C 中的 typedef 结构 inheritance

Question

Let's say I want to create two sub-types of a type in C. For example:

typedef struct Car {
    char *make;
} Car;

typedef struct Book {
    char *title; 
    char *author;
} Book;

What are the options for doing this? I come from a python background, so am used to being able to do something like:

class Item:
    pass

class Car(Item):
    ...

class Book(Item):
    ...

The only thing that comes to mind for C is doing a union or enum but then it seems like it will have a ton of un-used fields. For example:

typedef struct Item {
    enum {Car, Book} type; // hide the class here
    char *make;
    char *title; // but now will have a bunch of null fields depending on `Item` type
} Item;

Or:

typedef struct Item {
    union {
        Car;
        Book;
    } item;
} Item;

What options are there to do this sort of pseudo-subclassing in C? My goal here is to be able to pass 'multiple types' to the same function, in this case Car and Book .

Answer 1

Put the common superclass as an initial member of each subclass.

typedef struct Car {
    Item item;
    char *make;
} Car;

typedef struct Book {
    Item item;
    char *title;
    char *author;
} Book;

You can then cast a Book* or Car* to Item* when calling generic Item functions.

Another option is a discriminated union.

typedef struct Item {
    // general Item stuff goes here
    enum {Car, Book} type;
    union {
        Car car;
        Book book;
    };
} Item;

But if you need to do a lot of this, maybe you should use C++ instead of C, so you have real class hierarchies.