char*) /* Error: pointer */
define_list(unsigned long) /* Error: several words */

typedef char* char_ptr;
typedef unsigned long ulong;
define_list(char_ptr) /* OK */
define_list(ulong) /* OK */

You will have to resort to the same trick if you want to create nested lists.