How to create non-convertible C types without duplicating code?

Question

I've got some C code with various functions that each take a different type of "handle" object as an argument. The implementation of all of these handles is the same (it's just a struct with a void-pointer and an item-count), so it seems logical to just declare a single implementation -- but I also want the C compiler to generate a compile-time error when the user passes in a handle of the wrong type to a function.

My current approach uses typedef to create the various handle-types, which works to document which handle-type a function should accept, but the compiler does auto-conversion and so it doesn't flag type-mismatches as errors. Is there a recommended way to implement this, that doesn't require manually duplicating my handle-struct-declaration for every type?

Toy example code follows:

typedef struct _FruitHandle {
   int _numItems;
   void * _items;
} FruitHandle;

typedef FruitHandle AppleHandle;
typedef FruitHandle BananaHandle;
// imagine a number of other fruits as well

void EatAnApple(AppleHandle a) {}
void EatABanana(BananaHandle b) {}
// and so on -- each function should ONLY except its own fruit-handle-type as an argument!

int main(int argc, char ** argv)
{
   AppleHandle apple;
   BananaHandle banana;

   EatAnApple(apple);  // ok -- types match
   EatABanana(banana); // ok -- types match

   EatAnApple(banana); // type mismatch -- I want this to be a compile-time error, but it isn't!
   EatABanana(apple);  // type mismatch -- I want this to be a compile-time error, but it isn't!

   return 0;
}

You could write a macro that defines those structs. I don't think it gets any better than that. — HolyBlackCat, Jul 06 '18 at 20:22
Indeed. `typedef` is an alias and what you need are *mutually incompatible types*. — Antti Haapala -- Слава Україні, Jul 06 '18 at 20:24
The weakness seems to be when you bring a fruit that the program does not know. Known fruits are hard coded but soft fruits are not acceptable. — Weather Vane, Jul 06 '18 at 20:25
I would rather create structures having a `FruitHandle` field. So you can extend each `Fruit` with it's unique properties... — Eugene Sh., Jul 06 '18 at 20:27
And, if your struct is truly on 12-bytes, then the benefit of type checking by the compiler far outweighs saving 24-bytes of storage. — David C. Rankin, Jul 06 '18 at 20:42
@EugeneSh. each fruit will have its own unique properties, but the handles are really just a sort of opaque-pointer-to-a-fruit, so the handles will never have any unique properties of their own (or at least I'm not planning for them to have any :)) — Jeremy Friesner, Jul 06 '18 at 20:42

score 4 · Accepted Answer · answered Jul 06 '18 at 20:32

You might find the answers to this question helpful. While there is no explicit type inheritance in C, you can use the idiom described in the top answer to create AppleHandle and BananaHandle.

typedef struct {
    int _numItems;
    void *_items;
} FruitHandle;

typedef struct {
    FruitHandle fruit_handle;
} AppleHandle;

typedef struct {
    FruitHandle fruit_handle;
} BananaHandle;

...

score 2 · Answer 2 · answered Jul 06 '18 at 20:25

2

As mentioned in the comments, you can do this with a macro:

#define FRUIT(name) typedef struct _##name {\
   int _numItems;\
   void * _items;\
} name

FRUIT(AppleHandle);
FRUIT(BananaHandle);

This expands to:

typedef struct _AppleHandle { int _numItems; void * _items;} AppleHandle;
typedef struct _BananaHandle { int _numItems; void * _items;} BananaHandle;

answered Jul 06 '18 at 20:25

dbush

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Identifiers that begin with an underscore followed by an uppercase letter are reserved for any use. I'd just let the struct tag be the same as the type name. They are in different name spaces anyway. – StoryTeller - Unslander Monica Jul 06 '18 at 20:58

How to create non-convertible C types without duplicating code?

2 Answers2