C99-C23 functors

functor.h

typedef struct functor_s functor_t;
typedef struct functor_s {
    void (*callback)(functor_t* f);
    // other fields here
} functor_t;

typedef struct functor_ex_s {
    union {
        functor_t base;
        struct functor_s; // open unnamed struct to push field names up
    };
    // extra fields here or pointer to extended data:
    void* ex_data; // this is what 'that' is for
} functor_ex_t;

// nameless union is not necessary but pushing fields up prevents field names reuse and confusion later

void foo(functor_t* f) { 
    functor_ex_t* fx = (functor_ex_t*)f;
}